Monarch PABX Technical Description

The * Monarch 120 B Compact

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TECHNICAL DESCRIPTION PART 1

This manual was prepared by B R Horsfield C. Eng., F.I .E.E. Technical Consultant, with assistance from technical staff associated with Monarch 120B Compact.

Whilst every effort has been made to ensure that the information in this manual is current, subsequent changes may have taken place which are not recorded in this issue (8/82)

© 1982 British Telecommunications.

*Monarch is a Registered Trademark of British Telecommunications.

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PREFACE

The Monarch 120B Compact Call Connect System embodies many of the design principles used in the highly successful Monarch 120 System, large numbers of which are now installed and giving excellent service. It is a fully electronic, stored program controlled, digital switching system, specifically designed for use as a Private Automatic Branch Exchange at customers' installations with 50 extensions or less, yet having the ability in most situations to expand and cater economically for up to 120 extensions and a total of 32 exchange lines and Inter-PBX circuits. The maximum number of exchange lines is 26 and at an installation with one operator's console the maximum number ofInter-PBX circuits is 12. The system is provided in two table top units: a basic unit, which is sufficient by itself to meet the normal requirements of installations having up to 56 extensions, and an add-on unit which is required for larger installations. The add-on unit fits on top of the basic unit and may be provided at any time to accommodate growth.

The system offers a wide range of user facilities and incorporates switching principles and physical design features which ensure that installations will be capable, during their working life, of responding to new requirements resulting from evolutionary changes in the public or private networks to which they are connected. The system also takes full advantage of modern technology, making extensive use oflarge scale integrated circuits, microprocessors and other advanced design features, to ensure a high standard of reliability, virtually silent operation, a small accommodation requirement, low cost, and low power consumption.

The technical description in this manual is divided into 2 parts.

Part 1 Which provides an outline system description for senior engineering managers and others who wish to acquire a general appreciation of the system architecture, operating principles and physical construction, but do not have time to study the more detailed design features.

Part 2 Which provides additional design detail for those who have a specialised interest in the switching field. This part assumes from the beginning that the reader is fully conversant with the content of Part 1 .

Where appropriate, certain material has been removed into appendices to avoid disturbing the continuity of the main description.

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MONARCH 120B COMPACT CALL CONNECT SYSTEM

TECHNICAL DESCRIPTION - PART 1 CONTENTS

1 INTRODUCTION

2 SYSTEM CHARACTERISTICS 2.1 General 2.2 Physical Features 2.3 System Capacity 2.4 Electrical Characteristics 2.5 Automatic Performance Checks 2.6 Maintenance Philosophy 2.7 Control of Facilities 2.8 Installation Principles 2.9 Documentation

3 OUTLINE SYSTEM OPERATION 3.1 System Organisation 3.2 Line Units 3.3 The Concentrating Shelf Interface 3.4 The Shelf Multiplex 3.5 The Speech Switch 3.6 The Signalling Circuit 3 .7 The Address Decoder 3.8 Control Equipment 3.9 The Services Card 3.10 The Console 3 . 11 The Power Supply

4 PHYSICAL ENGINEERING

5 IN-BUILT DIAGNOSTIC CHECKS

6 MAN-MACHINE INTERFACE CAPABILITIES

7 INTEGRATING THE SYSTEM INTO THE ORGANISATION OF THE TELECOMMUNICATIONS ADMINISTRATION 7.1 General 7.2 Selling and Ordering 7.3 Assembly, Testing and Installation 7.4 Maintenance and Repair 7.5 Consultative Assistance

APPENDICES

1 GLOSSARY OF TECHNICAL TERMS

2 LEVELS OF RESPONSIBILITY FOR FACILITY CHANGES

3 PULSE CODE MODULATION (PCM) TRANSMISSION AND SWITCHING

4 THE HEXADECIMAL NUMBERING SYSTEM

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1 INTRODUCTION

1 . 1 Part 1 describes the general characteristics of the Monarch 120B Compact system and gives an outline description of the system operation, together with references to those technical features which contribute to the attainment of a high standard of reliability, ease of maintenance and minimum on-site installation costs. A quick appreciation of the system may be obtained by reading Section 2 and sub-Section 3 . 1 .

1 .2 The term "Central Equipment Unit" (CE U) used in this description refers to the basic unit at installations which do not have an add-on unit, or to the combined units where both are provided.

1 .3 Where reference is made in the description to storage capacity, normal computer/microprocessor practice has been followed by using a capital "K" to represent 210, ie 1024. Thus 64K = 65,536. A small "k" which is used extensively elsewhere in the manual, represents 1000.

1 .4 A glossary of technical terms used in Parts 1 and 2 of the manual is given in Appendix 1 .

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2 SYSTEM CHARACTERISTICS

2.1 General

2.1 . 1 The basic unit of the system normally caters for up to 56 extensions, 14 exchange lines, 2 inter-PBX circuits and one operator's console. The add-on unit normally caters for up to 64 extensions, 12 exchange lines -and 10 inter-PBX circuits; if desired a second console may be provided in place of two of the inter-PBX circuits. The maximum number ofinter-PBX circuits may be reduced at early installations when certain types of signalling are used. Within certain limits the relative numbers of extensions and exchange lines may be adjusted to suit the requirements of different installations. In this manual it is assumed that the units are equipped to cater for the numbers of extensions and exchange lines referred to earlier in this paragraph, in which case the two units combined will cater for up to 120 extensions, 26 exchange lines, 12 inter-PBX circuits and a console. There is an over-riding requirement that the total number of exchange lines and inter-PBX circuits shall not exceed 32.

Part 1 Figure 1 (a) The Basic Unit Page 7

2.1 .2 The system design, which utilises PCM transmissions, solid state time division switching with microprocessor control, printed circuit boards and plug-in units with two-part gold plated connectors, offers the following advantages:

u

The use of an operator's console with touch sensitive keys and a visual display which keeps the operator fully informed at all times and encourages maximum operating efficiency.

A wide range of user facilities for extensions and the console operator, and freedom to change many of these facilities at will.

iii High reliability, supported by an extensive program of in-built automatic diagnostic routines, resulting in long period of trouble free service.

iV Virtually noise free connections within the system.

v Effectively silent operation which enables the switching unit to be installed in an office environment.

Vi Low initial cost.

vu Low power consumption.

Vlll Ease of maintenance, which together with (vii) results in low running costs.

iX Low installation costs.

x Full compatability with future evolution towards integrated digital transmission and switching in the public network to which the system is connected.

xi Flexibility to enable the system to be readily adapted to meet unforeseen requirements in the future.

Part 1 Figure 1 (b) The Basic and Add-On Units Combined

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2.2 Physical Features

2.2. 1 The Equipment units are designed for use on a desk or table. Figure 1 (a) shows the basic unit only and Figure 1 (b) shows the basic and add-on units combined. Both units have the same nominal dimensions which are:-

Height 297 mm Width 807 mm Depth 4 77 mm

(approx 1 1 .7 inches) (approx 2ft 8inches) (approx 1ft 6.75 inches) (front to back)

These dimensions facilitate easy transportation through normal door openings, lifts, etc. prior to installation. The maximum weight of each unit when fully equipped is approximately 37 kg (80 lbs). In both cases the weight is supported by two plinths, each of which has a bearing surface 782 mm long and 12.5 mm wide. Front and rear access is provided, although in most circumstances access is only required from the front. The dimensions and materials have been chosen to combine lightness with rigidity. Fans are provided in both units to ensure adequate cooling.

2.2.2 The operator's console unit is attractively styled for use on an office desk or table, as may be seen from the illustration in Figure 2. Its nominal dimensions are:-

Height 130 mm (approx 5. 1inches) Width 4 70 mm (approx 18.5inches) Depth 250 mm (approx 9.8inches) (front to back)

No moving parts are used in the normal operation of the console which also has its own in-built microprocessor providing sophisticated facilities and self-checking capabilities.

Part 1 Figure 2 The Operator's Console Page 9

Operator-to-machine communication is by means of touch sensltlve keys, and machine-to-operator communication is by way of a Visual Display Unit (VDU ) and a limited number oflamp indicators. The VD U displays, in alpha-numeric form, such information as the calling circuit identity, the extension number keyed, the class of service of the called extension,· the extension status, etc. The system may also be programmed to provide an accurate display of the time and date on the VDU in response to a keyed request from the console operator. A typical display of call information is shown in Figure 3.

The console can be used to effect changes in extension numbers, extension Class of Service , etc, and to obtain displays of fault information, or to run diagnostic tests.

Part 1 Figure 3 Typical Display on VD U

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2.3 System Capacity

2.3.1 Line circuits are connected to "ports" at the periphery of the system; extensions are connected to plug-in wiring boards, each of which caters for 8 line circuits; exchange lines are connected to boards which cater for 4 line circuits; inter-PBX circuits are connected to boards which cater for 2 circuits (or at some early installations possibly one circuit, depending on the method of signalling used) . Each operator's console requires a printed board for its exclusive use. MF keyphone receivers, which appear at the periphery of the system, are mounted on a separate "Services Card" which also contains other types of functional unit, as described later in the manual.

2.3.2 The maximum capacity of the system and the incremental growth steps of the equipment are shown in Table 1 .

Type of Circuit System Capacity

Extensions Minimum 8, Maximum 120 Exchange Lines Minimum 0, Maximum 26

(Note 1 ) Inter-PBX Circuits Minimum 0, Maximum 12

(Notes 1 and 2) Consoles Minimum 0, Maximum 2 MF Keyphone Receivers Minimum 6, Maximum 6

Notes 1 The total number of exchange lines plus inter-PBX circuits must not exceed 32.

Incremental Step

8 4

1 or 2

1 -

2 The maximum number of inter-PBX circuits is reduced by 2 if a second console is provided. At early installations the total number of inter-PBX circuits may be reduced when certain types of signalling are used on the circuits.

Table 1 System Capacity and Incremental Growth

The data in Table 1 relate to normal usage of the system. In exceptional circumstances additional extensions may be obtained by plugging 8 port cards into 4 port positions, but in these cases only 4 of the ports can be used.

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2.4 Electrical Characteristics

2.4. 1 The principal electrical characteristics of the system are listed in Table 2.

1 Extension line voltage 2 Extension line current (off-hook)

(Constant current feed) 3 Extension loop resistance

(including instrument resistance) 4 Extension insulation limit 5 Extension ringing 6 Extension telephone 7 Maximum number of bells per

extension (in series) 8 Extension wiring 9 Exchange line loop resistance

limit (includes resistance of exchange equipment)

10 Exchange line insulation limit 11 Exchange line signalling

12 Transmission bandwidth 13 Call Records 14 Total system traffic (at P = 0.01 )

(a) Basic Unit (b) Combined Units

15 Average traffic per extension (at P = 0.01) (a) Basic Unit (b) Combined Units

16 Power Supply (a) Voltage

17 Power drain

(b) Frequency (c) Maximum break in

main supply

18 Normal operating ambient temperature range

19 Normal operating relative humidity range

The actual number depends on the characteristics of the bells.

50V ± 0.5V

25mA to 35mA

1 ,200 Ohms, maximum 10k Ohms, minimum 75V, 25 Hz Dial type or MF Keyphones

4 bells (Note 1 ) 2 wires plus earth

1 ,400 Ohms, maximum 10k Ohms, minimum OIG earth seizure ( or loop seizure) ; IIC A C 25 Hz 300-3400 Hz 32

10.7 Erlangs (385ccs) (Note 2) 21 .5 Erlangs (774ccs ) (Note 3)

0.22 Erlangs (7.9ccs) (Note 2) 0.21 Erlangs (7.5ccs) (Note 3) 230V + 10% - 20% 45Hz-65Hz

10 ms approx 380 watts per cabinet (max)

5'C to 35'C (Note 4)

30% to 70% (Note 4)

Notes 1 2 3 4

Assumes 30% internal traffic, 56 extensions and P = 0.05 on exchange lines. Assumes 30% internal traffic, 120 extensions and P = 0.05 on exchange lines. For applications outside the UK please consult the supplier.

Table 2 Principal Electrical Characteristics of the System

2.5 Automatic Performance Checks

2.5.1 The system incorporates an extensive range of automatic diagnostic checks and, in addition, continually confirms that the main Central Processing Unit (CPU) is functioning. The comprehensive set of automatic tests, which includes checks of the speech and signalling paths through the switchblock, is made one test at a time about every 15 seconds. The time taken to test the complete exchange depends on the size of the particular installation and the traffic loading. If a fault is encountered during one of these tests, the equipment automatically initiates a fault analysis routine to check all the other items of equipment involved in the test, before recording the identity of the faulty unit; if the fault has already been detected on a previous test cycle the fault record is up-dated but no other action is taken. If the fault is detected for the first time, details are recorded in the fault record and, depending on the nature of the fault, an urgent or non-urgent alarm indication is given at the operator's console, and the details are displayed within the switching unit. In response to touching a single key at the console, the identity of the

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faulty unit, and further details, are displayed on the VDU and the operator can pass this information to the fault control point. Provision is also made for on-site maintenance staff to initiate a test of any unit at any time. Continuous tests are made of the link circuit to the operator's console and the console itself has in-built test routines to check the keypad, VDU display and supervisory indicators.

2.6 Maintenance Philosophy

2.6 . 1 The basic maintenance philosophy is one of on-site replacement of plug-in printed cards, and this, in conjunction with the automatic identification of faulty cards described in Paragraph 2.5.1 results in the rapid rectification of faults. Faulty cards can be returned to centralised repair depots to be dealt with by a small number of specialised staff.

2.7 Control of Facilities

2.7.1 The system provides a very extensive range offacilities for the extension user, the console operator, senior management in the business served by the PABX, and the operating administration or company responsible for the installation and maintenance of the equipment. These facilities are fully described in the Customer Facilities Book PH3253. The use of processor control enables many of the facilities to be changed solely by alterations to the information contained in the customer Data Base and others to be changed by using different plug-in units of physical equipment (hardware) in conjunction with software changes. The operator's console or a Master Telephone or Teletype terminal may be used to effect Data Base modifications and hence to implement those facility changes which depend only on such modifications. Provision is made for 3 levels of responsibility for facility changes :

Level 1 Changes which the customer (eg console operator or master phone user) is permitted to make.

Level 2 Changes which the normal maintenance staff are permitted to make.

Level 3 Changes which only specialised maintenance staff are permitted to make.

Lists of the changes which are appropriate to these different levels of responsibility are given in Appendix 2. Access to the system to make changes is obtained by keying passwords known only to those entitled to use them. Passwords which give access to the higher levels of responsibility automatically confer the power to make changes appropriate to the lower levels of responsibility. The passwords may be changed at any time.

2.8 Installation Principles

2.8 .1 In the system design, particular attention has been paid to the need to reduce on-site installation time to a minimum. Line cabling is plug-ended and connected to the rear of the units. All that is necessary, after delivery and siting of the cabinets, is to connect up the power supply leads and plug-in the external cables . For British Telecom applications the units are supplied with cables 3 metres long already connected. The operator's console is connected to the system via a single cable and plug, and may be placed in any location subject to the requirement that the total cable length should not exceed 1 .2 km. Longer distances are possible by using heavier gauge cable (or bunched pairs) to supply the power, or by supplying power to the console locally. Once the power is connected to the system installation testing can commence. Use is made of the in-built diagnostic routines to identify any major faults, and facility checks are run to confirm the correct operation. If desired, the complete provisioning procedure can be managed using computer based records. With this technique a customer's ordering requirement in terms of system size and facilities is entered into the computer where it is used to derive the necessary operational data. This includes such information as stock lists, configuration, layout, installation charges and rental, customer Data Base and user instructions. The customer Data Base, together with the main program, is then transferred to the system memory boards prior to installation using a specially programmed control unit known as the Automatic Memory Board Loading Equipment (AMBLE) . The whole process can thus be automated so that the time between placing an order and delivery on site, is reduced to a minimum.

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2.9 Documentation

2.9.1 Complete documentation is supplied with each installation and includes:

For the user and British T elecom Publicity Leaflet PH3235 System Description PH3244 Customer Facilities Book PH3253 Extension Users' Handbook PH3236 Operators' Handbook PH3237 Customers' MMI Instructions PH3238 Dial and Press Button Extension Facility Cards

For British Telecom only Technical Description, Parts 1 and 2 Monarch Marketing Support Package Completion of Sales Documentation for a Full Program Data Base (Booklet) Engineering Completion and use of Customer Requirement Forms (Booklet) Planning Handbook Installation and Commissioning Handbook System Maintenance Handbook

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3 OUTLINE SYSTEM OPERATION

3.1 System Organisation

3 . 1 . 1 Pulse Code Modulation (PCM) transmission and switching techniques are employed thoughout the system, and for those not familiar with these techniques, a brief description is given in Appendix 3. A simplified block schematic diagram of the system is given in Figure 4 . This figure shows the arrangements on the concentrating line shelf positions on which extensions and exchange lines terminate; the alternative arrangements used on the non-concentrating line shelf positions for inter-PBX circuits and the console(s) are described later. For ease of presentation, the 2 extensions shown in Figure 4 appear at opposite sides of the exchange, but there is no physical significance in this; indeed, when the 2 extensions in communication are connected to line circuits in the same unit (basic or add-on) the same Concentrating Shelf Interface is used by both extensions, although the call is, of course, routed in and out of the Speech Switch and Signalling Circuit.

3 . 1 .2 Each extension and exchange line terminates on a line circuit which incorporates an individual PCM coder/decoder (codec) and signalling elements to send and receive standard line signals. The codec has access to two 32 channel PCM highways (one for each direction of transmission) which are common to all the extension and exchange line circuits in the concentrating shelf positions in the same unit (basic or add-on) . These highways each carry 8 bit PCM encoded speech, and since the sampling rate is 8 KHz, the total bit rate is 2.048 Mbit/s (32 X 8 X 8000) . The transmission characteristics of the encoded speech on the highways fully conform to CCITT standards (�Law encoding can be supplied if required) . In the case of extensions, 8 line circuits are mounted on a single plug-in Line Unit, and in the case of exchange lines, 4 line circuits are mounted on a unit. In both cases each Line Unit also accommodates a micro-computer which acts as a common control for all the line circuits on the unit, and is in permanent communication, over an 8 wire bothway signalling bus, with the Concentrating ShelfInterface (CSI) which is common to all the concentrating shelf positions in the same unit (basic or add-on) . When an outgoing, or incoming, call is originated, the micro-computer in the CSI informs the micro-computer in the Line Unit which channel has been allocated for the call, and the latter micro-computer sends a control signal to the codec to cause it to make connection to the highways during the time slot corresponding to that channel. Thus, the CSI, jointly with all the Line Units in the concentrating shelf positions, forms a distributed time switch that enables any of the line circuits (ports) in these shelf positions to be connected temporarily to any channel ( timeslot) on the speech highways. At the CSI the speech highways are extended to the Speech Switch.

3 . 1 .3 All signals between the Line Units in the concentrating shelf positions and the CSI pass in 8 bit parallel form over the bothway signalling bus. The CSI is also connected over separate single wire signalling-in and signalling-out highways to the Signalling Circuit. The bits used on these highways are assembled into groups (bytes) of 8 bits, designated A to H, each having its own timeslot. The highways each provide 32 such timeslots, corresponding to the 32 timeslots on the speech highways, but running at one-eighth of the speed (256 kbit/s) . A simple coding system is used to give different signalling meanings to different bit combinations within the bytes. During call set up and release some signals are restricted to the signalling bus between the Line Unit and the CSI, but once a call has been established subsequent signals, conforming to the coding used on the signalling highways, are relayed by the CSI to and from the Line Unit. S ince the method of transmitting signals over the signalling bus (but not the signal codes) differs from that on the signalling highways, it is necessary for the CSI to store and reformat the signals in both directions between the Line Unit and the Signalling Circuit.

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SIGNALLING 8US

SIGNALLING 256 kbit/s

SPEECH HIGHWAYS

2 ·048 Mbit/s ENCODED SPEECH SAMPLES I

NOTES, 1. The address decoder is mounted on the same board as the signalling circuit and the Speech Switch. 2.ln Monarch 1208 Compact. up to 76 line circuits may have access to the speech highways.

SPEECH

2·048 Mbit/s ENCDDED SPEECH SAMPLES \

SIGNALLING 256 kbit/s

SIGNALLING BUS

PART t FIGURE 4. BLOCK SCHEMATIC DIAGRAM OF THE SYSTEM (CONCENTRATING SHELF ARRANGEMENTS).

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TO AND FROM OTHER LINE

CIRCUITS ON THE SAME

SHELF � TO SIGNALLING

CIRCUli AND CONTROL EQUIPMENT

TO OTHER SHELF MULTIPLEX CIRCUITS

10R EQUIVALENT)

� TO SIGNALLING

CIRCUIT AND CONTROL EQUIPMENT

PART 1. F IGURE 5. NON-CONCENTRATING SHELF ARRANG EMENTS.

3.1 . 4 Inter-PBX circuits and console line circuits terminate on Line Units in non-concentrating shelf positions which are served by a Shelf Multiplex in place of the CS!. Depending on the type of Line Unit concerned, each inter-PBX line circuit has one or two separate paths in each direction to and from the Shelf Multiplex provided for its exclusive use, and each console line circuit always has two separate paths in each direction. Each plug-in unit accommodates 2 inter-PBX line circuits (at some early installations, possibly one, depending on the type of signalling) , or one console line circuit. The resultant modifications to the block schematic diagram in Figure 4 , necessary to show the Shelf Multiplex and inter-PBX/console Line Unit arrangements are indicated in Figure 5. The transmission paths in each direction, between each line circuit and the Shelf Multiplex, carry 8 bit PCM encoded speech plus a single bit per sample for signalling purposes, ie a total of9 bits per sample. With a sampling rate of8 kHz, the total bit rate on these transmission paths is 72 kbit/s (9 X 8000) .

3 . 1 .5 The Shelf Multiplex is designed to serve up to 32 transmissions paths to and from the Line Units, although in Monarch 120B Compact this number of transmission paths is not required and the Shelf Multiplex i� not used to its full capacity. At the Shelf Multiplex circuit, combined speech and signalling bits from all the line circuits enter the equipment over the 72 kbit/s paths. The signalling bits are separated from the speech bits and the latter are multiplexed to form a standard 32 time slotPCM group which is extended over a 2.04 8Mbi t/s (8 X 8000 X 32) highway to the Speech Switch. The signalling bits are extended over a separate 256 kbit/s ( 1 X 8000 X 32) highway to the Signalling Circuit. The speech and signalling formats at this point are illustrated in Figure 6. In the opposite direction of transmission, speech bits are received over a 2.04 8 Mbit/s highway from the Speech Switch, and signalling bits are received over a 256 kbit/s highway from the Signalling Circuit. These are then demultiplexed and combined to provide speech and signalling at 72 kbit/s from the shelf multiplex to each line circuit. The formats on the speech and signalling highways to and from the Shelf Multiplex are identical to those on the highways to and from the Concentrating ShelfInterface used for the concentrating shelf positions and so far as the Speech Switch and Signalling Circuit are concerned, the highways are inter-changeable.

TO AND FROM OTHER LINE

CIRCUITS ON THE SAME

SHELF

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SPEECH PLUS SIGNALLING BETWEEN PORT CIRCUIT AND SHELF MULTIPLEX 72 kbit/s

SPEECH BETWEEN SHELF MULTI PLEX AND SPEECH SWITCH 2048 kbit/s

,� ________ � r ________ �A� ____ ____ � r ________ �A�_________ _--------�I V V V PORT 0 PORT 1 PORT 2

S IGNALLING BETWEEN SHELF M U LTIPLEX AND SIGNALlNG CIRCUIT 256 kbit/s

,� ________ � r---------JI\�--------� r--------Jf\�--------� �------�I V V V

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PORT 0 PORT 1 PORT 2

PART 1. F IGURE 6. SPEECH AND S IGNALLI NG BIT FORMATS ASSOCIATED WITH THE SHELF M U LTIPLEX.

3 .1 .6 The Speech Switch and Signalling Circuit are multi-purpose designs which each have a capacity for up to 8 highways. In Monarch 120B Compact, 3 highways are used in the basic unit; one for the concentrating shelf positions, one for the non-concentrating shelf positions and one for the Services Card, and in the add-on unit 2 highways are used; one for the concentrating shelf positions, and one for the non-concentrating shelf positions. Thus, when both units are provided, use is made of5 of the 8 high ways and the remaining 3 are spare. The descriptions of the operation of the Speech Switch and Signalling Circuit given in the following paragraphs assume that all 8 highways are used and relate to the concentrating line shelf positions, where a channel is allocated to a port for each call, as already described, and different channels are allocated to the same port for different calls. Since in the Monarch 120B Compact application less than 8 highways are used, the surplus input and output highway terminals are left spare. In the case of the non-concentrating shelf positions, each port is permanently allocated a channel for its exclusive use. Thus, in this latter case, the storage locations in the Speech Switch and Signalling Circuit each represent not only a channel but a particular port. The resultant slight modifications to the operational description in the following paragraphs should be readily apparent.

3 . 1 .7 The 8 highways connected to the Speech Switch are collectively capable of carrying speech samples corresponding to 256 (8 X 32) channels. Within the Speech Switch each of these channels is permanently allocated an 8 bit location in a 256 X 8 bit speech store which takes the form of a Random Access Memory (RAM) . The bits representing the speech samples arrive in serial form over the highways, and after conversion to parallel form are entered into the appropriate parts of the speech store. After each frame period, the bits stored against a particular channel are replaced by the next sample. When a call is in progress, the control equipment has already received the keyed, or dialled, digits and recorded the identities of the calling and called

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line circuits, and the channels which have been connected to these line circuits. The identities of the 2 channels which need to be interconnected to maintain the connection are passed from the CPU to another 256 X 8 bit RAM (the Connection Store) in the Speech Switch. In Figure 4 it is assumed that line circuit A is connected to line circuit B. The information in the connection store causes the speech sample held in the speech store of the channel to which line circuit A is connected to be transferred to the output of the Speech Switch at a time which will cause it to be sent out in the timeslot of the channel to which line circuit B is connected. Similarly, the information held in the speech store of the channel to which line circuit B is connected, is transferred to the output of the Speech Switch at a time which will cause it to be sent out in the timeslot of the channel to which line circuit A is connected. Thus the Speech Switch, under the control of the connection store provides a single stage 256 X 256 non-blocking, full availability switch for inter-connecting channels in the 8 incoming and 8 outgoing highways connected to it.

3 . 1 .8 Every channel in the eight 256 kbit/s signalling-in highways connected to the Signalling Circuit is permanently allocated an 8 bit store in one half of a 512 X 8 bit RAM in this circuit. After conversion from serial to parallel form, each successive byte of signalling information from the channel is placed in this half of the store. Thus, a change of signalling condition is registered in the store within 1 ms (See paragraph 3 . 1 . 3 ) . The control equipment scans this signalling-in half of the store and on detecting a change of condition, takes appropriate action. In the case of channels which are not in use, the scanning interval is 128 ms, but while a channel is connected to a line circuit in the dialling state the interval is reduced to 8 ms.

3 . 1 .9 The other half of the RAM in the Signalling Circuit has a permanently allocated 8 bit location for each of the 256 channels provided by the 8 signalling-out highways. When the control equipment requires to send a signalling instruction to a Concentrating ShelfInterface (usually for onward transmission to a line circuit) it places an appropriately coded signalling byte in the store location corresponding to the channel concerned. The Signalling Circuit, after converting the information from parallel to serial form, sends the byte to the CSI during the channel timeslot period.

3 . 1 .10 The system control equipment, which consists of a Central Processing Unit (CPU) with appropriate program and data stores, communicates with different parts of the system via 2 multi-wire paths referred to as the "address bus" and the "data bus". The data bus uses 8 bits throughout, whereas 20 bits are used on the address bus between the CPU and its associated memories and the Address Decoder, and 8 bits between the Address Decoder and the Signalling Circuit and the connection store in the Speech Switch. The capacity of the address bus between the CPU and its associated stores and the Address Decoder is increased to 20 bits by a technique known as "bank switching" described later in the manual. From Figure 4 it will be seen that the Signalling Circuit and the Speech Switch, form the interfaces between the control equipment and the rest of the system. When the CPU needs to send information it connects to the address bus, in parallel binary coded form, the identity of the point to which the information should go, and after receiving an acknowledgement signal sends the information, also in parallel binary coded form, over the data bus. Similarly, if the CPU needs to obtain information it sends the identity of the information source over the address bus and after receiving an acknowledgement signal accepts the information in parallel binary coded form over the data bus. The use 0[20 bits on the address bus provides a theoretical capacity for over a million destinations and the use of8 bits on the data bus enables up to 256 different bytes of information to be sent to or received from each destination. A separate control bus is provided to indicate whether a read or write function is to be performed and to enable the acknowledgement signalto be returned, but this has been omitted from Figure 4 in the interests of clarity.

3. 1 . 1 1 The CPU itself consists of a micro-processor contained in a single integrated circuit. The information which it requires to fulfil its control functions is contained in 3 different stores:-

A Main Program store which contains the information on which the basic operation of the exchange depends. This is recorded on Erasable Programmable Read Only Memory (EPROM).

11 A Customer's Data Base store which contains information relating to the particular installation (extension numbers, class of service information, short code dialling data, etc) . This is recorded on EPROM and copied into Random Access Memory (RAM) to enable the customer to make changes, as described in Paragraph 2.7.1

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III A Call Record Store which contains all the information relating to each call in progress at any particular time. This is recorded on a RAM store.

In addition to the main bus connections, referred to in Paragraph 3. 1 . 10, three data input/output channels are provided for connection to external items of equipment:-

To a portable teletype, or similar machine for use in interrogating the exchange, either directly or over a modem link.

11 To data logging equipment.

III To provide specialist maintenance information. A special circuit, known as the "Watchdog". is also included to monitor the operation of the CPU. The watchdog should receive a pulse signal from the CPU every 100 ms, when the latter is functioning correctly. If the signal is not received, the watchdog causes a "warm start" interrupt procedure to be applied to the microprocessor, which results in the suspension of the software process running at the time and activates the scheduling of the next process waiting to be run. The watchdog is reset by the receipt of256 good pulses from the CPU. If 12 warm starts occur without the watchdog being reset a "cold start" interrupt procedure is applied, and this forces a complete re-start of the program which, in turn, clears all calls and working data storage in the exchange. before attempting further processing. If a further 3 signals to the watchdog are missed without a reset occurring, then, after 2 more cold starts, the watchdog halts the processor and forces the exchange to go into the dropback state. In this latter state, an urgent alarm is activated and a number of designated extensions are connected directly to exchange lines to provide an emergency service. The CPU printed board provides a 2 digit hexadecimal display on its front edge. This is used to indicate any tests which are failing and also the current state of the system. A brief description of the hexadecimal numbering system is given in Appendix 4. The system maintenance handbook defines the 2 digit codes.

3 .1 .12 The operator's fully electronic console is probably the most advanced unit of its kind in the world for small private exchanges. It contains its own micro-processor and program memory. has a touch sensitive keypad with depressions for finger location in place of the usual keys, and contains a Visual Display Unit (VDU), which gives an alpha-numeric display of 64 characters, each of which is formed from a 5 X 7 element matrix. It also contains a limited number of Light Emitting Diodes (LEDs) to give lamp type signals to the operator. A block schematic diagram of the console is given in Figure 7. All the equipment, apart from that in the console line unit and a supply current limiter, both of which are in the Central Equipment Unit (CEU) , is contained within the console itself. The console line unit occupies the whole of a 2 port line card in the CEU. Two 4-wire analogue speech circuits are provided between this unit and the console, to enable the operator to speak to both parties on a call, and to have separate access to either party if, and when, required; another 4-wire circuit is also provided for signalling purposes. Signalling over the links in each direction is effected by means of asynchronous digital transmission at 2 .4 kbit/s, with automatic error detection. The same format is used on the signalling link for all messages - one 8 bit information byte from the console to the CEU, and three 8 bit information bytes from the CEU to the console, plus start/stop signals, sequence checks and parity bits. When there is no activity on the link a continuous interchange of "idle" signals takes place between the console and the CE U. If this ceases, calls are directed to alternative answer points. Thus, any failure of the link is automatically detected and appropriate action taken. A queueing capability is·included in the console system to ensure that, under normal conditions, calls are dealt with in the order in which they arrive. The basic control program for the microprocessor is stored in an Erasable Programmable Read Only Memory (EPROM) with capacity for 20K eight bit bytes. Working data, such as messages received over the signalling link. are stored in a Random Access Memory (RAM) with capacity for 2K eight bit bytes. The VDU is actuated by a drive circuit containing another RAM in which the information to be displayed is stored. The microprocessor communicates with its own memories, the VDU drive, the touch sensitive keypad, the operator's speech circuit and the signalling link terminal, over the 16 bit address bus and 8 bit data bus in a similar way to that in which the microprocessor in the CEU communicates with its associated memories and other units.

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Page 21

3 .1 . 13 Two proprietary designs of power unit, made by different manufacturers, are available to supply the CEU cabinets (and the -sOv power feed to the consoles in most situations ) . Each cabinet has its own power unit and either design may be used in any cabinet. Both are designed for AC mains supplies of230V having tolerances not exceeding + 10% and -20% at 45 Hz - 65 Hz. In both designs the mains supply is passed through a full wave rectifier which produces a DC output of approximately 330V. In one design (170A) this output drives a single DC - DC convertor which provides 4 regulated outputs of +SV, +12V, -12V and -50V. In the other design ( 171A) the 330V DC output drives 4 separate DC - DC convertors which deliver regulated outputs of the same voltages. In addition to normal fuse protection of primary voltages, protection against current overload and over-voltage, is provided, partly by a common monitor board and partly within each DC - DC convertor. A small filter is provided in the mains supply to eliminate mains borne radio interference. Ringing current at 75V, 25 Hz is generated by a separate oscillator circuit mounted adjacent to the power unit. In addition to the ringing voltage, this circuit generates a timing signal (T6) which is distributed round the Extension Line Units to ensure that the ringing current is connected and disconnected from the lines when the AC waveform is at approximately zero voltage. This is important to reduce the voltage transients which would otherwise occur when the ringing is applied and removed by the line current relay to provide the ringing cadence, and when the ringing is tripped. The T6 signal is also used to monitor the power supply and the ringer. If the ringer fails, the T6 signal is no longer generated, and in the event of an excessive voltage occurring on one, or more, of the DC supplies the T6 signal is also disconnected. Failure of the T6 signal due to either cause results in drop-back conditions being established (see paragraph 3 . 1 . 1 1 ) . Provision canbe made to feed the cabinets from a standby power supply to guard against mains failure if, and where, such precautions are considered necessary. The standby power supply, which maintains a no-break supply when the mains fail, need only be provided when continuity of normal service must be maintained during mains breakdowns and the drop back feature, whereby exchange lines are connected to nominated extensions is insufficient.

3 . 1 . 14 The console incorporates a DC - DC convertor which provides an output of +SV. An additional -5V supply, required internally by the console processor board is obtained from a small DC - DC convertor mounted on that board. Exceptionally, where the distance between the CEU and the console exceeds the limit for a power lead, a local mains supply may be used at the console site.

3 . 1 . 15 All the clock pulse supplies for the CEU are derived from an 18.432 MHz crystal oscillator, the output of which is passed through frequency dividers to obtain the different pulse rates required. The pulses are then distributed to:-

The Central Processor Unit and Memory Cards

The Speech Switch

The Signalling Circuit

The Concentrating Shelf Interfaces

The Shelf Multiplexes

The Line Units

The Services Card

The additional pulse (T6) is obtained from the Ringer Unit and distributed to the Line Units.

3.2 Line Units

3.2.1 The following types of Line Unit are used in concentrating shelf positions with the line circuits connected into the system as shown in Figure 4:-

Eight Port Extension Line Unit

Four Port Exchange Line Unit

and the following types ofline unit are used in non-concentrating shelf positions with the line circuits connected into the system as shown in Figure 5 :-

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One/Two Port Inter-PBX Line Unit

Single Port Console Line Unit

3.2.2 Each of the different designs of Line Unit is considered separately below:-

Eight Port Extension Line Unit

A block schematic diagram of the Extension Line Unit is given in Figure 8. It will be seen that each line circuit splits into 2 main areas; one associated with line current feeding and signalling, and the other with speech transmission. The line feed is of the constant current type, varying only within the limits 25 mA to 35 mA over the whole range of line resistances. The use of a constant current feed has the following advantages :-

It results in reduced power consumption with resulting operational economies.

ii It provides an improved side-tone performance.

111 It facilitates the removal of power from the transmission elements in the line circuit when the extension is not in use.

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Page 23

iV It prolongs the life of the carbon microphones in the extension telephones.

v It results in space saving on the printed card.

Vi It enables simplified matching to be achieved with the different telephone instruments of foreign administrations.

The circuit arrangements are such, however, as to ensure than an unregulated low current is fed to line to charge standby batteries, etc, when required.

An unbalanced 2-wire/4-wire conversion circuit is used, with resultant design simplification, and a miniature transformer is inserted between this conversion circuit and the line to maintain a balanced termination. The constant current line feed circuit, which offers a very high impedance to AC currents, is connected to the line side of the miniature transformer, and a capacitor in series with the line winding provides the necessary blocking of the DC currents. On terminating calls, ringing is connected by a miniature relay, the changeover contacts of which are inserted between the line feed connections and the line. The line feed elements detect the presence, or absence of a loop from the telephone and send appropriate DC signals to the micro-computer, where they are converted to binary encoded form and sent over the 8 wire signalling bus to the Concentrating ShelfInterface (CSI) . Similarly, incoming ringing instructions are received over the signalling bus by the micro-computer, which sends a DC condition to the miniature relay to cause application of 25 Hz ringing current to line. Bothway signalling is effected over the signalling bus by allocating a 2 ms period for signalling in one direction and a 2 ms period for signalling in the other direction every 8 ms.

The micro-computer incorporates a Read Only Memory (ROM) on which its program is stored and a Random Access Memory on which the status of each port in the unit is stored.

As soon as the line circuit is taken into use for an originating call, the micro-computer informs the CSI over the signalling bus and the CSI sends back over the signalling bus, the identity of the channel in the speech and signalling highways which has been allocated to the call. The micro-computer then powers up the codec and sets it so that, until further instructions are received, it makes connections to the speech highways at the appropriate slot time. Thus, in addition to functioning as a coder/decoder, the codec also functions as part of a distributed time switch. As soon as the codec is powered up it sends a signal to the band limiting filters to cause these to be powered up. In the idle condition power is disconnected from the codec and band limiting filters to reduce the power consumption of the system. In the case of an incoming call, the Central Processing Unit (CPU) informs the CS! which channel has been allocated for the call in the speech and signalling highways, and the CSI informs the micro-computer in the line circuit. This micro-computer then sends a control signal to the codec as in the case of an originating call.

The 2-wire/4-wire conversion in the speech transmission path is achieved by a simple impedance bridge arrangement using active circuits, in which the balancing impedances are chosen to obtain a good match to a varity of cables, line lengths and telephone instruments. Filters are inserted in the "transmit" and "receive" paths to limit the analogue signals to frequencies below 3400 Hz. Removable straps are provided on each Extension Line Unit to give a gain adjustment for long/short extension working.

A proprietary design of codec is used in the unit. The characteristics of the compandors used in the codec conform to the "A-Law" specified by the Conference of European Postal and Telegraph Administrations (CEPT), but alternative codecs using " Il-Law" coding are available when required. For protection against extraneous voltages, the line circuits incorporate fusible series resistances to limit the current which can flow, and for extensions which are considered to be subject to sufficient risk, Zener diodes are fitted on the cards. For protection from lightning the line circuit relies on the use of gas discharge tubes on the distribution frame. Only those extensions considered to be at risk need be provided with gas discharge tube protectors.

An Extension Line Unit is shown in Figure 9.

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Part 1 Figure 9 An Extension Line Unit

Four Port Exchange Line Unit

With minor modifications in the signalling area, the block schematic diagram in Figure 8 is applicable to the Exchange Line Unit. The subscriber's line interface circuit is replaced by an interface circuit which contains equipment for detecting AC ringing and DC signals from the exchange, and for sending DC loop/earth signals to the exchange; these changes also lead to an increase in the number of connections between the interface circuit and the micro-computer. The standard British Telecom method of preventing a dialling-out extension or operator fro� seizing an exchange line which has not completed its release from a previous call, is to use the existence of battery potential on one of the exchange line conductors, and a disconnection on the other, as a "circuit free" indication. This necessitates the use of an earth calling condition from the Exchange Line Circuit at the PABX. The earth is replaced by a holding loop as soon as the seizure sequence has been completed. The Exchange Line Circuit can readily be converted to loop calling, if required.

The terminating equipment in the Exchange Line Circuit uses the same design of miniature 2-wire transformer with series connected capacitor, as the Extension Line Circuit, to provide a balanced termination; the signalling connections are made to the line side of this transformer. The holding loop is applied by a 28-50 mA constant current sink circuit which offers a very high impedance to AC signals and accordingly does not degrade the transmission performance. Dialling is effected by switching the constant current circuit on and off under the control of a mercury wetted relay contact. Circuit design precautions are included to ensure that the shunt inductance of the miniature transformer does not distort the dial-pulses. The speech path is extended to the extension, or operator, during each inter-digit pause, to enable the user to hear any supervisory tones or verbal announcements returned by the network during the setting up of a call . If the local exchange applies a reversal to the exchange line polarity when the called

Page 25

party answers, this is detected and a signal sent to the micro-computer and thence to the CPU via the CS!.

On incoming calls, when the constant current loop circuit is switched off, other line circuit elements detect the ringing current (or the reversed polarity during the intervals between ringing current) and send a signal to the CPU via the CSI causing the line to be busied against outgoing calls and the call to be routed to the operator's console. When the operator answers the call a signal is received from the CPU to cause the micro-computer to switch in the constant current loop and trip the ring.

British Telecom makes private meter recording equipment available to customers who wish to check the charges being levied while an outgoing call is in progress. The equipment is operated by a 50Hz earth return pulse applied to both line conductors, via low pass filters, each time a driving pulse is sent to the exchange meter. The exchange Line Circuit in the Monarch 120B Compact system is designed so that the necessary detection equipment can be provided when required. The metering information is recorded in software and printed out during call logging operations. It may also be displayed on the console VDU if required.

As in the Extension Line Circuit, provision is made to disconnect the power from the band limiting filters and the codec when the exchange line is not in use.

The transmission area of the block schematic diagram in Figure 8 is equally applicable to that in the exchange line unit, but there are some minor differences in the equipment contained in certain of the boxes. In particular, the 2-wire/4-wire conversion circuit contains different terminating and balancing impedances to match the different transmission characteristics of exchange lines, and different amplifier gains are provided to ensure that the transmission requirements of the public network are met. To cater for the wide range of transmission conditions resulting from different combinations of exchange line and extension line lengths in different P ABX locations, provision is made on the line unit for the insertion and removal of simple straps for each direction of transmission, the effect of which is to modify the gain in the circuit.

Arrangements similar to those described for extension circuits are provided to protect the Exchange Line Circuits from surge interference including lightning and direct mains- contacts. Since exchange lines normally involve external line plant the Zener diodes are fitted as standard on all uni ts.

Inter-PBX Line Circuits

The following types of Inter PBX Line Unit are, or will be, available for Monarch 120B Compact (the terms "2-wire" and "4-wire" refer to the transmission conditions presented at the PABX port; a 2-wire presented circuit may use 4-wire line transmission) : -

a. SSDC No. 5 (2-wire) . This provides signalling over "E" and "M" wires.

b. SSDC No. 5 (2-wire/4-wire) . As for (a) with the option of 4-wire connection.

c. Interim SSDC No. 10 (2-wire) . This provides Single Commutation DC signalling and pulsing.

d. Interim Loop-Disconnect (2-wire ) . This provides standard loop-disconnect pulsing and the associated signalling.

e. Multiple DC Signalling (2-wire/4-wire) . This is a multi-purpose line unit capable of providing any of the types of signalling referred to in (a) to (d) above, together with other less frequently used types of DC signalling.

f. Interim SSAC No. 13 (2-wire) . This provides pulse code in-band signalling.

g. Interim SSAC No. 15 (2-wire presented but using 4-wire connected signalling ) . This provides continuous tone in-band signalling using "tone on idle".

h. Multiple AC Signalling (2-wirel 4-wire) . This is currently under development and will be capable of providing either of the types of signalling referred to in (f) and (g) above.

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The features of these line units are described separately below:-

2-WIRE I NTER- PBX LIN E

SPEECH PATH

a. SSDC No. 5 (2-wire)

A block schematic diagram of the 2-wire SSDC No. 5 Inter-PBX Line Circuit is given in Figure 10. Two such circuits are mounted on a line unit. The line termination contains a minature 2-wire transformer to provide a conversion from the balanced circuit on the line side to the unbalanced connection through the 2-wire/4-wire conversion circuit and the band limiting filters to the codec; a blocking capacitor and DC connections are provided to supply a loop "wetting" current. The 2-wire/4-wire conversion circuit and the band limiting filters are electrically similar to those in the 8 port Extension Line Unit, but the codec is to a British Telecom design which uses an intermediate digital code, referred to as "delta-sigma" modulation. This intermediate stage is used for design convenience and does not alter the overall function of the codec, which is to convert analogue speech currents to and from the standard 8 bit PCM code, as described in Appendix 3. The compandors have the same characteristics as those in the Extension Line Unit, and "IJ. Law" encoding can be provided if required. The Line Unit Signalling Interface (LUSI) , under the control of the CPU, causes attenuation to be inserted when an exchange line is extended via the Inter-PBX line, or the Inter-PBX line is used to form part of a tandem inter-PBX connection.

Separate wires, designated "E" and "M" are provided for signalling in each direction. Signals are sent over the M wire and received over the E wire. The signals connected to the M wire consists of the presence or absence of an earth potential applied via a resistor and a noise eliminating capacitor. The E wire is connected to a potential of -SOY via a resistive detector circuit, and a proportion of the voltage developed within the circuit is compared with a reference voltage to indicate when earth connected signals are being received. These signalling conditions may be used by direct interconnection of the M leads to the E leads when the resistances of the paths over which they are connected do not exceed 25 ohms, but are primarily designed for use with some form of signalling conversion equipment (eg in the channel equipments of Frequency Division Multiplex (FDM) or PCM transmission systems, or in separately mounted AC in band or DC conversion units) .

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72 kbil/s STREAMS TO ANO FROM

SHELF MULTIPLEX EQUIPMENT

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PART 1. F IGURE 10. BLOCK SCHEMATIC D IAGRAM OF THE 2-WIRE SSDC N O. 5 INTER- PBX LI N E CIRCU IT.

Page 27

The power is disconnected from the 2-wire/4-wire conversion circuit, the band limiting filters and the codec when the inter-PBX line is not in use.

The LUSI converts the DC signals received over the E wire into binary encoded form and passes them to the codec for inclusion in the serial bit stream on the 72 kbit/s connection to the shelf Multiplex. Similarly, signalling instructions from the CPU are received in the serial bit stream from the shelf Multiplex, and passed to the LUSI which causes the appropriate DC conditions to be extended over the M wire. The signalling bits A to H transmitted in each direction between the LUSI in the line circuit and the CPU, via the Shelf Multiplex and the Signalling Circuit, are assembled in groups (bytes) of 8 bits coded as described in Part 2.

As the Inter-PBX Line Unit can be connected only to short lengths of internal wiring of up to 25 ohms resistance, it is designed to be secure against any of the 4 wires encountering direct earth or -50V contacts. Zener diodes on the secondary winding of the transformer protect the 2-wire/4-wire conversion circuit and subsequent circuits against overload.

h. SSDC No. 5 (2-wire/4-wire)

The 2-wire/4-wire SSDC No. 5 Inter-PBX Line Circuit is identical to that shown in Figure 10 apart from the connections to the line and the 2-wire/4-wire conversion element. The differences in this part of the circuit are shown in simplified form in Figure 1 1 . The detailed strapping arrangements, which are more complicated, are described in Part 2.

(2-WIRE � )

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NOTE 1 I TO SWITCHED ATTENUATOR.

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Page 28

NOTES: 1 For 4-wire operation provide straps x x and omit straps - - - -For 2 -wire operation provide straps - - - - and omit straps x x

PART 1. FIG URE 11 . S IM PLIF I ED D IAGRAM OF THE 2-WI RE/4-WIRE CONNECTIONS IN SSDC NO. 5 I NTER- PBX LI NE CIRCU IT.

c. SSDC No. 10 (2-wire) .

A block schematic diagram of the 2-wire SSDC No. 10 Inter-PBX Line Circuit is given in Figure 12. This is an interim design for use pending the availability of the multiple DC signalling unit (item e) , and occupies the whole of one line unit, using 2 ports (ports 0 and 1 ) . The unit provides two unidirectional circuits (one outgoing plus one incoming) or one bothway circuit. When used for unidirectional circuits the incoming circuit is connected to port 0 and the outgoing circuit to port 1 . When used for a both way circuit the line is connected to port 1 and port 0 is left spare.

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PART 1. F IGURE 12. BLOCK SCHEMATIC D IAG RAM OF THE 2-WIRE/4-WIRE SSDC NO. 10 INTER-PBX LIN E CIRCU IT.

72 kbit/s STREAMS

TO ANO FROM

THE SHELF MULTIPLEX EQUI PMENT

72 kbitls STREAMS

TO ANO FROM

THE SHELF MULTIPLEX EQUIPMENT

Page 29

Within the unit the Single Commutation DC (SCDC) signalling elements required at the incoming end of a circuit are connected to port 0 and the SC DC signalling elements required at the outgoing end of a circuit are connected to port 1 . On bothway circuits the signalling connections made within the line unit depend on the direction of traffic usage and are controlled by relay le. When relay IC is unoperated the connections are those required for an outgoing call, and with it operated they are those required for an incoming call; the relay is permanently operated when the circuit is in the idle condition.

The operation of the line circuit is controlled by a micro-processor, the program for which is stored on a Read Only Memory (ROM) accommodated for convenience in the expansion unit. The principal function of the expansion unit is to provide an interface in the path for the control signals between the micro-processor on the one hand and the signalling elemen ts and the le relay on the other, and to provide an interface in the path to the micro-processor from a group of manually controlled switches (not shown in the figure) , one of which sets the direction of traffic usage (incoming, outgoing or bothway) and others the duration of any pre-sending pause required on the circuit. Received line signals are detected by current sensing circuits in the signalling elements, the outputs of which are directly connected to the micro-processor. The micro-processor converts these signals to 8 bit binary encoded form and sends the resultant bytes, via the PISO converter to the codec serving the 2-wire path in which the signalling elements receiving the signals are located. The bits are then forwarded from the codec to the Shelf Multiplex as bits A to H in the 72 kbit/s stream. Similarly, instructions from the CPU are received via the Shelf Multiplex in the 72 kbit/s stream, extracted at the SIPO converter and forwarded to the micro-processor.

The 2-wire/4-wire conversion circuits, the band limiting filters and the codec, all of which are continuously powered up, are similar to those used in the SSDC No. 5 circuit.

d. Loop-Disconnect (2-wire)

This is an interim design for use pending the availability of the multiple DC signalling unit (item e) . The block schematic diagram in Figure 12 is equally applicable to the 2-wire Loop-Disconnect Inter-PBX Line Circuit provided the incoming and outgoing SCDC signalling elements are replaced by the corresponding loop-disconnect signalling elements. The functions of the remaining circuit elements are similar to those in the SSDC No. 10 Inter-PBX circuit.

e. Multiple DC Signalling (2-wire/4-wire)

This Inter-PBX Line Circuit is designed to provide any of the following methods of signalling on a 2-wire or 4-wire presented circuit:-

SSDC No. 5

SSDC No. lO

Loop-Disconnect

Balanced Battery

Balanced Battery Signalling is a manual signalling system, used on certain overseas circuits, and is a DC equivalent of the old generator signalling system. Only a seizing signal is provided by the system and this takes the form of a single timed pulse of battery potential connected to both signalling wires. No supervisory or release signals are provided.

A block schematic diagram of the 2-wire/4-wire multiple DC Signalling Inter-PBX Line Unit is given in Figure 13 . Each unit accommodates two line circuits (which may be 2-wire or 4-wire presented) and a micro-processor to control both circuits. Each of the line circuits may be used independently to provide an outgoing circuit, an incoming circuit or a both way circuit. The junction interface circuit contains sensing resistors to measure the current flowing in each line conductor, switching transistors which, under the control of the micro-processor, cause any required combinations of signalling conditions to be applied to the line conductors, and variable line feed resistors which, under the control of the micro-processor, alter the value of the current being fed to line. The values of the line currents in the A and B leg sensing resistors are sent to the

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CONVERSION CI RCUIT

N OTES: ! On 4-wire presented circuits the dotted connections are made and the 214 wire conversion circuit is disconnected.

PART 1. F IGURE 13. BLOCK SCHEMATIC D IAGRAM OF THE MULTI PLE DC S IGNALLI NG I NTER- PBX LINE U N IT.

micro-processor in 8 bit linear PCM encoded form; instructions to set the switching transistors which determine the signalling mode are sent by the micro-processor over each of the signalling mode control leads, individually, as required; instructions to set the current feed resistances in each of the A and B line feeds are sent in 3 bit binary encoded form by the micro-processor over the 3 leads to each line resistor. Signals from the micro-processor also switch the attenuators in or out of the transmission path, in the circumstances described for SSDC No. 5, but the 8 different instructions possible over the 3 leads, using binary encoding, allow a finer control of the losses introduced on different types of connection.

Signals received by the micro-processor are sent via the PISO conversion circuit to the codec and thence to the Shelf Multiplex, as described in the case of the SSDC No. 10 Inter-PBX Line Unit. Instructions from the CPU to apply signalling conditions are received via the Shelf Multiplex in the 72 kbit/s stream, and after passing through the SIPO conversion circuit are received by the micro-processor.

The 2-wire/4-wire conversion circuit, band limiting filters and codec are of the same design as in the SSDC No. 5 Inter-PBX Line Unit; all are powered up continuously.

The line unit is made suitable for 2-wire or 4-wire applications by the insertion and/or omission of straps, similar in principle to those shown in Figure 1 1 . Different programs, recorded on ROMs, are provided for the micro-processor for each different type of signalling.

Page 31

A multiple DC signalling Inter-PBX Line Unit is shown in Figure 14.

f. SSAC 13 (2-wire)

As an interim measure, a proprietary design ofSSAC No. 13 Inter-PBX Lin·e Unit is being used, pending the availability of the Multiple AC Signalling Line Unit (Item h) , referred to below. SSAC No. 13 is a purse code, in-band, signalling system using a single frequency of2280 Hz at a send level of -6dBmO. The unit design is such as to enable the line unit to be used only on 2-wire presented circuits.

In this interim design a common printed wiring board, which fits into a line circuit position, is required for every 4 line units, or less, and connection is made between the common board and the line units by means of short lengths of plug-in ribbon cable which are provided across the fronts of the units. A more detailed description of this unit is given in Part 2.

g. SSAC No. 15 (2-wire)

As an interim measure, a temporary design of line unit is being used to cater for inter-PBX circuits employing SSAC No. 15 pending the availability of the Multiple AC Signalling Unit (Item h) . Signalling System AC No. 15 is an in-band system which uses a single frequency of 2280 Hz in a continuous ("tone-on idle") signalling code. The temporary design of line unit combines the circuit elements of the SSDC No. 5 line circuit described in (a) above with an existing design of SSAC No. 15 conversion unit. The system can only be used over circuits having 4-wire transmission and the AC signalling elements are connected in the 4-wire transmission paths. A block schematic diagram of the complete unit is given in Figure 15 .

Considering received signals first, in the idle condition 2280 Hz tone at a sent level of -20 dBmO 1S received continuously over the incoming transmission path and the

Part 1 Figure 14 A Multiple D.e. Signalling Inter-PBX Line Unit

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2280 Hz

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PERSISTENCE SWITCH -- CHECK

A r - - I I 2280 Hz RECEIVER

2/4 WIRE

CONVERTOR

SPEECH PATH ! (2-WIRE) I

4-WIRE LINE PERSISTENCE

CHECK

E I I 'I�I

2280 Hz OSCI LLATOR

t HIGH/LOW

LEVEL CONTROL

LINE SPLIT

B

+ .------. .. ----�-� �--� OUTGOING DIAL

PULSE CORRECTOR

t INCOMING

DIAL PULSE

CORRECTOR

t 1 -50V

E

PERSISTENCE CHECK

C

I M-WIRE

N OTES: 1 This relay contact i s symbolic. No relay contacts are used at this point.

CIRCUIT ELEMENTS

OF SSoC N O.5 LINE CIRCUIT (FIGURE 10)

•

72 kbitls STREAM TO AND FROM THE SHELF

MULTIPLEX EQUIPMENT

PART 1. F IGURE 1 5 . BLOCK SCHEMATIC D IAGRAM OF THE I NTER IM SSAC NO. 15 I NTER-PBX LI NE CIRCU IT.

Page 33

2280 Hz receiver holds relay E operated, thus disconnecting the earth signal from the E wire to the SSDC No. 5 circuit elements. When a seizing/holding or answer signal is applied at the distant end the tone is removed, the receiver responds, and after a nominal delay of 40 ms (persistance check B) , the holding circuit for relay E via the incoming dial pulse corrector is removed. Relay E releases and applies an earth to the E wire into the SSDC No. 5 circuit elements. On incoming calls, the incoming dial pulse corrector ensures that dial pulses are repeated to the E relay with an acceptable make/break ratio. When the signal at the distant end is removed, the 2280 Hz tone is reconnected, the receiver operates and after a nominal delay of30 ms (persistence check B) , relay E re-operates and removes the earth from the E wire. After the 2280 Hz receiver has been operated for a nominal period of 12.5 ms (persistence check A) , the 2280 Hz filter is also re-inserted in the incoming transmission path. On outgoing calls, the presence of this filter prevents the signalling tone returned during the unanswered condition being heard while supervisory tones are being received from the distant equipment. The 12.5 ms persistence check minimises the risk of impairment of the transmission path by transient insertions of the filter due to voice operation of the receiver during conversion.

Considering transmitted signals, in the idle condition no earth is connected to the M wire from the SSDC No. 5 circuit elements. In the absence of this earth the transmission path is disconnected by the line split element, and 2280 Hz tone is sent continuously at a level of -20 dBmO from the oscillator to line. When a seizing/holding signal, or an answer signal requires to be sent ("tone off" ) , an earth is connected to the M wire, and after a persistence check of 10 ms (persistence check C), this causes the tone to be disconnected and the outgoing transmission path to be extended by the split element to the 2-wire/4-wire conversion circuit. When the signalling condition is terminated, the earth condition is removed from the M wire and after a persistence check of10 ms has been satisfied, the transmission path is split and the 2280 Hz signalling tone is re-applied. The high/low level control causes the first 425 ms (nominal) of the tone to be applied at a level of -10 dBmO, after which the level drops to -20 dBmO. Dial pulses, and certain auxiliary pulse signals which may be used, all have durations less than 300 ms and accordingly are transmitted at the higher level. The use of the higher level for these signals, and for the initial periods of continuous tone, ensures that the receiver will respond in the presence ofline noise.

h. Multiple AC Signalling (2-wire/4-wire)

The Multiple AC Signalling Inter-PBX Line Unit, which is still under development, will provide SSAC No. 13 and SSAC No. 15 signalling capabilities. Each unit will accommodate two line circuits (which may be 2-wire or 4-wire presented) and a micro-processor to control both circuits. Each of the line circuits may be used independently to provide an outgoing circuit, an incoming circuit or a both way circuit. The design of the unit will closely follow that shown in the block schematic diagram in Figure 16. The 2280 Hz sending and receiving elements in the circuit will be controlled by the micro-processor and depending on the program provided, will send and receive the pulse signals used in SSAC No. 13 or the continuous signals used in SSAC No. 15. The persistence timers will measure the durations of received 2280 Hz signals, and each will deliver an output after the prescribed time has elapsed. Except in the case of the short persistence timer, which causes the insertion of the 2280 Hz band stop filter, the outputs go to the micro-processor. After decoding, the received signals are sent by the micro-processor as standard signalling bytes (bits A to H) via the PISO conversion circuit to the Shelf Multiplex and thence to the Signalling Circuit and the CPU. Instructions to apply signalling conditions are received via the Shelf Multiplex from the CPU as standard signalling bytes (bits A to H), and after passing through the SIPO conversion circuit are received by the micro-processor. The micro-processor converts the instructions to the equivalent line signalling code and controls the sending of the 2280 Hz line signals by applying appropriate conditions to the 3 leads to the "level select and transmit split" circuit elements.

As in the case of the Multiple DC Inter-PBX unit, the micro-processor will switch the attenuators in or out of the transmission path using the 3 leads to each attenuator to provide 8 different settings. The 2-wire/4-wire conversion circuit, band limiting filters and codec are of the same design as in the SSDC No. 5 Inter-PBX Line Unit; all are powered up continuously.

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FIlTER � PORT 0 RECEIVE

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A 2280 Hz t LONG PORT 0 SEND / / 2/4 ' -.... � RECEIVER I--.... � PERSISTENCE

2-W I R E WIRE , TIMERS - - - -< CONVERSION > NOTE 2 , /

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2280 Hz OSCILLATOR

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- BAND LIMITING FIlTERS

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mID PORT 0 SEND I SELECT AND l' �� -----... --+-....01 TRANSMIT 1--+--+-+-4 SPLIT

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- SWITCHED ( ) ATTENUATOR ___ +-__ +....j�+-_....j�+....jl-l r ______ -. CONTROLS

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HIGH LEVEL -re -) SEND < -

ON/OFF

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PULSE ' --t' BLAN KING ... ..L._---JL..L..L.L..a...L._�-L....Ii_.l.�l.._....L_.L_..I_a,

MICROPROCESSOR

NOTES: 1 , Th is contact is symbolic, No relay contacts a re used in this circuit.

-PARALLEL- I N SERIAL- OUT � CONVERSION �

CIRCUIT

2, On 2- l ine prllsented circuits the dotted connections are made and the internal send and receive circuits terminate on the 2-wire/4-wire conversion circuit.

PART 1, F IGURE 16, BLOCK SCH EMATIC D IAG RAM OF THE MULTIPLE AC S IGNALLI N G I NTER- PBX LI NE UN IT.

t 72 kbit/s

STREAMS TO AND

FROM THE SHELF

MULTIPLEX EQUIPMENT

Page 35

4-WIRE ANALOGUE

SPEECH CIRCUIT TO THE

CONSOLE

4-WIRE ANALOGUE

SPEECH CIRCUIT TO THE

CONSOLE

Console Line Unit

A block schematic diagram of the console line unit, which caters for one console, is shown in Figure 17. The connections to the cable interconnecting the line unit and the console correspond to the link in Figure 7.

The line interfaces on which the two 4-wire speech circuits terminate contain miniature 2-wire transformers to provide conversion from the balanced circuits on the line side to the unbalanced connections through the band limiting filters to the codec. Resistors are connected to the line sides of the transformers to provide DC "wetting" currents for any contacts in the lines .

The band limiting filters and the code cs are similar to those in the Inter-PBX Line Circuits already described. A gain adjustment capability is included to allow compensation for variations in loss due to the tolerances permitted in the codec and filter areas. Only one setting is required for all lengths of console cable, and this is made during manufacture.

-- LINE X -- INTERFACE - '\...;

-

BAND 72 kbit/s STREAM TO AND FROM LIMITING CODEC SHELF MULTI PLEX FILTERS EQU IPMENT

LINE r::0 .. - .. -- INTERFACE - '\...;

-"" LINE 0(., -- INTERFACE - � - ..

BAND 72 kbit/s STREAM LIMITING CODEC � TO AND FROM

SHELF MULTI PLEX FILTERS EQUIPMENT

I 0(., I ..

-- LINE - INTERFACE .. �

ASYNCHRONOUS 2'4 kbit/s SIGNAL

TRANSMISSION

SIGNALLING FROM CONSOLE

SIGNALLING TO CONSOLE

Page 36

...... LINE - L SIGNALLING .... SIGNALLING SENDER --

CONVERSION AND I --- RECEIVER -- -

PART 1 , FIGURE 1 7 . BLOCK SCHEMATIC DIAGRAM OF THE CONSOLE LINE CIRCUIT

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The signalling link consists of two 2-wire circuits, one for each direction of signal transmission. Information is sent in balanced asynchronous digital form in both directions at 2.4 kbitl s. Each message consists of a start signal, followed by an 8 bit information byte, followed by a stop signal. A parity bit is also added for error detection purposes. When the circuit is not being used a single ("idle") byte is transmitted from the CEU to the console every 8 ms and a single byte is sent in the opposite direction every 137 ms as long as the console is active. Failure to receive this signal automatically indicates the existence of a fault and appropriate action is taken.

A Console Line Unit is Illustrated in Figure 18.

3.3 The Concentrating Shelf Interface

3.3.1 The Concentrating ShelfInterface (CSI) allocates a free channel (time slot) in each of the 2 speech highways, for use by a Line Unit, immediately it becomes involved in an outgoing call, and causes connection to be made to a designated time slot when the Line Unit is involved in an incoming call. The time slot allocated to a call is the same for both directions of speech transmission, and the same time sl(')t is used in the signalling highways between the CSI and the Signalling Circuit. The CSI is transparent to the 2048 kbit/s speech samples on the speech highways, although buffers and retiming elements are inserted in the highways within the unit. The principal signalling functions of the CSI are to exchange control messages with the Line Units over the shelf signalling bus while connection is being made to a time slot at the beginning of a call and while the time slot is being released at the end of a call, and to act as an interface between the signalling bus and the signalling highways to and from the Signalling Circuit during the remainder of the call.

Part 1 Figure 18 A Console Line Unit Page 37

FROM LINE U N ITS

• TO L INE U N ITS

...

NOTES 1 & 2

OIRECTIONAL SWITCH

2048 kbit/s - - - -

SPEECH H IGHWAYS 2048 kbit/s

- - - -

8 BITS IN PARALLEL

RANDOM ACCESS

MEMORY

H DATA TO

SIGNALLING CCT (SIGNALLING - IN)

- - - - -.-

TO SPEECH SWITCH

• FROM

SPEECH SWITCH ...

TO SIG NALLIN G PARALlEl- IN CIRCUIT SERIAL-OUT ""

CONVERSION 256 kbit/s CIRCUIT

t I

REAOIWRITE CONTROL I

SHELF DATA FROM NOTE 2 SIG NALLING CCT I

SIG NALLI NG BUS (SIGNALLI NG-OUT) I TO AND FROM LINE 6 BITS IN U N ITS. 8 BITS '- PARALLEL I I N PARALLEL NOTES 1 & 2 I �� J- --I

SWITCH

8 BITS IN PARALLEL

/ 6 BITS IN PARALLEL

MICRO COMPUTER

H

NOTE 1 �r ...

SELECTOR H COUNTER J

NOTES 1 & 2 SERIAL- I N -""'" PARALLEL- O UT

8 BITS IN PARALLEL CONVERSION SWITCH CIRCUIT

FROM SIGNALLI NG

CIRCUIT

256 kb�/s

N OTES: 1 . These contacts are symbolic. No relay contacts are used in th is un it. 2 . This item is switched by the micro-computer as required.

Page 38

PART 1. F IGURE 19. BLOCK SCHEMATIC D IAG RAM OF THE CONCENTRATING SHELF I NTERFACE.

3.3.2 A block schematic diagram of the CSI is given in Figure 19. Control of the unit is vested in a micro-computer which incorporates an internal Random Access Memory (RAM) having a status store for every port on the shelf. The micro-computer also incorporates a Read Only Memory (ROM) on which its program is stored. The micro-computer communicates over an internal 8 bit data bus with:-

1 the shelf signalling bus to and from the Line Units,

and

11 the CSI RAM which stores data to and from the Signalling Circuit.

Information received over the signalling-out highway is transferred via the Serial-In-Parallel-Out conversion circuit, at an appropriate time, over the internal data bus to the signalling-out store in the CS! RAM, from which it is read out asynchronously by the micro-computer. Information in the CS! RAM that requires to be sent to the Signalling Circuit

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is read out via the PISO conversion circuit at the correct slot times for transmission over the signalling-in highway. Switches controlled by the micro-computer are included in the internal data bus to ensure that data which is being exchanged between particular locations on the bus cannot obtain irregular access to other locations. As already described in sub-Section 3.2, the bothway shelf signalling bus to and from the Line U nits consists of8 conductors, each operating at 64 kbit/s, and functions for periods of2 ms in each direction once every 8 ms.

3.3.3 When a call originates from a port, the Line Unit causes a calling condition to be sent over the shelf signalling bus to the CSI during the first 2 ms signalling period in the required direction. Within the CSI it is detected by the micro-computer which allocates the first free time slot (starting from slot 0) and sends the identity of the port concerned over that time slot in the signalling-in highway to the Signalling Circuit and thence to the CPU. The CPU notes the port number and time slot number, and returns an acknowledgement signal to the CS!. The CSI then sends the identity of the time slot, together with a power up signal, back to the Line Unit during the next 2 ms period allocated for signalling in that direction over the signalling bus. This causes the codec to power up and make connection to the allocated time slot on the speech highways, and to power up the band limiting filters. From this stage onwards, signalling information from the Line Unit passes directly from the shelf signalling bus over the internal data bus to the signalling-in store in the CSI RAM, from which it is read out by the PISO conversion circuit to the signalling-in highway. Similarly, information received over the signalling-out highway is transferred to the signalling-out store in the CSI RAM, from which it is read and sent by the micro-computer over the signalling bus to the Line Unit. Speech and signalling are now working in the same time slot to the Speech Switch and the Signalling Circuit. Speech bits pass directly over the transparent speech highways and call set up proceeds by means of signals relayed by the CSI between the Line Unit and the Signalling Circuit. At the end of the call, the CPU recognises the clear condition (from the first party to clear in the case of an extension to extension call) and initiates clear down by sending a continuous "idle" message to the CS!. The CSI clears the time slot and relays the idle signal to the Line Unit which, if the port has cleared, returns the idle signal to the CSI, causing the "busy" indication to be removed from the port status store in the micro-computer RAM. If the port has not cleared, the CSI does not receive an idle signal and the port is parked pending the receipt of such a signal.

3.3.4 Signals relating to incoming calls are handled in a similar way to those relating to outgoing calls. The sequences involved are briefly as follows. On receiving a request to set up a call to a port, the CPU selects the first free time slot (starting from Slot 31 ) in the highways to the shelf concerned, and sends the number of the required port, in that time slot, over the signalling out highway to the CS!. The micro-processor in the CSI notes the port number and sends out over the shelf signalling bus, a request to the Line Unit concerned to power up the codec associated with the required port and connect it to the allocated time slot; this is acknowledged by the Line Unit. The speech channels are now connected through the Speech Switch and the signalling paths are established via the CSI RAM to the Signalling Circuit. Progressing of the call now proceeds by direct communication between the Signalling Circuit and the Line Unit. Clear down takes place in the same way as in the case of an originating call.

3.4 The Shelf Multiplex

3.4.1 A block schematic diagram of the Shelf Multiplex, which is used instead of the CSI for the non-concentrating shelf positions, is given in Figure 20. The Shelf Multiplex receives combined speech and signalling bits in a 72 kbit/s stream from each line circuit, separates the speech bits from the signalling bits and assembles the speech bits in multiplexed form on the 2048 kbit/s highway to the Speech Switch and the signalling bits on the 256 kbit/s highway to the Signalling Circuit. In the opposite direction it receives 2048 kbit/s multiplexed speech over the highway from the Speech Switch and 256 kbit/s multiplexed signalling over the highway from the Signalling Circuit, separates out the speech and signalling bits destined for each line circuit, and combines them in 72 kbit/s streams to these line circuits.

3.4.2 Every 125 micro-seconds, the 72 kbit/s stream from each line circuit produces a nine bit byte comprised of a signalling bit (bit O) and 8 speech bits (bits one to 8) . All the 32 line circuits on a shelf operate simultaneously and the first task of the Shelf Multiplex is to combine the bit streams from these circuits in sequence on one highway operating at 2304 kbit/s. This is done by a shift register forming a 32 bit Parallel-In-Serial-Out (PISO) conversion circuit. The 32 stages of

Page 39

FROM LINE

CIRCUITS 72 kbitls

TO L INE

CIRCUITS 72 kbitls

Page 40

2304 kbit/s

PORT O ::::; TO SPEECH :::::: PARALLEL·IN SWITCH 2048 kbit

SERIAL·OUT .. Is

CONVERSION CIRCUIT SPEECH

FROM SPEECH REFORMATTER SWITCH 2048 kbit -PORT 3 1 Is

-

TO SIG NAlllNG INPUT 256 kbit/s

SIGNALLING FROM SIGNAlllN G

REFORMATTER OUTPUT 256 kbitl

:= � = SERIAL·IN LATCH PARALLEL·QUT

, ,Ir PORT O

(STATICISER) CONVERSION CIRCUIT DATA

SELECTOR CLOCK - I-PORT 3 1

,� 2304 kbitls

PART 1. F IGURE 20. BLOCK SCHEMATIC D IAGRAM OF THE SHELF MULTIPLEX.

the PISO conversion circuit are loaded in parallel at 72 kbit/s from the 32 line circuits and the stored data is then clocked out serially at 2304 kbit/s. The 2304 kbit/s serial output from the PISO conversion circuit contains the speech and signalling bits from all the line circuits in bit organised form, ie bits 0 from all the 32 ports are sent first, then bits one and so on. The signalling bits are then separated from the speech bits at the inputs of the speech reformatter and the signalling reformatter by normal gating techniques.

3.4.3 The function of the speech reformatter is to receive the speech bits at 2304 kbit/s in bit organised form from the PISO conversion circuit, and to transmit them to the Speech Switch in byte organised form at 2048 kbit/s. To achieve these changes in the speed and order of bits, a complete frame of speech bits is written into a memory at 2304 kbit/s and the data is read out of the memory during the next frame in the desired order at 2048 kbit/s. Two memories are required so that the data can be written into one while the other is being read. At the end of each frame (125 micro seconds) the roles of the memories are interchanged.

3.4.4 The signalling reformatter performs a similar function with the signalling bits. These arrive from the PISO conversion circuit in bit organised form and a complete frame (signalling bits A to H) is written into the store in one millisecond (8 speech frames) . The data is read out of the memory during the next signalling frame at 256 kbit/s and sent over the signalling highway to the Signalling Circuit. As in the case of the speech reformatter, 2 memories are used alternately for the reading-out and writing-in functions.

3.4.5 In the receive direction, the inverse process is required for both the speech and signalling bits. Speech samples arrive from the Speech Switch destined for a particular port. This data must be changed from byte organised form at 2048 kbitl s to bit organised form at 2304 kbitl s with gaps for the signalling bits to be inserted. Similarly, the signalling bits from the Signalling Circuit are received at 256 kbit/s in byte organised form and must be changed to bit organised form and inserted in the gaps between the speech bits. The process by which this is achieved are the inverse of those used in the send direction and, in fact, it has proved possible to use the same pairs of memories for both directions of transmission, with resultant economies. The memories used in the Shelf Multiplex shift registers and reformatter are all of the Random Access type.

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SAMPLES

-OUTGOING SPEECH

SAMPLES

S PEECH BITS REPRESENTING STORE SPEECH SAMPLES

IS 1\

7 6 5 4 3 2 1 \ CHANNEL 1

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L'-- -r-

CHANNEL CHANN EL 33 0 1 0 0 1 0 1 1 I� I NPUT TIME ---

SLOTS

CHANNEL 64 [TH 0 1 1 ill fj CHANN EL 256FT t I I l iB

CONNECTION IDENTITY OF SAMPLES TO BE

CHANNEL OUTPUT

TIME SLOTS

STORE READ OUT DURING PORT OUTPUT TIME SLOTS

1\ IS 7 6 5 4 3 2 1\ CHANN EL 1

�i CHAN NEL 2

,/ -

CHANN EL 33 0 1 0 0 0 0 0 0 ...........

CHANN El 64� ! �m CHANNEL 256� I ��

t DATA FROM CENTRAL PROCESSOR UN IT FOR I NSERTION I N CONNECTION STORE

---

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ENCODED S PEECH SAMPLES

STRUCTIONS WHEN TO READ OUT

SPEECH SAMPLES

BINARY CODE FOR 64

BINARY CODE FOR 33

NOTE: The information in the connection store is for a cal l between Channel 33 and Channel 64.

PART 1. F IGURE 21. OPERATING PR INCI PLE OF THE SPEECH SWITCH.

Page 41

3.5.3 A block schematic diagram of the Speech Switch is given in Figure 22. The 2.048 Mbit/s highways from the line unit shelves, the 2.048Mbit/s highway from the Services Card and the remaining spare highway inputs are connected to a Serial-In-Parallel-Out (SIPO) conversion circuit, the output of which appears on an 8 wire parallel highway to the Speech Store. The SIPO conversion circuit accepts the standard 2.048 Mbit/s serial, byte organised, bit streams from the 8 incoming highways and delivers them at the output in such a manner that all the first bits appear on wire 1 , all the second bits on wire 2, and so on. Thus, although the 8 bits for each speech sample are received at the input in serial form, and hence at different times, they are reformatted so that they appear on the highway to the Speech Store at the same time. The bits relating to channels on the different shelves are interleaved so that those relating to channel 0 are followed by those relating to Channel 32, then those relating to channel 64, and so on. On arrival at the Speech Store all the 8 bits relating to a particular channel are written into the store for that channel during a single bit period. On each of the wires on the parallel highway between the SIPO conversion circuit and the Speech Store, 256 bits (one per channel) appear 8000 times a second and the rate of information flow over each wire is 2.048 Mbit/s.

3.5.4 The output counter, via the connection store selector, addresses the locations in the Connection Store sequentially at 2.048 MHz during the first halves of the clock cycles and causes the contents to be read out, at the appropriate times, via the speech store selector to the address input of the Speech Store. During the second half cycles the connection store selector connects the Connection Store to the CPU address bus, and whenever a connection requires to be set up or released, the CPU puts out the outgoing channel number (or deletion instructions if the call is to be released) over the address bus during one of these half cycles and the incoming channel number over the data bus. Thus during one half cycle the Connection Store indicates to the Speech Store which location should be read out, and during the other half cycle the Connection Store is available to the CPU for new instructions to be written in as, and when, required.

3.5.5 The 2.048 MHz pulses from the input counter are connected via the speech store selector to the Speech Store during the first half of each cycle and cause the incoming speech samples to be inserted into their correct locations in the store. During the second half cycles the Speech Store selector connects the output from the Connection Store to the Speech Store to cause the appropriate speech sample to be read out and sent to the switch output via the PISO converter. It is to be noted that the data which is read out of the Speech Store during the second half of the clock pulse relates to a different connection from that for which data was written in during the first half. Thus the Speech and Connection Stores, loaded and controlled as just described perform the functions referred to in paragraph 3.5.1 and illustrated in Figure 21 .

3.6 The Signalling Circuit

3.6. 1 A block schematic diagram of the Signalling Circuit is given in Figure 23. This circuit stores the serial information received over the 256 kHz signalling-in highways and makes it available to the CPU, in parallel form, when required. The circuit also stores signalling information received from the CPU in parallel form and extends it over the signalling-out highways at the appropriate channel times. A 512 x 8 bit RAM provides the storage capability. Half the RAM provides a separate 8 bit location for every channel in the signalling-in highways and the other half provides a similar location for every channel in the signalling-out highways.

3.6.2 The signals on each of the eight signalling-in highways are received in byte organised form (bits A to H) . These bits are fed into the SIPO conversion circuit and appear at the output in shelfinterleaved parallel form, by a process similar to that in the SIPO conversion circuit in the Speech Switch. At an appropriate time, as described later, the bits are transferred to the signalling-in portion of the RAM, from which they are read out over the data bus to the CPU.

3.6.3 Signals which require to be sent over the signalling-out highways are received over the data bus from the CPU, and at an appropriate time are extended from the switched buffers to the signalling-out portion of the RAM from which they are later read out, in shelf interleaved parallel form, via the PISO conversion circuit to the signalling-out highways, where they appear in serial byte organised form.

3.6.4 The transfers of information are controlled by a 256 kHz waveform. During one half cycle, the selector connects the CPU address bus to the RAM and information may be transferred from the signalling-in portion of the RAM to the CPU, or from the CPU to the

Page 43

SPEECH H IGHWAYS

F R OM SHELVES

AND SERVICES

CARD

SPEECH H IGHWAYS

T O SHELVES

A N D SERVICES

CARD

Page 42

,

/

3.5 The Speech Switch

3.5 . 1 The principle of operation of the Speech Switch is shown in Figure 21.

Each of the 256 channels in the system is allocated its own time slot, which occurs at the same time in the Concentrating ShelfInterface or Shelf Multiplex for both directions of transmission. The CPU, acting upon the call connection instructions received (eg dialled digits) inserts into the Connection Store the identities of the outgoing channel time slots to which the speech samples received from each incoming channel by the Speech Store should be repeated. In the example shown it is assumed that Channel 33 is connected to Channel 64, thus the binary equivalent of33 is written into the Connection Store of Channel 64. In the Speech Store 8 bits capacity is permanently allocated to each channel and if a channel is involved in a call, incoming speech samples from the 2.048 Mbit/s highway, after processing, are inserted into the channel store during its time slot periods. During each output time slot the content of the Connection Store location corresponding to that time slot indicates to the Speech Store which speech sample should be read out. Thus, during output time slot 64 the speech sample corresponding to input time slot 33 in the Speech Store is read out and vice versa.

3.5.2 Speech can be transferred separately in both directions between any 2 channels regardless of the number of other channels in use, and accordingly the switch behaves as a full availability non-blocking 4-wire switch. As mentioned in Section 2.3 and elsewhere, one highway is used to obtain access to the Services Card which contains the MF Keyphone Receivers, Tone supplies and the Conference Unit. Each of these items has its own channel (time slot) and the principle of connecting them is identical with that described above, except that in the case of tone supplies a connection in only one direction is required, and in the case of MF Keyphone Receivers, connections in the 2 directions are made to different channels, one of which sends dial tone to the extension and the other receives MF signals from the extension. Since the reading out of speech and tone samples is non-destructive it is possible to send the same sample to many destinations. For example dial tone may be sent to several ports simultaneously simply by writing the channel number corresponding to dial tone into the connection store locations for all the channels serving the ports that are to receive it.

l/ CPU [lATA BUS

I\. "'f CONNECTION

STORE O UTPUT

� (256 x 8 BIT COUNTER CLOCK 8 BITS IN PARALLEL

RANDOM PULSE

SHELF ACCESS I

SERIAL· IN INTERLEAVED MEMORY)

PARALLEL· OUT .� CONNECTION CONVERSION STORE

CIRCUIT -v SELECTOR

t (NOTE 1)

SPEECH SPEECH l' t CLOCK

STORE (256 x 8 BIT 1-Lr.. STORE

RANDOM SELECTOR PULSE

ACCESS (NOTE 1)

8 BITS IN MEMORY) fCLOCK CPU ADDRESS BUS PARALLEL PULSE

PARALLEL· IN SHELF

SERIAL·OUT INTER LEAVED IN PUT

CONVERSION COUNTER

C IRCUIT t CLOCK PULSE

N OTES: 1 . This contact is symbolic. No relay contacts are used in this circuit.

PART 1 . F IGURE 22 . BLOCK SCHEMATIC D IAGRAM OF THE SPEECH SWITCH.

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S IGNALLI NG- OUT H IG HWAYS

TO SH ELVES

AND SERVICES CARD

Page 44

PARALLEL- OUT CONVERSION � RANDOM

SELECTOR AND ACCESS NOTE 1

INTERLEAVING M EM ORY

CIRCUIT STORE

CLOCK PU LSE

INPUT COUNTER

STORE ENABLE SELECT

! t CLOCK 8 BITS IN PARALLEL � PULSE

OUTPUT COUNTER

CLOCK PULSE

PARALLEL- IN SERIAL-OUT

CONVERSION SWITCHED AND BUFFERS

DE-I NTERLEAVING ...- � CI RCUIT ---.. CPU DATA BUS

LATCH

NOTES: 1 . This contact is symbolic. No relay contacts are used in this circuit.

PART 1, F IGURE 23 . BLOCK SCHEMATIC D IAG RAM OF THE SIGNALLI NG CIRCUIT.

signalling-out portion of the RAM. During the other half cycle the selector connects the counters to the RAM and during the early part of the half cycle information is transferred from a time slot on one of the signalling-in highways to the signalling-in portion of the RAM under the control of the input counter and during the later part of the half cycle, information is transferred from the signalling-out part of the RAM to a time slot on one of the signalling-out highways under the control of the output counter.

3.7 The Address Decoder

3.7.1 A block schematic diagram of the Address Decoder is given in Figure 24. The 20 bit address bus is connected to the Address Decoder, and only those leads which carry the 8 least significant bits are extended to the Speech Switch and the Signalling Circuit. This 8 bit extension is capable of identifying any of the 256 locations in the Speech Switch or in the outgoing portion or the incoming portion of the Signalling Circuit RAM. It is necessary to supplement the address on the 8 bit extension with an additional indication over a separate lead to show to which of these locations the address applies. Although 2 bits would be sufficient to provide this indication to the

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ENABLE SPEECH SWITCH

LEAST S IGN IFICANT 8 BITS " )

ENABLE S IGNALLING- IN

ELEMENTS

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ENABLE SIGNALLI NG-OUT

ELEMENTS

V' '---v---/ '--v--/ '--v--/

TO SPEECH SWITCH

TO SIGNALLI NG- IN ELEMENTS OF

SIGNALLI N G CIRCUIT

TO S I GNALLING-OUT

ELEM ENTS OF S IGNALLI NG CIRCUIT

PART 1. F IGURE 24. BLOCK SCHEMATIC DIAGRAM OF THE ADDRESS DECODER.

Address Decoder, it is convenient to derive the information from the remaining 12 bits in a structured manner which is consistent with the bank swiching arrangements described in paragraph 3.8.4, thus leaving flexibility to permit re-allocation of storage addresses within the system if required in the future. Strapping fields are provided within the decoding equipment to enable the address to be changed at any time.

3.7.2 The Signalling Circuit, Speech Switch, Address Decoder and the main clock circuit are all mounted on the same printed wiring board, which is illustrated in Figure 25.

3.8 Control Equipment

3.8.1 All the functions carried out by the exchange are controlled by the Central Processing Unit (CPU) acting on data contained in the system programs and working information stores (ie stores containing information relating to calls in progress at any particular time, and other activities of a non-permanent character) . The way in which the CPU extends its control throughout the system, by means of address and data busses, is illustrated in Figure 4, where it will be seen that the interfaces between the control equipment and the rest of the system are the Signalling Circuit and the Input Time Switch. A block schematic diagram of the CPU, and its associated memories� is given in Figure 26. The microprocessor communicates with the memories containing its various program stores and working stores over the same address, data and control busses as are extended to the rest of the system. The section of the address bus

Page 45

Part 1 Figure 25 The Signalling Circuit, Speech Switch, Clock Board

between the CPU and its associated stores as far as the Address Decoder, uses 20 bits to enable the capacities of the stores to be increased by "Bank Switching" - a technique which is described later in this sub-section. The Read Only Memories (ROMs) are of the erasable programmable type (EPROMs) and the Random Access Memories (RAMs) are of the Complementary Metal Oxide Silicon (CMOS) type for the low power requirements, or "N" channel Metal Oxide Semiconductor (NMOS) type for normal read/write storage.

3.8.2 The 6K ROM and lK RAM containing the off-line maintenance and diagnostic program and working data are contained within the CPU printed board. The remaining memories are mounted on a separate board. The main program, on which the basic operation of the system depends, is written into the 106K ROM store at the time of manufacture. The 16K RAM provides the working store in which all the processor data relating to each call in progress at any particular time is recorded. The customer Data Base (see Paragraph 3.8.3) and fault record information are stored in a separate 8K RAM and to safeguard against the loss of this information if a power failure should occur, a back-up power supply consisting of nickel cadmium cells is provided on the board which accordingly becomes a non-volatile (NV) RAM.

3.8.3 The customer's Data Base, which contains all the information relating to the particular location at which the system is installed is prepared initially by the Administration and permanently stored in a separate 4K ROM on a small printed board ("Daughter Board") which is

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PART 1, F IGURE 26. BLOCK SCHEMATIC DIAGRAM OF CENTRAL PROCESSING UN IT AND ASSOCIATED STORES.

supplied as a plug-in item on the NVRAM (the "Mother Board") . When the system is first made operational, the information in the daughter board is automatically transferred to the mother board, where it is accessed by the CPU as, and when, required. Facility changes made by the customer, using the Man-Machine Interface described later, effect alterations to the Data Base information in the NVRAM so that after a time this information will differ from that in the daughter board. However, the information permanently stored in the daughter board ensures that if, in spite of the battery back-up, the NVRAM loses its information, an adequate Customer Data Base is available to enable the installation to function satisfactorily as soon as it is restored to service. The battery back-up in the NVRAM ensures that the customer modified information is retained for at least 4 days. The administration can, at any time, replace the daughter board in an installation by an unprogrammed one and cause the current Data Base information in the NVRAM to be transferred automatically to the new daughter board which will then provide a permanent updated customer Data Base. The NVRAM store with its daughter board is illustrated in Figure 27.

3.8.4 The use of a 16 bit address bus throughout would restrict the total number of addresses to 64K. However, within the CPU and its associated stores there are well over lOOK bytes of information to be accessed and, in addition, there are further bytes stored in the Signalling Circuit and the Speech Switch. To increase the capacity of the address bus, an additional 4 bits are used between the processor and its associated memories and the Address Decoder. The bits

Page 47

concerned are in the 4 most significant positions in the binary encoded numbers comprising the address and each 4 bit combination, known as the "Bank Number", identifies the group of stores (or "Bank") to which access is required. In principle, this increases the capacity of the address bus from 64K to 1024K, but in practice it is convenient, for design reasons, to make some areas of memory common (memories in these areas are addressed independently of the bank number) and this reduces the capacity of the address bus below 1024 K. By choosing the correct balance between common memory and "bank switched" memory a useful gain in addressing capacity is still obtained.

3.8.5 The hexadecimal display and sense switches, to which reference is made in Figure 26 are mounted on the front edge of the CPU printed board. The hexadecimal digits, which identify faults recorded by the CPU are presented as a 2 digit illuminated display; the use made of this display is referred to later in the manual. There are 7 sense switches, also mounted on the front of the CPU panel, which are used by maintenance personnel to control the application of certain types of teSt. The "power-up and reset" element functions when the power is first connected, or restored after a mains failure. The "power-up" element checks that the principal (SV) DC supply is within its tolerances 'before giving a signal to the Watchdog to cause the CPU to be brought into operation. The function of the Watchdog has already been described in Paragraph 3 . 1 . 1 1 . The Central Processing Unit is illustrated in Figure 28.

3.8.6 The CEU software consists of a number of processes, each of which has a specific function. The processes are loosely coupled so that changes to one process have a minimal effect on the other. Communication between processes is effected by messages passed through the Operating System, which decides which process should be run at any particular time; i t also

Part 1 Figure 27 The Random Access Memory/Non-Volatile Memory and Daughter Board

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provides common services, such as maintenance of the time of day clock. There is no communication between the different processes by the use of shared memory.

3.8.7 There is a further division of each process into parts - initialisation, which runs when the system is restarted to put the process data into a known state; foreground parts (fast and slow) which are run on periodic interrupts generated from the system clock; and a background part which is run on demand. Thus the background part is only active when work requires to be done. The application processes can be divided into 2 types: the telephonic type, which is concerned directly with controlling calls through the exchange, and the non-telephonic type, which is concerned with functions auxiliary to the main business of controlling calls. The CEU software structure is illustrated in Figure 29. The initialisation part, which is only run when the system is restarted, has been omitted for ease of presentation.

3.8.8 The telephonic processes consist of:

Exchange Scan (XSCAN)

The foreground parts of this process scan the exchange, looking for new events, and the background part controls the exchange hardware on command from other processes. The slow scan looks at the entire exchange every 128 milliseconds, to detect significant changes such as a user lifting his handset. The fast scan looks at calls already in progress which have reached the stage where the fast recognition of changed conditions is essential - for example, the recognition of pulses during dialling. The exchange equipment does not interrupt the processor; only the real-time clock does this. The XSCAN process acts as an interface between the exchange hardware and the Call Processing functions. It translates

Part 1 Figure 28 The Centra/Processing Unit Page 49

BACKGROUND PROCESSES

SOFTWARE PROCESSES

OPERATING SYSTEM

INPUT QUEUE

INPUT QUEUE

ASSOCIATED HAROWARE

SIGNALLING OUT

- 1- - - - -1- - - -

Page 50

I , _ ..L _ .., -i 1 2BmS INTERRUPT ...J' L.. - T -

MISCELLANEOUS : MTCE & DIAGNOSTICS

LIST PROCESS TRAFFIC RECORDING

C ALL LOGGING

MAN· MACHINE INTERFACE

NOTES: 1 limited use is also made of a lOOmS interrupt but this has been omi«ed for clarity.

CALL PROCESSING

ICPRO)

2 The Slow Scan and Fast Scan parts of processes constitute the Foreground Parts.

EXCHANGE SCAN

IXSCAN)

PART 1. F IGURE 29 . THE CEU SOFTWARE STRUCTURE.

hardware signals into telephonic messages within the CPU. For example, a certain bit set in a signalling byte from an Extension Line Unit may be translated to a "new call" message.

Call Processing (CPRO)

This has no foreground part; it runs on demand in response to messages from other processes, to control requirements for both basic telephone service and special facilities. The CPRO process contains the main intelligence of the system.

3.8.9 The non-telephonic processes consist of:

The Man-Machine Interface (MM!)

The function of the MMI is described in Section 6. The MMI software process has no foreground part. It runs on demand, in response to messages from other processes, to provide a means of examining the state of exchange resources, altering the Customer's Data Base, and invoking diagnostic tests.

The Maintenance and Diagnostic Process (MAD)

The foreground parts monitor the output of the ringing generator and act as a Watchdog to ensure that the background part runs, and that the background part is scheduled

SIGNALLING IN

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periodically to test the exchange hardware. MAD can also be invoked, when required, by maintenance personnel through MMI.

Call Logging (CLOG)

This process has no foreground part. It runs on demand, in response to messages from CPRO, to log details of outgoing calls as they are completed.

List Process (LISP)

The foreground parts of this process handle character output to the MMI, call logging and system monitor printers, and accept input from the MMI keyboard. The background part reformats data from other processes for output to the appropriate printer.

Traffic Recording (TREC)

The process has no foreground part. It receives messages from CPRO and XSCAN which enable it to record the use of common resources, both software and hardware, within the exchange, and the occasions when calls fail due to a shortage of resources.

All the above processes except MMI have been included in the miscellaneous area of Figure 29.

3.8.10 The Operating System schedules foreground routines on interrupt from the system real-time clock. Every 128 ms, on completion of the slow foreground routines, the background scheduler is run and the highest priority executable background process is entered. A background process is executable if it has been interrupted or if it has a task waiting in its queue. The priority order of processes is fixed; XSCAN has the highest priority and LISP the lowest.

3.8.1 1 It is to be noted from the foregoing that the 2 processes CPRO and XSCAN together perform all the telephonic functions of the system. CPRO is the master and XSCAN is the slave. CPRO receives from XSCAN notice of the telephonic events it has detected, and in return issues commands to be executed by XSCAN. Typical events are the detection of CLEAR, DIGIT, NEWCALL, and RECALL signals. Typical commands are to MARK an extension, SET STATE of termination, CONNECT a path, RING a bell etc. XSCAN has no memory of the progress of a call or of the relationship between the various ports it is monitoring. Thus, when XSCAN detects an event and sends it to CPRO it takes no further action other than the minimum necessary to maintain its own functions. Hence no conflict in control between CPRO and XSCAN can arise.

3.8.12 Since the system must support many calls in progress at any one time, a record of the progress of each call must be kept. This information is kept in the Call Records. Each Call Record is identified by a Call Record Number (CRN) which is used to access the correct data and to direct external events, such as those detected by XSCAN, to the appropriate call. Typical of the data stored are the Equipment Numbers (ENs), ie the Port Numbers, of the parties involved in the call, the identities of any hardware or software resources employed, dialled digits , and information describing the type of call being made and its progress so far.

3 .8 .13 The progress of a simple own-exchange call through the system starts as follows : The arrival of the port number of the calling extension in a time slot from the CSI is detected by XSCAN which sends an acknowledgement to the CSI, causing it to become transparent to signalling in both directions via that time slot. XSCAN then detects the loop condition in the time slot and a NEWCALL message together with the EN of the extension concerned is sent to CPRO. The message is recognised by CPRO as a NEW CALL type and a CRN is allocated to it. An entry is made in the EN-CRN translation table so that any subsequent messages referenced by the same EN will be directed to the same CR. The CR is activated and the class of service of the originating extension examined to determine the type of signalling required. XSCAN is instructed to provide the appropriate digit receiver( s) and CPRO enters a wait state to await reception of digit messages from XSCAN. When a digit message is received and CPRO re-started, the EN is referred to the EN-CRN translation table and the CRN is obtained . The digit is then placed in the CR stores. CPRO instructs XSCAN what action should be taken in the hardware at each stage as the call proceeds. This type of procedure is repeated until the call finally terminates.

Page 51

SPEECH H IGHWAY FROM SPEECH SWITCH

(32 CHANNELS) SPEECH

2048 kbit/s

IGN ALLlNG -OUT S H IGHWAY FROM

S IGNALLI NG C IRCUIT

(3 2 CHANNELS)

2 5 6 kbitls

SPEECH (NOTE 1)

SPEECH (NOTE 1)

DROPBACK INPUT

I SPEECH (NOTE 1) SIGNALLI N G

S IGNALLI N G ..

CONFERENCE SPEECH UN IT

TONE SPEECH (NOTE 1) "" GEN ERATOR SPEECH H IGHWAY

TO SPEECH SWITCH (32 CHANNELS)

2048 kbit/s

DIAL TONE SIGNALLING RECEIVER

M F S IGNALLING BI NDER RECEIVERS

(SSMF4)

S IGNALLIN G (NOTE 2) SPEECH (NOTE 1)

SIGNALLING- IN TEST U N IT H IGHWAY TO

SIGNALLI N G .. SIGNALLING

C IRCUIT S IGNALLI N G (32 CHANN ELS)

(NOTE 2) ,� 2 5 6 kbit/s TO REMOTELY

PATTERN SITED ALARM G EN ERATOR BOX (OPTIONAL)

SSM F4 SPEECH (NOTE 1) ..

SENDER

NOTES : 1 . In this figure the term "speech" includes digitally encoded tones/A.C. signals in the speech frequency band. and speech-like test patterns.

Page 52

2. The signal ling outputs from the Keyphone Receiver and the Pattern Generator utilise circuit elements in the Test Unit IC to ach ieve maximum design economy.

PART 1. F IGURE 30. BLOCK SCH EMATIC DIAGRAM OF THE SERVICES CARD.

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3.8.14 The system program is large and complex, and during the lifetime of the system i t may need to be modified from time to time to provide enhanced capabilities, or as a consequence of maintenance requirements. Further, in view of the forecast life of the system, it has to be expected that modifications will need . to be carried out by people other than the original programmers. For these reasons a high level language (CORAL) has been used for most of the program, but in time critical areas, where it is important to achieve maximum processor efficiency, assembly language has been used. The general rule is that all background procedures are programmed in CORAL, and the remainder, which account for about 15% of the program, are written in assembly language.

3.9 The Services Card

3.9.1 The Services Card, which interfaces with the speech switch and signalling units in a similar way to the Concentrating Shelf Interface, provides the following functional items:-

The Conference Unit

The Tone Generator

The Dial Tone Receiver

- The MF Signalling Receivers

S PEECH H IGHWAYS SIGNALLING HIG HWAYS

I NCOMING FROM SPEECH SWITCH

OUTGO ING TO SPEECH SWITCH

INCOMING FROM SIGNALLING CIRCUIT

O UTGOING TO S IGNALLING CIRCU IT

PART 1. FIGURE 31. ALLOCATION OF PORTS IN THE SERVICES CARD. Page 53

The Test Unit

The Pattern Generator

The SSMF No. 4 Sender (not used with the present processor program)

3.9.2 A block sch<:matic diagram of the Services Card is given in Figure 30. All incoming information to the card arrives over a 2048 kbit/s highway from the Speech Switch or a 256 kbit/s highway from the Signalling Circuit. Similarly, all information leaving the card is sent over a 2048 kbit/s highway to the Speech Switch or a 256 kbit/s highway to the Signalling Circuit. These highways provide 32 time slots (ports) which are allocated to the various items on the card, as shown in Figure 31 . It will be noticed that most items do not require ports in all 4 highways, indeed, only the Test Unit makes use of all the different highways. Thus it is possible to use the same port on different highways for different items. For example, the MF Signalling Receivers use ports 0 to 7 only on the incoming speech and outgoing signalling highways, and these ports on the outgoing speech highway are used for outputs from the Tone Generator. Other similar examples can be found in Figure 31 .

3.9.3 Each functional item on the card which requires to receive information from an incoming highway incorporates timing features at its own input which enable it to capture the required bit (or bits) in the time slot allocated to that item. Each item is responsible for collecting its own input data.

3.9.4 Depending on the function performed by an item, its output may be in the form of A-Law encoded speech (or equivalent, eg audible tones) at 2048 kbit/s, or in the form of

Part 1 Figure 32 The Services Card

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signalling bytes (bits A to H) at 256 kbit/s, or both, as in the case of the Test Unit. The outputs, which are in serially encoded form in all cases, are fed into a "Binder" circuit, the function of which is to ensure that output information is connected to the outgoing highways at exactly the correct port slot time. Depending on the design of the item concerned, its output may appear at the input to the Binder continuously at every port slot time, or only at a time corresponding to the port slot time allocated for use by the item. The Binder is not involved in the receipt of information by the Services Card (see Paragraph 3.9.3) and it does not insert any information of its own into the outgoing bit streams; it performs only the output data selecting function.

3.9.5 It will be seen that the signalling outputs from the MF Signalling Receivers and the Pattern Generator pass to the Binder via the Test Unit. This is not due to any functional relationship between the three items, but arises from the fact that economies have been achieved by making use of spare circuit elements in the Test Unit IC to perform the Pattern Generator function and to provide certain circuit elements required in the MF Receiver outputs.

3.9.6 An illustration of a Services Card is given in Figure 32.

The Conference Unit

3.9.7 A block schematic diagram of the Conference Unit is given in Figure 33. The 16 ports allocated to the unit are divided into 4 groups, each comprising a conference "bridge" capable of interconnecting 4 ports. Thus the unit can handle 4 conference calls simultaneously, each involving up to 4 ports. The conference unit performs its internal functions continuously regardless of whether the CPU is making use of them, and it is una ware of the occasions on which it is taken into use. The service that each bridge provides is to deliver an output to each of its 4

SPEECH H IGHWAY FROM SPEECH

SWITCH SERIAL· IN 8 BITS IN INPUT 8 BITS IN A· LAW 13 BIT S IN

LLEL (PORTS 16·31) PARALLEl PARALLEL PARA • PARALLEl· OUT RANDOM .. TO

CONVERSION ACCESS L INEAR 2 · 048 Mbit/s C IRCUIT MEMORY CONVERTOR SER IAL STREAM

ACCUMULATOR -

TO SPEECH H IGHWAY

TO SPEECH SWITCH

VIA B INDER PARALLEL· IN L INEAR (PORTS 1 6·31) 12 BITS IN .. SERIAL-OUT TO PARALLEL CONVERSION - A-LAW 2 · 048 Mbit/s C IRCU IT 8 BITS IN CONVERTOR SERIAL STREAM PARALLEL

PART 1. F IGURE 33 . BLOCK SCHEMATIC D IAGRAM OF THE CONFERENCE UN IT.

Page 55

ports which is the sum of the inputs received from the other 3 ports. The summing operation is linear and introduces no gain or loss.

3.9.8 When the CPU receives a request for a conference call it connects the parties concerned to the time slots corresponding to one conference bridge on the 2048 kbit/s incoming and outgoing highways between the Speech Switch and the Services Card. The incoming highway from the Speech Switch appears at the input to the Serial-In-Parallel-Out Conversion Circuit which extracts each of the 8 bit speech bytes corresponding to the ports to be interconnected, and sends them in sequence to the Input RAM, where each of the 16 conference ports has its own store. The Accumulator receives the bytes from the Input RAM via an A-Law to Linear Convertor, which transforms the 8 A-Law encoded speech bits into 13 bits linear encoded PCM. The thirteenth bit is required to identify the centre point of a quantum level and thus ensure that the quantising distortion during this process does not exceed half a quantum level.

3.9.9 The speech bits from each combination 00 ports in a bridge are successively summed in the Accumulator, and the results are sent in 12 bit parallel form to a linear to A Law converter: the thirteenth bit is discarded (rounded up) . The sum values, now in 8 bit parallel A-Law coded form, are converted to serial form and each sent via the Binder, to the remaining port of the bridge on the outgoing highway to the Speech Switch. Thus, the bits from Ports 1 , 2 and 3 in the bridge are added together and sent to Port 0, the bits from Ports 0, 2 and 3 in the bridge are added together and sent to Port 1 and so on. The intermediate conversion to linear encoding is necessary to permit simple addition of the sample values in the Accumulator.

3.9.10 If only three parties are connected together on a conference call, the unused port in the bridge must be connected to silence, and the CPU effects this automatically by connecting the port via the Speech Switch to Port 10 on the Services Card (see Paragraph 3.9. 13, Table 3 ) . Similarly, if any party clears down from a four party conference call, the CPU must connect the port which it was using to silence. However, if a party clears down from a 3 party conference then the two remaining parties are connected directly to each other, and the conference bridge is returned to the idle pool.

The Tone Generator

3.9 . 11 All the supervisory tones used by the system originate from the Tone Generator, where they are stored as digital patterns on an Erasable Programmable Read Only Memory (EPROM) . The cadences for the different tones are hard wired. The outputs are fed via the Binder onto the 2048 kbit/s highway to the Speech Switch. The Tone Generator has 12 ports, each of which can supply a different type of tone. The bit patterns corresponding to the tone supplied by each port are continuously sent over the speech highway to that port in the Speech Switch. To connect a tone to a line it is only necessary for the CPU to cause the speech switch to make a connection between the port supplying the required tone and the time slot serving the line concerned. If more than one line requires the same tone, one Speech Switch connection is made to each time slot from the tone port. No problems of tone supply loading arise because of the nature of the Speech Switch. A block schematic diagram of the Tone Generator is given in Figure 34 .

3.9.12 The digitally encoded tones are read out from the EPROM on which they are stored, under the control of clock pulses, and passed in 8 bit parallel form to a Parallel-In-Serial-Out conversion circuit which extends the bits in serial form via the Binder to the highway to the Speech Switch. The EPROM contains a table of silence samples, so that in effect silence becomes another tone. The cadences for the different supervisory tones are provided by the cadence logic which causes silence samples to be read out during the "off' periods of the tones.

3.9.13 Table 3 shows the various tones currently assigned to the different ports allocated to the Tone Generator, and the levels of tones as measured at the Main Distribution Frame (MDF) . Lower levels have been adopted for the 1000 Hz and 1400 Hz tones to take account of psophometric weighting considerations. The 1000 Hz tone is not used at present. The interrupted dial tone reminds the user that calls to his extension are being diverted. Different tones required in other networks, can be provided as an alternative by suitably programming the EPROMs and, where cadence changes are required, by replacing one of the ICs.

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TONE 8 BITS IN PARALLEL SER IAL-OUT VIA B INDER (PORTS 0-10) SAMPLES CONVERSION (READ 2048 kbitls

ONLY C IRCUIT MEMORY)

TONE SELECT

COUNTER

CADENCE 3 BITS I N PARALLEL CADENCE GENERATE SELECT LOGIC

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COUNTER COUNTER S ILENCE

DUR ING "OFF' PERIODS OF

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PART 1 . F IGURE 34. BLOCK SCHEMATIC DIAG RAM OF THE TONE GEN ERATOR.

Page 57

Port Tone Basic Tone Cadence Level at No. Store MDF

0 Continuous ringing 400+450 Hz Continuous On

1 Ringing 400+450 Hz � On .4s; Off .2s;

On .4s; Off 2.0s -10 dBmO

2 Inverted ringing 400+450 Hz � Off .4s; On .2s

Off .4s; On 2.0s 3 Confirmation tone 1400 Hz Continuous On � -20 dBmO 4 Spare 1000 Hz Continuous On 5 Number

Unobtainable 400 Hz Continuous On

6 Switching 400 Hz � Off .4s; On .2s

Off .4s; On 2.0s -10 dBmO 7 Engaged 400 Hz On .375s; Off .375s 8 Dial 350+400 Hz Continuous On 9 Interrupted dial 350+400 Hz On .75s; Off .75s 10 Silence - - -

11 Unused - - -

Table 3 Allocation of Tone Generator Ports

The Dial Tone Receiver

3.9.14 In some of the older designs of public exchange, a significant delay may occur between seizure of an exchange line and the establishment of dialling conditions at the exchange. To cater for these cases, pulsing out from the P ABX must be inhibited until dial tone has been received, and accordingly, a Dial Tone Detector is required at the PABX. The characteristics of dial tone in respect of amplitude, fundamental frequency and harmonic content vary so widely throughout the network as a whole that the following criteria have been found the most satisfactory for recognising the tone:-

a. it must reach a level of -21 dBm within every 8 ms sampling period over a nominal period of 600 ms.

b. the sign of the AC signal comprising the tone must have alternated at least 8 times.

When the CPU recognises that a call requires to be routed via the public exchange, it selects an outgoing exchange line and sets up a connection between the "receive" path of the line and Port 13 on the Speech Highway to the Services Card. As soon as conditions (a) and (b), above, are satisfied in the Dial Tone Receiver, it sends a signal to the CPU indicating that it may be released and pulsing out may start.

3 .9 .15 A block schematic diagram of the Dial Tone Receiver is given in Figure 35. The amplitude detector examines the PCM encoded bits in the time slot corresponding to Port 13 in the incoming speech highway, to determine whether the signal level is above or below -21 dBm, and the alternator detector similarly examines the bits to determine the sign changes in the received AC signal. As long as the requirements of the amplitude detector continue to be satisfied, the persistence timer continues to count for approximately 600 ms, after which a signal is extended to the check circuit. Meanwhile, providing the required number of alternations has been detected by the alternation detector, this also sends a signal to the check circuit, and provided both signals are being received, the check circuit extends a signal to Port 13 of the Signalling-In highway, via the Binder, and thence to the Signalling Circuit and CPU . If, at any time during the 600 ms recognition period, the received tone fails to satisfy the requirements of the amplitude detector, this detector connects a signal to the drop-out lead, which causes both the persistence timer and the alternation detector to be reset; it is then necessary for acceptable conditions to be received for a further 600ms before an output signal can be extended to the Binder.

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SPEECH H IGHWAY FROM

SPEECH SWITCH (PORT 13)

2048 kbit/s

AMPLITUDE DETECTOR

DROPOUT

ALTERNATION DETECTOR

PERSISTENCE TIMER

CHECK CIRCUIT

TO S IGNALLING- IN H IGHWAY

VIA B INDER (PORT 13)

256 kbit/s

PART 1. F IGURE 35 . BLOCK SCHEMATIC D IAGRAM OF THE D IAL TON E RECEIVER.

FROM OTHER KEYPHONE RECEIVERS

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PART OF I � I

OTHER I SIGNAL I KEYPHONE ENCOOING RECEIVER I CIRCUIT I

(EXPLANATORY I r -ONLY) I L_ L_-.J_ U I I I SPEECH HIG HWAY

FROM SPEECH SWITCH

(PORTS 0-7 )

BINARY ENCODED

D IG ITS

- TO SIGNALLING- IN PCM TO MF SIG NALS SIGNAL

H IGHWAY VIA TEST UNIT AND B INDER (pORTS O· 7 ) ----.. � ANALOGUE

SSMF4 SIG NAL

RECEIVER I---"'� ENCODING I-------.... �

2 048 kbit/s DECODER ANALOGUE DC CIRCUIT

PART 1 . FIGURE 36. BLOCK SCHEMATIC D IAGRAM OF THE MF SIGNALLI NG RECEIVER.

The MP Signalling Receivers

256 kbit/s

3.9.16 The MF Signalling Receivers are designed to accept signals conforming to the internationally agreed system (referred to in British Telecom as Signalling System Multi-Frequency No. 4 - SSMF4) in which each signal is represented by a combination of2 frequencies - one in the Band 697 Hz to 941 Hz, and the other in the Band 1209 Hz to 1633 Hz. On receipt of an "off hook" signal from an extension provided with an MF keyphone Class of

Page 59

Service, the CPU immediately sets up a connection in the "send" direction to an MF Receiver port on the Services Card, and in the "receive" direction to dial tone (Port 8 on the Services Card) , thus connecting dial tone to the extension user. The MF digits are accepted by the receiver and forwarded to the CPU, via the Binder and Signalling-In highway, in appropriately coded form using the 8 signalling bits A to H, as described in Part 2. When the CPU concludes that all the digits have been received, it releases the connection to the MF Receiver and sets up a call from the originating extension to the required extensions, or other destination, in the usual way.

3.9.17 The Services Card can accommodate up to 6 MF Receivers and 2 spare ports are left to accommodate a further 2 receivers for possible later applications. A block schematic diagram of a Services Card MF Receiver is given in Figure 36 from which it will be seen that the outputs of the receivers are connected in series and are passed in succession over a single path to the Binder and thence to the Signalling-In highway. The output from the receiver shown in full lines is sent to Port 0, the output from the receiver shown in dotted lines is sent to Port 1, and so on. The required time delays are inserted automatically as the receiver outputs pass down the chain.

3.9. 18 Considering now the operation of one receiver, the incoming MF signals, in b inary encoded form are captured from the incoming speech highway at the appropriate port time, and after conversion to analogue are applied to the input of a standard MF signal receiver. Provided that one signal, and only one signal is present in each of the signalling bands, and provided the levels of the 2 signals are both within the prescribed limits, and their presence exceeds a predetermined duration (the "Recognition Time") , DC signals are passed to the Signal Encoding Circuit, where they are converted to binary and sent in serial form via the Binder to the appropriate port on the 256 kbit/s Signalling-In highway.

The Test Unit and Pattern Generator

3.9 .19 The functions of the Test Unit and the Pattern Generator are described in Section 5.

The SSMF4 Sender

3.9.20 An SSMF4 Sender is provided on the Services Card to enable outgoing calls to be set up over exchange lines that terminate on public exchanges equipped with SSMF4 keyphone receivers but the main processor program does not enable this to be used at present. When the CPU recognises that an outgoing call requires to be routed over a circuit which employs MF signalling, it immediately sets up a connection between Port 13 on the outgoing Speech Highway from the Services Card and the "send" path of the cir.cuit concerned. A connection is set up at the same time between the Dial Tone Receiver (Port 13 on the incoming Speech Highway of the Services Card) and the "receive" path of the circuit concerned. When enabled by the Dial Tone Receiver the CPU then forwards each digit as it is received over the Signalling-In highway to Port 13 on the Services Card, and the SSMF4 Sender converts the digit from the binary form in which it is received over the signalling highway to the appropriate MF tone combination for transmission over the outgoing circuit. As soon as the CPU concludes that dialling has been completed it releases the connection to the SSMF4 Generator and sets up a connection between the calling extension and the outgoing circuit.

SIG NALLlNG· OUT TO SPEECH HIGHWAY H IGHWAY FROM TO SPEECH SWITCH

SIG NALLING CIRCUIT BINARY VIA (pORT 13) TO 8 BITS LOW BINDER (PORT 13)

KEYPAD TONE PASS CDDEC

256 kbit/s SIGNAL DC GENERATOR

ANALOGUE F I LTER ANALOGUE 2048 kb it/s CONVERTOR ENCODED MF MF

KEYPAD SIG NALS SIGNALS D TA A

PART 1. F IGURE 37. BLOCK SCHEMATIC DIAGRAM OF THE SSMF4 SENDER.

Page 60

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3.9.21 A block schematic diagram of the SSMF4 Sender is given in Figure 37. The binary to keypad signal converter captures the required binary encoded signalling bits from the signalling-out highway during the appropriate port slot time, and converts them into a DC encoded form identical to that produced by the key pad in an MF keyphone. The signals then enter a standard tone generator, as used in MF keyphones, and the resultant MF signals are fed via a low pass filter, (to eliminate any unwanted frequency components) into a standard analogu� to PCM coder, the output of which is connected via the Binder to the outgoing speech highway at the appropriate port time.

3.10 The Console

3.10.1 Block schematic diagrams of the operator's console and the Console Line Unit are given in Figures 7 and 17. Excluding the signalling system used between the console and the CE U, and

CONTROL PROCESSES ASSOCIATED HARDWARE

I LINK HANDLER I J SPEECH CIRCUIT I I

Data Interrupt

SCHEDULER

BLEEP LEDs ROUTINE AND

AUDIBLE

FLASH TONE

ROUTINE CIRCUIT

I VDU

VDU HANDLER

MEMORY TESTING RAM ROUTINE

KEYBOARD KEYBOARD HANDLER

DATA TO CENTRAL ( QUEUE ) ., OUTGOING EQUIPMENT

LINK DATA FROM

CENTRAL

r '\ INCOMING EQUIPMENT

"- QUEUE J LINK ..

PART 1. F IGURE 38. BLOCK DIAGRAM OF THE CONSOLE CONTROL STRUCTURE.

Page 61

described in sub-section 3.2.2 (the Console Line Unit) , the console has 4 main areas offunctional interest: the Central Processor Unit (CPU) and associated memories; the keyboard; the Visual Display Unit and the Operator's Speech Circuit.

3.10.2 A block diagram of the console CPU control structure is given in Figure 38. The blocks at the lefthand side represent software packages stored in the Erasable Programmable Read Only Memory (EPROM) which accordingly it is not appropriate to show as a separate item in the figure; the blocks on the righthand side represent hardware items, controlled by the software packages, plus the Random Access Memory (RAM) to which the CPU applies for temporary data relating to calls etc, being handled at any particular time. The software packages consist of a number of routines known as the Link Handler; the VDU Handler; the keyboard Handler; a Bleep Routine and a Flash Routine. In every case but one the routines are run periodically under the control of the Scheduler; the exception is the Link Handler which runs on an interrupt basis. MMI messages are passed to a subsidiary routine, which prepares them for display on the VDU.

3.10.3 When the Central Equipment Unit (CEU) receives a call which has to be directed to the Console, a 3 byte message is received over the signalling link and the Link Handler forwards the data to the other routines to cause appropriate action to be taken. In the case of an incoming exchange line call, the Bleep and Flash routines cause a cadenced audible tone to be given and a Light Emitting Diode (LED) , adjacent to the appropriate key, to flash. When the key is operated, the data output from the keyboard is recognised by the Keyboard Handler, which causes a 1 byte message to be sent over the signalling link to the Console Line Unit, and thence to the processor in the CEU. Receipt of this message causes the processor in the CEU to connect the exchange line port to one of the Console Line Unit ports via the Speech Switch. The processor in the CE U also instructs the Console Line Unit to send a 3 byte message to the console, where the Link Handler operates as before and causes the correct connections to be made within the Operator's Speech Circuit; the VDU handler causes the data relating to the call to be displayed on the Visual Display Unit. At this stage the operator can speak to the caller and the identity of the line used, and the status of the call, are displayed. The operator proceeds to deal with the call and subsequent keyboard operation causes further interchange of messages over the signalling link. Other types of call are handled in a similar way.

3.10.4 The control program for the Console does not vary with different sizes of P ABX. Information regarding system configuration is transmitted to the console by the CEU and stored in the RAM. This occurs each time the console is brought into use (eg when the console is first connected up). Thus a console recovered from one Monarch 120B Compact installation can be immediately re-used at another similar site.

3.10.5 As in the case of the processor in the CEU, new and enhanced facilities can be introduced during the life of the equipment by making appropriate changes to the control program. The Read Only Memory (ROM) is mounted on a detachable daughter board, similar in principle to that used for the Customer's Data Base ( see Paragraph 3.8.3) and can be readily removed, re-programmed and re-inserted into the mother board.

3. 10.6 The keyboard has 46 keys arranged in 3 groups: a. The central group contains a 12 digit keypad (including the symbols * and tt: ) and 4 commonly used function keys - the HOLD, RETRIEVE, CANCEL and WITHDRA W keys.

b. The lefthand group includes 4 keys for OUTGOING GROUPS and INCOMING GRO UPS; 4 other keys -the ASSIST, CALL-IN, W AITIN G RETURN and SERIES RETURN keys - are used for special categories of incoming call. Each of the 8 keys concerned with the incoming calls has an associated LED on the keyboard. The left hand group of keys also includes the 3 other function keys - TRUNK SELECT, SERIES CALL and VOLUME keys.

c. The righthand group contains 3 keys concerned with speech circuit control: these are the SPEAK 1, SPEAK 2/KEY INTERN AL and JOIN keys. Nine keys are also associated with special operator facilities - the EXTENSION STATUS, LAST NUMBER REPEAT, METER, CALL BACK, LAST CALL RECOVER, RING, STEP-ON, INTRUDE and TIME keys. The 3 remaining keys in the righthand group are the RECEIVING ATTENTION key (used under alarm conditions) , the CONSOLE

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TEST KEY (which implements a shelf testing routine) and ' a key marked MMI (Man-Machine Interface) which enables the system to be used to effect facility changes and to initiate diagnostic testing routines. A close-up illustration of the keyboard is given in Figure 39.

3 .10.7 The keys have no moving parts and operate on the capacitive touch principle. When a key is operated, the operator's finger forms the second plate of a capacitor, the first being the conductive pad on the printed wiring board. The keyplate acts as the dielectric. The additional capacitance to earth thus connected to the base of a transistor is sufficient to cause a shift in DC output level which is recognised by the scanning circuit. Because there are no moving parts to the key it is necessary to provide a substitute for the mechanical feedback given by conventional keys. This is done by/sounding an audible tone to confirm each key operation; operator reaction to this method has proved very favourable.

3.10.8 The VDU circuit contains a proprietary design of Liquid Crystal Display (LCD) , which incorporates its own oscillator and scanning elements. The viewing panel of the LCD module consists of 2 parallel glass plates, between which a fluid containing liquid crystals is introduced. The orientation of the molecules in the crystals is such that ambient light is normally reflected and the plates present a silvery effect. Horizontal rows of transparent conductors are printed on one glass plate and vertical columns on the other. When an electric potential of suitable value is applied between a horizontal and a vertical conductor, the orientation of the crystals at the cross point is changed and the ambient light is no longer reflected, thus making the affected area appear black. Each character is formed by darkening the appropriate position on a 5 X 7 crosspoint matrix, there being 64 such matrices in 2 rows of32. The VDU has its own RAM (external to the LCD module) in which the identity of the characters to be displayed in every position at any one time are stored, and a character generator (also external to the LCD module) which, having been supplied by the RAM with the identity of the character to be displayed, indicates to the LCD module which crosspoints should be activated as each row of the display is scanned. The LCD module runs continuously, extracting the display information from the

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Part 1 Figure 3 9 Close Up Illustration of the Console Keyboard Page 63

character generator, and refreshing the display at intervals which are short in relation to the decay time of the LCD. When it is required to change a display, the circulating path via the RAM and character generator is temporarily blocked and the console writes the new information into the RAM. As soon as the information has been inserted, the circulating path is reconnected and the new information is set up on the display and continuously refreshed as before.

3. 10.9 The two 4-wire analogue speech circuits from the CEU interface with the operator's speech circuit through transformers which perform the same functions as those in the Console Line Unit. The operator's speech circuit consists of resistors, Complementary Metal Oxide Semiconductor (CMOS) bilateral switches and operational amplifiers. The CMOS switches are controlled by the Console CPU and are used to connect either, or both, of the 4-wire circuits to the transmitter and receiver in the handset under the control of the SPEAK 1 , SPEAK 2 and JOIN keys, thus providing the normal operator facility of call splitting and joining. Another CMOS switch is used to select different gain settings in an amplifier associated with the handset receiver. This enables the operator to increase the loudn�ss of calls by 6 dB if required. Measures are incorporated to provide the correct level of sidetone in the handset.

3.11 The Power Supply

3 . 11 . 1 The information in this sub-section supplements that already given in paragraph 3. 1 . 13. The output ratings of the two proprietary designs of power unit available for use in the main and add-on units are given in Table 4.

Normal Output Maximum Output Maximum Power Voltage (Volts) Current (Amps) Output (Watts)

+5.175 18 -12.00 3 Note 1

302 +12.00 4 Note 1 -50.00 2.5

Note 1. In the 1 70A design these outputs rise to a maximum of7 Amps when the + 5Vatld -50v rails are notfully loaded.

Table 4 Output Ratings of the Power Units

3 . 1 1 .2 Six plated copper busbars, mechanically "polarized" to ensure correct fitting, are ' provided to distribute the four power supplies and two earth connections (analogue and digital) around each of the two units. Two earth connections are provided - one for the 50v line feed return, which is intrinsically noisy ( the "analogue" earth) and one for the printed wiring boards (the "digital" earth) . The two earths are joined together in the power supply unit and only one external earth connection is required.

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4. PHYSICAL ENGINEERING

4.1 The basic and add-on units have the same external dimensions, and each consists of a single shelf of equipment within a mild steel case, the general layout being as shown in Figure 40. With the exception of the power unit, the equipment is mounted on Printed Wiring Boards (PWB) which are 200.66 mm (7.9 inches) high and 323.2 ( 12.72 inches) deep from back to front. The PWBs are connected by high density, 2 part, gold plated connectors to Printed Wiring Backplanes, two of which are provided at the rear of each shelf. Each of the units has its own power supply unit, but the Speech Switch, Signalling Circuit, Control Equipment and the Services Card in the basic unit also serve the add-on unit, when provided. Both units have sections of their shelves allocated to concentrating and non-concentrating line equipments.

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PART 1 . F IGURE 40. GENERAL LAYOUT OF CENTRAL EQU I PMENT UN ITS.

Page 65

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4_2 The detailed layout of equipment in the basic unit is shown in Figure 41 (a) _ It will be noted that in the concentrating section of the shelf on the immediate right of the power supply unit, ports 0 - 95 appear in 12 PWB positions, the first six of which are allocated to 8 port line units, the next to a drop-back unit when required, a further two to 8 port line units and the remaining three to 4 port line units_ When the unit is catering for the number of extensions and exchange lines referred to in paragraph 2 _ 1 . 1 , the last of the 8 port positions is equipped with a 4 port exchange line unit in which only 2 of the 4 line circuits are used_ Thus, in these circumstances, this section of the shelf serves up to 56 extensions and 14 exchange lines_ To the right of the Concentrating Shelf Interface, the non-concentrating section of the shelf contains two 2 port PWB positions, the first of which is allocated to inter-PBX circuits and the second to the console line circuit. To the right of the Shelf Multiplex serving these two positions are the Services Card, the combined Signalling Circuit/Speech Switch/Clock Card, and then the CPU and its associated memories_ The 128K PROM board includes some spare capacity at the present time_

4_3 The detailed layout of the equipment in the add-on unit is shown in Figure 41 (b ) _ In the concentrating section of the Shelf, on the immediate right of the power supply unit, ports 0 - 95 appear in 12 PWB positions, the first six of which are allocated to 8 port units, the next to a drop-back unit if required, the next two to 8 port units and, the remaining three to 4 port units_ Thus, this section of the shelf serves up to 64 extensions and 12 exchange lines_ To the right of the Concentrating Shelf Interface, the non-concentrating section of the shelf contains five 2 port line positions which at most locations will be used for up to 10 inter-PBX circuits (depending on local requirements and the type of signalling used) but at some locations may be used to serve up to 8 inter-PBX circuits and a second console_

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4.4 At the rear of the units, cables to the distribution frame are terminated on insulation displacement type connectors. For British Telecom applications the units will be supplied with 3 metres of cabling already connected, thus further facilitating rapid and easy installation. Within the units (and between the unit when an add-on unit is provided) ribbon cables are used to carry the digital speech and signalling highways and timing waveforms. Two connectors are provided on the CPU to give access to the peripheral equipments referred to in paragraph 3 . 1 . 1 1 . One connector, mounted on the front of the PWB provides facilities for the connection of the portable teletype, or similar, machine; and the other, mounted at the rear of the PWB functions as a combined connector, providing access for both the data logging equipment and the extraction of specialist maintenance information.

4.5 A view of the front of the two cabinets with the covers removed is given in Figure 42.

4.6 The operator's console consists of 6 main items ofhardware:-

1 The Power Unit

2 The Processor and Miscellaneous Circuits Board

3 The Slide Switch Assembly

4 The Keyboard

5 The VDU Assembly

6 The Handset

Part 1 Figure 42 Front View of the Central Equipment Units with the Covers Removed Page 67

The console is constructed of2 parts - a lower part, or base, and a detachable upper part which is connected to the base by a ribbon cable. Items 1 , 2 and3 are mounted on the base, and Items 4 and 5 on the upper part. The console remains operational when the 2 halves are separated provided the ribbon cable is not disconnected, and accordingly it can continue to be used while certain types of maintenance work are in progress. An illustration of the console in the opened position is given in Figure 43.

4.7 The 2 parts of the console shell are moulded in a polycarbonate material and are held together by 4 screws.

4.8 A handset rest is provided to the left of the keyboard and there are 2 jacks on the front of the console for the handset plug. The lefthand jack position is for normal use; the righthand jack position allows a second handset to be connected during periods of operator training. Both the case and the handset are in stone colour. Optionally, a headset may be used instead of a handset. The keyboard and VDU surround plate have an anti-reflective black finish, with white lettering to identify keys and the associated LEDs.

4.9 The 4-wire cable connecting the console to the Central Equipment Unit enters the console through the rear of the bottom half of the case. Adjacent to the cable entry point is a switch which enables the operator to initiate drop-back service for the whole PABX if, in exceptional

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circumstances, telephone service should be severely interrupted by an equipment or power failure. The switch is located where there is little chance of accidental operation.

4.10 The keyboard has a polycarbonate plate in which there are depressions for finger location above the sensitive area of each key. This keyplate protects the circuits from the ingress of dirt or harmful fluids and also carries the legends identifying each key. A printed wiring board carrying the key detection electronics is bonded directly to the underside of the keyplate. A second printed wiring board carrying a 110kHz oscillator, together with key encoding circuit connects with this assembly. The complete keyboard is 26 mm thick and includes the header connection to the console wiring form.

Page 69

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5 BUILT-IN DIAGNOSTIC CHECKS

5.1 The Central Processor Unit (CPU) is programmed to apply an extensive repertoire of "on-line" tests (carried out while the system is operational and carrying live traffic) and "off-line" tests (carried out while the system, although connected to the power supply with the CPU operational, is not in service ) . Since the on-line tests must not interfere with the normal operation of the exchange, they are more limited in scope than the off-line tests. Nevertheless they are designed to apply continuously a searching range of tests to the working equipment. Their object is to identify a faulty printed board as soon as possible after the fault develops, and to bring the existence of the fault to the notice of the maintenance staff so that the board may be quickly replaced. Off-line tests are used when commissioning an installation before it is brought into service, or if, exceptionally, an installation is in the drop-back state due to a fault condition. The off-line tests are more disruptive, and therefore more exacting than their on-line counterparts.

5 .2 Each test exercises, as far as possible the facilities of a single board. The more important tests exercise the following units of hardware:

Central Processing Unit

Clock

Memories

Signalling Circuit

Speech Switch

Tone Generator

Line Units

Some items cannot be tested in isolation and their failure can only be detected by implication. An example of such a unit is the Concentrating ShelfInterface (CSI) and, in this case, if a specified number of the tests applied sequentially to the line units on a particular shelf fail, the CSI is considered to be faulty and appropriate action taken. The overall functioning of the CPU is continuously monitored by the watchdog referred to in Paragraph 3. 1 . 1 1 . Its detailed functioning is continuously monitored by the comprehensive sets of automatic tests, which depend on the CPU for their execution and are carried out one at a time every 15 seconds. Any faults within the processor should come to light during these tests. In addition the on-line tests include automatic memory checks. PROMs are tested by counting the number of bits in each store and comparing the total with a "check sum" stored at the time the memory was first programmed. RAMs are tested by writing known patterns into store locations, reading back the contents and comparing the results with the original patterns.

5.3 Certain tests require access to the Line Unit shelves, and for this purpose a special Test Unit, mounted on the Services Card, is used. A block schematic diagram of the Test Unit is given in Figure 44. The Test Unit carries out the following types of test under the control of the CPU:

1 Speech Path Loop Back (Tone Test)

This checks the transmission performance of the Line Unit by using a 400 Hz supervisory tone, and is applied by the CPU successively to each Line Unit in the exchange (extension, exchange line and inter-PABX line) . The conditions which exist when the test is being applied are shown in Figure 45 as one example of the type of test carried out by the unit. A speech path connection is set up from the 400 Hz tone supply (Port 5 on the outgoing Speech Highway from the Services Card) to the "Receive" path of the Line U nit under test, and another connection is set up from the "Transmit" path of the Line Unit to Port 14 on the incoming Speech Highway to the Services Card, thus connecting the path to the Tone Test element in the Test Unit. Due to mis-match in the 2-wire/4-wire conversion circuit of the Line Unit, while the receiver is "on-hook", the 400 Hz tone is reflected and received by the Tone Test element which detects the signal and sends a "pass" indication to the CPU via Port 14 of the outgoing Signalling Highway from the Services Card. If the tone is not received, a "pass" indication is not sent and a fault is recorded. During the

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PATTERN G EN ERATOR

S IGNALLI NG H IGHWAY FROM SIG NALLING

PORT 19

PORT 18

PORT 17 M U LTIPLEXER

PORT 16

FROM MF RECEIVERS

I PORTS 0-7 r__----.

SIGNALLI NG

PORTS 16-19 S IGNALLI N G

S IGNALLING SELECTOR

__ .. �CI_RC_U�IT .... _

P_0_RT_1_5""�1 SIGNALLING t-___ P_OR_T_15 ___ .... � TEST SIGNALLING 2 5 6 kbit/s S IGNALLIN G

RESULT SIGNALLING PORT 14

RESULT r--��_�O:UT:..-..J SIGNALLIN G

� SIG LOOP BACK

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TO SIG NALLING- IN H IGHWAY VIA

B INDER

SPEECH H IGHWAY FROM SPEECH

SWITCH PORT 14 2048 kbit/s SPEECH TONE

PORT 15 SPEECH

SPEECH TEST

WORD

SPEECH TEST

TONE TEST

L-__ -I

SPEECH TEST WORD

... 256 kbit/s

URGENT ALARM

NON-URGENT SITED ALARM }TO REMOTELY

ALARM BOX

PORT 14 SPEECH

(OPTIONAL)

TO SPEECH H IGHWAY TO SPEECH SWITCH

VIA B INDER

PART 1. F IGURE 44. BLOCK SCHEMATIC D IAGRAM OF THE TEST UN IT AND PATTERN G ENERATOR.

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CPU

course of this test, the following hardware in the Line Unit is checked; coder, decoder, filters, 2-wire/4-wire conversion circuit.

2 Time Switch Check (Speech Test)

A digital speech path is set up from Port 14 on the outgoing Speech Highway from the Services Card to Port 15 on the incoming Speech Highway to that card. Alternative speech patterns are sent by the Speech Test element via Port 14 and detected via Port 15. The hardware tested by the test includes: the speech store (partly by this test and partly by the tone test described in ( 1 ) above) ; the serial-to-parallel and parallel-to-serial conversion circuits ; the connection store; the counters. A separate test of the Speech Switch connection store is made by writing test patterns into spare locations and reading out the patterns as a check. The locations are then set back to their NULL value on completion of the test.

3 Signalling Check

The unit has the facility of looping back, via the Signalling Test element, any 8 bit pattern of signalling bits sent to it by the CPU. The test consists of sending 2 complete signalling bytes, one being the inverse of the other so that all the paths are checked. This test checks the serial-to-parallel and parallel-to-serial conversion circuits in the Signalling Circuit. The RAM in the Signalling Circuit is checked separately by the method already described.

I-"'� SIG NALLING-OUT t----..

SIGNALLING- IN �

I ND ICATES I F ---. TON E DETECTED

SIGNALLING .I' - I, PATHS ---. , 11

POWER -+-U P

SPEECH SWITCH

D IG ITAL SPEECH

PATH +

TEST UNIT

TONE GENERATOR

D IG ITAL _ -.... SPEECH -. ( � PATHS

TONE DETECTION

TRANSMIT

2-WIRE LIN E LINE U N IT U NDER TEST RECEIVE

PART 1. F IGURE 45. EXAMPLE OF TEST.

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5.4 In addition to performing the test functions just described, the Test Unit acts as an Interface between the system and an optional Alarm Box which may be sited remotely from the CE U, and replicates the alarm indications given at the operator's console. This box, which has its own mains supply, gives urgent and non-urgent alarm indications, under the control of the CPU, which are independent of the visual display on the operator's console so that a warning is given when the console is unattended, or when, exceptionally, it has been disabled by a fault. Physical pairs are provided between the control backplane and the Alarm Box for signalling purposes. Relays in the Test Unit control the application of DC conditions over the pairs and a "fail safe" technique is used for the urgent alarm. Two LEDs are provided on the Test Unit; one lights when the unit is in use and the other is spare.

5.5 The signalling codes used between the Test Unit and the CPU are described in Part 2. When the CPU is notified that a failure has occurred, it causes all the items of common equipment involved in the test to be checked. If the common equipment is functioning correctly, the fault is recorded against the board under test; if the failure is caused by a common equipment fault, it is progressively investigated until, eventually, the faulty board is identified and details of the fault are then recorded.

5.6 The Test Unit also provides a convenient location for a Pattern Generator, which supplies complementary combinations of signalling bits (A to H), used for testing the Signalling-in portion of the Signalling Circuit during automatic fault localisation tests by the CPU. During these tests, the CPU connects the Signalling-in portion in sequence to Ports 16 to 19 of the Services Card and checks the correct receipt of the bit patterns connected to each of these ports.

5.7 Automatic checks of the console are also embodied in the system. The Random Access memory in the console is tested using the technique already described in this Section, and the performance of the signalling link between the Central Equipment Unit and the console is continuously checked using idle messages every 8 ms as described in Sub-Section 3.2 (Console Line Unit) .

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6 MAN-MACHINE INTERFACE CAPABILITIES

6.1 Reference has already been made in Section 2.7 and Appendix 2 to the ease with which many of the service facilities provided by the PABX may be changed, and the 3 levels of responsibility for making such changes. Three input devices may be used for making the changes:

a Master Telephone (MF Keyphone type ) ;

the Operator's Console;

a Teletype Terminal, either connected directly or via a modem. While this device provides easier communication with the CPU, it is not necessary to provide a teletype at each installation.

All Man-Machine Interface (MMI) commands are locked out until a special command has been inserted to unlock the appropriate access level. To use the MMI capability, the user must first insert a command called "Open System Lock" which is a 3 digit code having different values for each level of access. The "Open System Lock" code is followed by a 3 digit "Key" if access is being made by the customer, or a 4 digit "Key" if access if being made by the Administration or Operating Company responsible for maintaining the system. As soon as the validity of this combination has been checked by the CPU, a 1 .4kHz tone is returned to notify the user of a master telephone, or an "MMI READY" display is given on the VDU if the call is from a console; an equivalent indication is given if the call is from a teletype terminal. The system is now ready to receive commands. Under normal operating conditions it is expected that several changes will be made at a time, and accordingly the MMI mode will remain in operation until it is terminated. To avoid contention between more than one user trying to change the facilities at the same time, MMI operation is made available to only one level at a time and to only one terminal at a time. Attempts by other users to invoke the MMI facility encounter Busy Tone instead of the l . 4kHz tone (or equivalent in the case of consoles and teletype terminals) .

6.2 Once MMI access has been obtained the required commands are inserted. These take the form of a Facility Code, which identifies the type of change required, followed by information necessary to enable the change to be effected. A customer level Man-Machine Interface Instruction Booklet is supplied with each installation to inform the customer of the various facility codes available and the way in which they should be used. Similar information, appropriate to the level of responsibility for changes, is made available to the maintenance staff. The detailed procedures vary slightly, depending on the type of input terminal used to make the changes and these are briefly described below.

6.3 From a Master Telephone, the system is unlocked by keying:

* Open System Lock * (3 digit code)

Key ++ (3 or 4 digit code)

On receipt of "confirmation" tone an instruction is inserted. For example to enter a new Short Dialling Code the following is inserted:

* 101 * ENTRY * DIGITS ++

The Digits 101 are the Facility Code for "Insert Short Dialling Code". The ENTRY number is the Short Code (2 digits ) . The DIGITS are the number to be sent by the equipment when the short code is keyed. The command will over-write any existing short code having the same entry number.

6.4 When using the console as an MMI terminal to effect customer level changes, the following procedure may be used:

1 Operate the "MMI" key

The VDU will then display "MM I ENTER PASS KEY"

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2 Key in the Customer Level Keyword preceded by * and followed by tt: The VDU will then display "MMI READY".

The command may now be entered by keying in a string of information, similar to that used from the master telephone, as described in Paragraph 6.3. Alternatively, a "prompt" technique may be used in which the user is prompted at each stage. An example of the use of this technique to enter a new Short Dialling Code is given below: .

1 Key in the Facility Code (in this case 101) preceded by * and followed by tt

The VDU will then display "ENTRY"

2 Key in the Short Code (2 digits) followed by tt: The VDU will then display "DIGITS"

3 Key in the number to be sent by the equipment followed by tt: The VDU will then display "READY", indicating the successful entry of the command and the equipment's readiness for another command.

The MMI key cannot be used for obtaining access from the console at the service and specialist levels. Maintenance personnel may obtain such access by keying the code for "Open System Lock" (at the service or specialist level) , whereupon the console will respond in the same manner as if the "MMI" key had been operated, except that the command sub-set available will be at the appropriate level.

6.5 A teletype terminal will not normally be present at the PABX location, but may be temporarily brought on-site for use by maintenance personnel when required. The terminal is connected by means of a D type, V24, multi-way socket on the CPU card. The machine is set to full duplex, 300 baud operation. The terminal operates in a very similar way to the console except that the procedures for invoking and closing down MMI access differ and some of the characters used for keying instructions differ. As soon as the CPU detects that the teletype terminal has been connected, it sends a message requesting the "Open System Lock" command. The user sends this information followed by the appropriate Keyword for the level of access required. As soon as this has been validated, the MMI commands may be putin, using procedures similar to those already described. The teletype terminal can be used to obtain complete listings of data stores within the system, when required.

6.6 The MMI access may be terminated at any time - in the case of a master telephone by clearing the connection, in the case of the console by operating the "WITHDRA W" key and in the case of the teletype terminal by unplugging it from the CPU card.

6.7 The MMI facility can be used by authorised maintenance personnel to make both on-line and off-line diagnostic tests; to initiate "warm starts" and to perform other maintenance functions (see Appendix 2, Sections 3 and 4 ) . When an MMI initiated on-line test is being made, the automatic diagnostic checking (background testing) carried out by the CPU under normal conditions continues to operate, but if the maintenance personnel wish to stop these tests this can be done by operating a switch on the CPU card. In the event of a fault being found, details are given on the illuminated hexadecimal display on the CPU card, etc.

6.8 Off-line tests require only a working CPU for their operation. They can be run in 2 modes:

Basic - without a teletype terminal

ii Interactive - using a teletype terminal

For the basic test, the system runs a full test, starting with the control shelf and moving on to the line shelves. The progress of the tests is indicated on the hexadecimal display on the CPU card. The system stops at the first fault found and gives details on the display. In the interactive mode,

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the maintenance officer can run a variety of tests in response to commands from the teletype terminal.

6.9 On-line faults which the system detects within itself, either as a result of the in-built diagnostic test sequences, or as a result of the monitoring functions of the Watchdog and the CEU-console signalling link checks, are brought to notice by the operation of Urgent or non-Urgent visible and audible alarms on the console and on the independent Alarm Box when provided ( see paragraph 5.4) .

6 .10 On the console an LED is provided adjacent to the VDU, to signal the 2 types of alarm and an audible tone is also given. The urgent alarm indication is given by flashing the LED at a fast rate, and the non-urgent alarm by flashing at a slow rate. The audible tone is suppressed while the console operator is dealing with a call. Operation of the RECEIVE ATTENTION key on the console cancels the alarm indication and causes details of the fault to be displayed on the VDU. The operator can relay this information to the service personnel when the fault is reported. In most cases the information will be sufficient to identify hardware faults down to the printed circuit board level so that the correct replacement item can be brought to site.

6 . 1 1 The optional Alarm Box, which can be either wall mounted or free standing on a suitable surface, is either sited in the vicinity of the console or remotely in, for example, a night attendant's room. It is powered by a separately fused mains supply and the alarms are given by 2 different coloured lamps together with a distinctive sounding buzzer. When the RECEIVING ATTENTION button is pressed the buzzer is silenced and a different lamp is lit. Two test push buttons are provided to enable all the lamps and buzzers to be periodically tested. If a standby battery is provided for the CEU (see Paragraph 3 . 1 . 13) a secure power feed from this battery to the Alarm Box is provided to ensure that the Alarm Box will continue to function under mains failure conditions.

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7 INTEGRATING THE SYSTEM INTO THE ORGANISATION OF THE TELECOMMUNICATIONS ADMINISTRATION

7.1 General

7.1 .1 A new product introduced by a Telecommunications Administration (or Operating Company) must be fully integrated into the organisation of the Administration to ensure that it realises its full potential. How this is done will depend on the particular product and Administration.

7.1 .2 Most of the functions required to support a modern PABX system will already be provided by an Administration. These comprise:

selling and ordering;

assembly, testing and installation;

maintenance and repair.

All 3 must interact with the customer, as the left hand side of Figure 46 shows in a simplified form. Within the Administration these functions need to be inter-linked to ensure that the information required to do a job is available when and where it is needed. As shown in the righthand side of Figure 46, these information flows for the Monarch 120B Compact Call Connect system can be provided by MDC - the Monarch Database Compiler - which maintains computerised records of all the information required by the Administration, both initially, and throughout the life of each installation. This ensures that the different functions are fully co-ordinated, w orking towards the goal of providing the customer with the requirements he specified, as quickly and efficiently as possible, with the minimum of disruption to his normal activities.

The optimum size and distribution of organisation units depends on the existing organisation and demand for the Monarch family of systems in a territory. The structure and interactions described here illustrate the principles being adopted by British Telecom.

7.2 Selling and Ordering

7.2.1 Local sales organisations are responsible for promoting the Monarch 120B Compact system by explaining its wide ranging capabilities and advantages to potential customers. The sales organisation must establish and record the precise requirements of each customer, for new systems and modifications to existing installations. The procedure is speeded up and simplified by using a specially developed set of documentation which enables the sales staff to record, concisely and unambiguously, a customer's requirements for the system as a whole, for each individual terminal, and for each external circuit. This documentation forms the basis of the interactive input of customer requirements to MDC.

7.3 Assembly, Testing and Installation

7.3.1 Having established the customer's requirements, the Telecommunications Administration needs to convert them into a valid working system at the customer's premises as quickly as possible. The first stage in this process is the transfer of the customer's requirements from the ordering documentation to MDC by means of a plain language interaction at a computer terminal. Engineering factors need to be considered at this point and MDC will check that it has complete and consistent information . Having established this record, MDC plays the central role of co-ordinating and providing information for all stages of assembly, testing and installation.

-

7.3.2 Assembly and testing of the Monarch 120B Compact Central Equipment Unit in British Telecom is being concentrated in a small number of centralised workshops. These are collocated with important equipment stores, thus ensuring that equipment is readily available. MDC schedules the work of each workshop according to target dates requested by the local sales staff, and priorities set at a regional or national level.

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I MONARCH DATABASE CUSTOMER ADMINISTRATION I

I COMPI LER I

Selling and Customer :' Sales and I Customer specification Ordering requirements engineering staff i record

I - - - - - - - - - - - - - - -� - - - - - - - - - - - - - - -� - - - - - - - - - - -

On-site cabl.ing -

Assembly Testing and Installation

Install central unit and console

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t I Wiring instructions

t I Equipment list L

Assembly instructions prepare database

- - - - - - - - - - - - - � - - - - - - - - - - - - - - - -r - - - - - ·

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Maintenance and Repair

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Fault report

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Restore service

: I I I : Fault I

� j � recording I I I I

Maintenance I centre i

I I I

Equipment I r-- I repair I

I

Hardware and software records

- - - - -

PART 1 . FIGURE 46. INTER-ACTIVE FUNCTIONS CO-ORDINATED BY THE DATABASE COMPI LER.

7.3.3 When required, MDC will print lists of parts and assembly instructions for a particular installation. When the central equipment has been assembled, the customer database is transmitted over a data link to the workshop, where it is loaded into the daughter board memory (see Paragraph 3.8 .3) using the Automatic Memory Board Loading Equipment (AMBLE) developed by British Telecom. The completed Monarch 120B Compact equipment can be tested by supplying it with power; the diagnostic procedures built into the system will then identify any faults in the equipment. More comprehensive interactive tests are then carried out, using test programs which make use of the system off-line maintenance and diagnostic capabilities .

7 .3 .4 In parallel with the centralised assembly and testing operation, local installers can prepare the wiring at the customer's premises, and install telephones. Internal and external wiring is connected to connection boxes equipped with standard sockets for the later connection of the central equipment and console. MDC prints out instructions on the required connections to these boxes.

7.3.5 When both these operations have been completed, the central equipment and console are delivered direct to the customer's premises and connected to the internal and external circuits

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by purpose designed plug ended cables. As the equipment is fully assembled and tested, service to the customer is provided very speedily.

7.4 Maintenance and Repair

7.4.1 The maintenance of the Monarch 120B Compact Call Connect System involves 3 functions:

fault recording;

service restoration;

equipment repair.

Fault recording is handled in a centre serving the entire telecommunications service for a geographical area. Faults involving the central equipment and console will be identified by the system's diagnostic facilities. In these cases the customer should pass on to the fault recording centre the fault details displayed on the Monarch 120B Compact console. In most cases this will enable a faulty slide-in unit to be identified before the maintenance staff visit the customer.

7.4.2 Maintenance field staff are controlled and co-ordinated from the fault reporting centre. They carry a small stock of slide-in units thus enabling most faults to be repaired without a special trip to collect replacement equipment. The combination of fault diagnosis before visiting the customer and carrying a nucleus of replacement equipment, enables service to be restored to normal very rapidly.

7.4.3 Faults on units containing memory can be dealt with in the same way, since the replacement units carried by the maintenance staff are programmed with the standard software required. The only software unique to an installation - the customer's database - is held in a permanent form on the daughter board. If the mother board needs to be changed, the daughter board can be removed from the old board and plugged into the new board in the majority of cases.

7.4.4 Faulty equipment is repaired at local centres which deal with a variety of electronic equipment. These will be equipped with programmable test equipment to speed up the identification of faults. Repaired units are returned to the stores for re-use.

7.5 Consultative Assistance

7.5.1 The main features of a possible organisation to support the Monarch 120B Compact Call Connect System within a telecommunications administration have been outlined. The flexibility of the system and its modern technology are reflected in the variety of possible approaches to providing this organisational support. In particular, the various functions can be easily combined or separated depending on demand and the geographical characteristics of the territory to be served. All the information flows can be transmitted over the telephone network and co-ordinated by MDC which in turn can be centralised or locally provided as appropriate.

7.5.2 British Telecom is fully committed to the supply of the Monarch family of systems to its business customers. It has developed its organisation and evolved new approaches to enable it fully to support the systems in the British Telecommunications Network. British Telecom has a wealth of experience to draw on and through its consultative service (British Teleconsult) will be pleased to study the features and requirements of a telecommunications administration and advise on the most appropriate organisational configuration, in full co-operation with the administration concerned.

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. Page 82

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APPENDIX 1

Glossary of Technical Terms

1 General

1 . 1 This appendix defines certain terms used in Parts 1 and 2 of the manual. The definitions relate only to the meanings of the terms in the context of this manual.

2 Definitions

Accumulator

Address

Alpha-Numeric

Analogue

AND Gate

Assembly Language

Asynchronous

Background Mode

Background Program

Binary

Bipolar PROM

Bit

Byte

Buffer

Bus

Central Processing Unit (CPU) or Central Processor

Checksum

A general purpose register which receives and stores the result of an arithmetic or logical operation.

A number identifying a memory location or an item of equipment.

A combination of letters and figures.

A variable characteristic (eg electrical voltage) which closely mId continuously represents the value of a different variable characteristic (eg a sound wave).

A logic circuit with two or more inputs whose output is binary 1 if, and only if, all inputs are binary 1 .

The language in which programs specifically intended for a particular design of processor (or family of processesors) are written, and in which labels replace addresses and mnemonics replace machine code instructions. There is normally a one to one correspondence between an assembly language instruction and the machine instruction generated by it.

Not synchronised by a clock. In this mode of operation, the completion of one event initiates the start of the next.

A mode of operation in which processing can be carried out during computer time not required for real time processing.

A program that is handled in the background mode.

A numbering system having a Radix or base of2.

A high speed Programmable Read Only Memory usually programmed by internalfuses. Often used in Monarch 120B Compact as an address decode component (see "Programmable Read Only Memory").

A binary digit. It has one of two possible states, 0 or 1 .

A group of 8 bits processed and addressed collectively.

(1) A storage device used to compensate for a difference in the rate of flow of information.

(2) An electronic device which links two circuits by providing electrical matching.

A common path along which information travels from one of several sources to one or more of several destinations. The common path tlOrmally consists of a number of conductors.

That part of a computing system which includes the arithmetic unit and circuits controlling the interpretation and execution of instructions.

A value which is the arithmetic sum of all the bits in a program or program segment computed when the program is known to be valid.

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Chip

Clock

Code

Cold Start

Complementary Metal Oxide Semiconductor (CMOS)

Compandor

Comparator

Counter

Customer's Data Base

Data Base

Delta-Sigma Modulation

Diagnostics

Digital

Equipment Number (EN)

Erasable Programmable Read Only Memory (EPROM)

Firmware

Hexadecimal

High Level Language

(1) An integrated circuit. (2) A single crystalline wafer of semi-conductor material (3) A small slice of potato heated to a very high temperature in

an oily substance derived from the corpses of animals.

A time keeping, synchronising device within a system or pulse stream.

A system of symbols or bits representing data or instructions.

A complete re-start of the system in which all calls and working data stored in the system are cleared before attempting further processing.

A semiconductor device characterised by low power dissipation, moderate speed and a high packing density capability.

A device, or process, which compresses the level range at the sending end of a transmission system and expands it at the receiving end. The device improves the signal to noise ratio when noise is injected between the compressor and the expander.

A device which compares the data in a received byte of information with that contained in a particular byte stored within itself and takes action only when the two are identical.

A device, or register, which can be set to an initial value and incremented or decremented by a predetermined value.

The information defining all the system requirements which are unique to a particular customer's installation.

See Customer's Data Base.

A modulation technique in which the amplitude of all analogue signal is represented by the density of pulses in a synchronous stream of pulse positions.

Procedures to pinpoint a malfuntion in hardware or software.

In the form of coded pulses as distinct from continuous (analogue) signals. In addition, those circuits which deal with binary signals alone are called digital circuits.

The number of the equipment terminal (port) to which a line circuit is connected. This normally differs from the directory number.

A Read Only Memory that can be erased (for example by ultra-violet light) and re-programmed. Also referred to as an Electrically Programmable Read Only Memory.

Computer system programs (software) which normally remain unchanged during the working life of a computer. In Monarch 120B Compact they are usually stored on memories incorporated in the same integrated circuits as the microprocessors which they control.

A numbering system based on the Radix 16 (see Appendix 4). A computer programming language which is close to the user's language and suitable for general use, but requires compiling into Assembly Code and/or Machine Code before it can be written into the program stores of a particular design of computer.

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Highway

Hybrid Package

Integrated Circuit

Interrupt

Large Scale Integration

Latch

Man-Machine Interface

Memory

MF Keyphone

Micro-computer

Micro-processor

Multiplexer

N-Channel Metal Oxide Semiconductor Device (NMOS)

Nibble

Non-Volatile Memory

Opto-Isolator

A major route along which signals travel from one of several sources to one of several destinations. See "Bus".

A flat substrate of insulating material (usually ceramic) containing an electrical circuit. The circuit is comprised of discrete components (which may be integrated circuits) and resIstIve elements. The resistive elements and inter-connections are deposited directly on the substrate.

A wafer of semiconducter material on which have been built and internally connected together, by successive manufacturing processes, a number of diodes, transistors, etc, to form one or more multi-element circuits (see "Large Scale Integration ").

A break in the execution of a program which requires that control should pass temporarily to another routine.

High Density integrated circuits for complex logic functions. Several thousand transistors may be packed into a one tenth inch square silicon chip (see "Integrated Circuit").

A device which staticises information and retains it as long as may be necessary to enable associated functions to be performed.

The totality of methods by which the users of a system convey instructions to, and obtain information from, the system.

An electrical store of information, usually in binary coded form.

A telephone instrument on which the dial has been replaced by a set of keys (push-buttons) from which numerical digits and other symbols are transmitted in the form of different combinations offrequencies. Referred to in some networks as a "Touchtone" telephone.

A micro-processor and its program stores all contained in a single integrated circuit.

A central processor unit fabricated in Large Scale Integrated technology and capable of performing sequences of arithmetic and logic operations under the automatic control of a stored program. To turn this device into a complete functional computer, it is usually necessary to add other devices such as memories and input/output ports.

A device which receives information from a number of independent sources and transmits the information over a single communication channel.

A semiconductor device characterised by relatively high power dissipation, high speed and very high packing density.

Half a byte = 4 bits. A storage element which will retain its contents when the external power supply is removed.

A device which contains an electrically controlled light source and a photo sensitive device assembled in such a way that switching the current in the circuit containing the light source causes the photo sensitive device to perform corresponding switching functions in a different circuit.

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OR Gate

Parallel

Parallel-Serial Conversion

Parallel Transmission

PCM Switching

Peripheral Equipment

Port

Program

Programmable Peripheral Interface (PPI)

Programmable Read Only Memory (PROM)

Pulse Code Modulation (PCM)

Random Access Memory (RAM)

Read Only Memory

Reformatter

Serial

Serial Parallel Conversion

Shift Register

A logic circuit with two or more inputs whose output is binary 1 if any, or all, of the inputs are binary 1 .

Simultaneous handling of all the bits in a word.

Accepance and storage of parallel information from a number of conductors, alld re-transmission in the serial mode over a single conductor.

The transmission of more than one bit simultaneously.

A technique which uses electronic gates and stores to switch multiplexed PCM communication channels without decoding the PCM information. (See Appendix 3.)

Equipmelzt for putting information into, or gathering information out of a system, and which is not part of the computer (processor).

A point at which a line circuit or item of equipment has access to the periphery of a system.

A group of instructions plus data which define the algorithm to be executed by a computer (processor).

An integrated circuit which converts parallel information received over a data bus into signals to control hardware. Similarly, signals from the hardware are converted for transmission over the data bus. The device is programmed by the processor so that groups of pins operate as input, output or control leads.

A Read Only Memory that can be programmed by the user (in this case the Telecommunications Administration or Operating Company).

A method of transmitting analogue information (eg speech waveforms) by means of coded binary pulses (see Appendix 3).

A memory where access to the next location is independent of the address currently being accessed. The Random Access Memories can be written into and read out of at any time as dictated by the functional requirements of the system. Except where a back-up battery is provided the Random Access Memories provide volatile storage.

A memory from which information can be read out but cannot be written in during the normal operation of the system (see "Programmable Read Only Memory" and "Erasable Programmable Read Only Memory").

A device which receives a serial train of bits in bit organised form (eg the first bits for all channels followed by the second bits for all channels, and so on) and transmits the bits in byte organised form (eg bits 1 to 8 for Channel 1 ,followed by bits 1 to 8 for Channel 2 and so on) or vice versa.

A method of data transmission in which data is transferred bit by bit over a single circuit.

Acceptance and storage of serial information over a single conductor, and re-transmission in the parallel mode over a number of parallel cOllductors.

A storage device which accepts a serial train of bits and puts the bits sequentially into a succession of stores.

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Teletype Terminal

Time Division Multiplex

Uncommitted Logic Array (ULA)

Universal Asynchronous Receiver Transmitter (UART)

Volatile Memory

Warm Start

Watchdog

Word

Zener Diode

The programs used in a computer system.

Any device, or part of a device, which can retain information. A Memory.

Low Speed peripheral equipment consisting of a transmitting keyboard and printer. It may have a paper tape punch reader.

A method of combining a number of independent communication channels on a single channel in which each channel occupies a particular time slot in a continually repeating succession of time slots referred to as a ((frame". (see Appendix 3.)

A chip containing a number of uncommitted logic elements. The interconnection of these elements is specified by the user in order that the final integrated circuit performs the functions required.

An integrated circuit which converts parallel binary information received over a data bus into serial asynchronous form and vice versa. The device may insert or remove start-stop signals and parity bits, and carry out error detecting functions when required.

A storage element the contents of which are destroyed when the power supply is removed.

Termination of the software program which is running at the time the start is made and activation of the next program to be run.

A device which monitors the operation of the CPU by checking the receipt of a pulse signal every 100 ms when the CPU is functioning. Failure to receive this signal initiates a ((warm" or "cold" start depending on circumstances.

A set of bits that occupies one storage location and is treated by the computer (processor) as a unit.

A diode which has a high resistance up to a certain voltage and a low resistance beyond that voltage.

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APPENDIX 2

Levels of Responsibility for Facility Changes

1 General

1 . 1 This appendix lists the changes which are permitted to be made to the facilities provided by the Monarch 120B Compact Call Connect System. Three levels of access to MMI are allowed. The first level allows the customer access to a considerable number of commands which enable changes to be made to the user facilities. The second level allows maintenance personnel access to all the first level commands, plus additional maintenance and diagnostic commands. The third level allows specialist staff access to the lower level commands, plus specialised "debugging" commands not normally needed by maintenance personnel. Unless otherwise stated, the commands are available from the master extension, the console and a teletype.

2 Changes which the Customer is empowered to make (Level 1)

Activate Extension This returns to service an extension that has been made manually busy by the "Deactivate Extension" command.

Activate Trunk This returns a trunk ( exchange line or inter-PBX line ) to service for outgoing traffic ifit has been made manually busy by the "Deactivate Trunk" command.

Add Directory Number This adds a second directory number to an extension. An extension may not have more than two directory numbers.

Cancel Alarms This resets all alarms activated by the Test Unit.

Cancel Barring of Extension This removes either specific route restrictions for a particular extension if any category codes are provided, or all route restrictions for a particular extension if no category code is provided. The significance attached to each of the category codes is installation dependent.

Cancel Dial 9 Block This allows extensions access to the public network. The command is available from the master extension and console only.

Cancel Log of Dialled Digits This suppresses any subsequent printing of the final dialled digits on the call logging printer so that a printed list would only give the exchange codes.

Cancel Special Facility This cancels specific special facilities set up for a specified extension by the "Set Special Facility" command.

Clear Hex Display This clears the hexadecimal display on the Central Processor Unit board.

Deactive Extension This removes a specified extension from service by setting it manually busy.

Deactivate Trunk This busies a specified trunk for outgoing traffic; the incoming direction remaim unaffected.

Interchange Directory Numbers This interchanges the directory numbers of two specified extensions. No other extension attributes are affected so that the telephones will retain such properties as group membership or diversion facilities. This command is available from the console and teletype only.

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Interchange Extensions This will interchange two extension numbers, each referred to by a specified input directory number, together with their associated class of service attributes. This command is available from the console and teletype only.

Invoke Dial 9 Block This prohibits access to the public network for all extensions which do not have the "Dial 9 Block Override" Class of Service. This command is available from the master extension and console only.

List Extension Attributes This provides a list of extension related information in one of six options eg "all categories", "Diversions", "Barring", etc. This command is available from the console and teletype only.

List Group Membership This lists all members of a specified group eg "Extension Hunt", "Extension Pick-up", "Trunk" ete. This command is available from the console and teletype only.

List Meter Count This causes the contents of the call logging meters to be printed for a specified extension. However, if no directory number is provided then the contents of all meters will be listed. The meter readings requested may refer to either the running total or the last call for the extensions. This command is available from the console and teletype only.

List Route Restrictions This causes all the routes in the route restriction table to be listed, in priority order, with either "OK" or "BARRED" printed to indicate the access for the category requested. The significance attached to each category code is installation dependent. This command is available from the console and teletype only.

List Spare Directory Numbers This provides a complete list of spare directory numbers in the system. If the optional parameter is specified then all spare directory numbers in logical directory number order after the parameter are listed. This command is available from the console and teletype only.

List System Keys This lists the "System Keys" at, and below, the user's access level. They are listed in the key order; "Specialist", "Maintenance" and "Customer". This command is available from the console and teletype only.

List System Short Codes This will list out the digits for each short code entry that is in use. If a parameter corresponding to a short code entry is input then the digits for that short code only will be listed. If the parameter is omitted then all the short codes will be listed. This command is available from the console and teletype only.

List Traffic Control Data This will list the various control information which has been entered for use by the Traffic Recording Data facility. This command is available from the console and teletype only.

List Traffic Records This lists traffic recording data, either for all resources or for a particular resource. This command is available from the console and teletype only.

List Trunk Attributes This provides a comprehensive list of trunk related information for either a specified trunk, or all trunks. Typical attributes are: trunk number, trunk type, direction, the digits used to obtain access to the trunk, whether the trunk has direct dialling access to extensions, etc. This command is available from the console and teletype only.

Remove Directory Number This will remove a reference to the second directory number for a specified extension so that this directory number will become spare. The second directory number would have been previously established by the "Add Directory Number" command.

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Reset Meter Count This resets the call logging meters, either for a particular extension if a directory number is input, or for all extensions. This command is available from the console and teletype only.

Revise Call Diversion This invokes a specified form of call diversion from the first specified directory number to the second specified directory number. Typical forms of diversion are: divert all incoming calls, divert calls on no reply, divert calls on busy. This command can be used to cancel a diversion by setting the second specified directory number to the same value as the first.

Revise Directory Numbers This changes the directory number for an extension from the first specified value to the second specified value. This results in the first value becoming a spare directory number.

Revise Extension Attributes At the customer level, this is effective only on the master extension and can be used to give that extension access to MMI or to debar such access. This command is available from the console and teletype only.

Revise Group Membership This amends the directory number allocated for a specific member within either a specified hunt or pick-up extension group. The original group number will be removed and if a new directory number is specified then that number is inserted as a substitute.

Revise System Key This is used to revise the particular system key which is appropriate to the user's level of access (customer, maintenance or specialist) . This command is available from the console and teletype only.

Set Barring of Extension This sets up route restrictions for specific categories of call from a specified extension as indicated by entering a maximum of eight category codes. If no category codes are entered with the command then all categories of restriction will be set up. The significance attached to each of the eight possible category codes is installation dependent.

Set Date This sets the system calendar to the specified date.

Set Hunt Group This is used to set up a hunt group. A hunt group is a set of up to 16 extensions which may be referred to by a collective directory number in addition to the extensions' individual directory numbers. There are two hunt modes which are set as specified by the input: sequential and cyclic. In the sequential mode the directory number list is searched sequentially for a free extension; in the cyclic mode the free extension search is continued from where it left off in the directory number list.

Set Log Categories This is used to set up specific categories of calls for the purpose of call information logging. Up to eight categories can be specified; these categories are identical to those used by the "Set Barring of Extension" command, the significance attached to each category code being installation dependent.

Set Log of Dialled Digits This causes all the dialled digits to be logged.

Set Pick-up Group This allows extensions to be installed in a group to allow the Group Pick-up facility.

Set Special Facility This sets up a specific special facility for a particular extension. Typical special facilities are: dial 9 bar override, divert to operator, intrude inhibit, ete.

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Set System Short Code This enables system short dialling codes to be set. The short code (from 0 to 19) and the corresponding directory number to be transmitted are required as input parameters. Any previously allocated directory number for the input short code will be over written with the new input value.

Set Time This sets the system clock to the specified time. Note that a 24 hour clock is used.

3 Changes which normal maintenance staff are empowered to make (Level 2)

Activate Port This returns to service a port which has been made manually busy by the "Deactivate Port" command.

Activate Traffic Days of Week This is used to revise the active days of the week for traffic recording. From one to seven days can be indicated by inputting an appropriate code combination.

Add Dial Tone Detector This enables operation of the dial tone detector for the specified port.

Cancel Barring of Trunk This removes route restrictions for specific categories of call from a particular trunk which would have been established by the "Set Barring of Trunk" command. The significance attached to each category code is installation dependent.

Cancel Log This disables the call information logging system.

Clear Facility Statistics This clears all MMI and Special Facility statistical counters. These counters are used to record the level of usage of individual commands.

Clear Fault Record This clears the contents of the fault record.

Clear Software Audit Record This clears the software audit record.

Clear Test Data This clears data used for statistical tests on such items as MF4 receivers.

Clear Traffic Records This clears the traffic record for either all resources, or a particular resource depending on the instruction keyed in.

Deactive Port This manually busies a specific port.

Delete Port This effects the deletion of an item of equipment from the system database. The command allows for the membership of the port concerned in any group definition.

Execute Diagnostic Test This causes the on-line maintenance and diagnostic routine to run the test specified by the input parameter, which must be inserted as required for the specified test.

Execute Warm Start This forces the system to restart. No call diversions, etc, are lost due to a warm start.

List Fault Record This requests a list offault records, either in their entirety or of a particular type. This command is available from the console and teletype only.

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List Software Audit Record This provides a list of the last eight software faults logged. This command is available from the teletype only.

New Extension This brings into service a new extension. The system allocates the lowest spare logical number. The equipment type will be set to dial. All Class of Service options will be enabled.

New MF4 Receiver This brings into service a new MF4 receiver. In Monarch 120B Compact this command will not normally be required, since six MF4 receivers are provided as standard.

New Trunk This will bring into service a new trunk, set its trunk type to ordinary, and place it in the first mcommg group.

Revise Equipment Group This changes the members of an equipment group. It allows old equipment to be reallocated into groups and new equipment to be installed in groups. The group number and index number of the member to be changed are required as input parameters. If the port number is omitted, the specified group member will be set to "spare" .

Revise Group Answer Point This sets up a group fail answer point.

Revise Timeout This allows the values of two of the system timeouts to be changed. The timeouts are the maximum duration for un-answered ringing, and the time after which an un-answered operator controlled call will be returned to the console.

Revise Extension Attributes At the maintenance level this is used to change the type of extension signalling. Three types are available: MF only (keyphone) ; loop-disconnect (dial phone) ; both. This command is available from the console and teletype only.

Revise Trunk Attributes This changes the attributes of a trunk, eg the type (2-wire, 4-wire, signalling method, etc ) ; whether DDI is provided; the direction of usage (outgoing, incoming or bothway) ; etc.

Set Barring of Trunk This sets up route restrictions for specific categories of call from a particular trunk. The significance attached to each category code is installation dependent.

Set Equipment Group This sets up groups of equipment. Values for group number, equipment type, hunt mode and port numbers are required as input parameters.

Set Fast Testing This reduces the period between maintenance and diagnostic tests to lOOms to enable a long test to be completed quickly. The effect of this command is automatically cancelled after about 30 minutes.

Set Log This enables the call information logging system.

Set Traffic Begin Date This enters the Traffic Begin Date control parameter for use by the Traffic Recording data facility.

Set Traffic Begin Time This enters the Traffic Begin Time control parameter for use by the Traffic Recording data facility.

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Set Traffic End Date This enters the Traffic End Date control parameter for use by the Traffic Recording Data facility.

Set Traffic End Time This enters the Traffic End Time control parameter for use by the Traffic Recording data facility .

Transfer Data Base This causes the data base held in RAM to be transferred to a PROM. Tests are performed to confirm that the PROM is blank before the transfer takes place.

4 Changes which can only be made at the Specialist Level

List Memory This allows the listing of consecutive memory locations in any chosen bank. The input parameters required are: the bank number, the start address and the number oflocations. This command is available only from the teletype.

Put in Junk for Garbage Collection This command is designed as a test aid. It sets up erroneous data in various tables used in the system in order to exercise the Garbage Collection routines. This command is available only from the console and teletype.

Put in Message Queue This generates an interprocess message. Input values are required for between five and eight message bytes. This command is available only from the console and teletype.

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APPENDIX 3

Pulse Code Modulation (PCM) Transmission and Switching

Contents

1 General

2 PCM Transmission Principles

3 Characteristics of the PCM System used in the Monarch 120B Compact System

4 PCM switching principles

1 General 1 .1 This appendix describes the basic principles of PCM Transmission, defines the main characteristics of the PCM System employed in Monarch 120B Compact and indicates how PCM transmission channels may be interconnected via a digital exchange.

2 PCM Transmission Principles 2.1 The impact of speech sound waves on the diaphram of a telephone transmitter results in the generation of a varying electrical current having a complex waveform which corresponds very closely to the waveform of the sound waves, and is, therefore, an electrical analogue of those sound waves. The use of this varying current to convey the speech information is accordingly referred to as "analogue transmission". In recent years it has been increasingly realised that the conversion of analogue speech currents to coded pulses offers transmission advantages in many situations. The most commonly used application of this technique is PCM, in which the instantaneous amplitude of the analogue waveform is sampled many times a second, and on each occasion a sequence of pulses ( bits) representing a binary number is transmitted to indicate the value of this amplitude. At the receiving end, the pulses are converted to analogue signals by decoding equipment which, on receipt of each binary number, causes a voltage, having a magnitude corresponding to that number, to be connected to the analogue line circuit. The principle is illustrated in Figure 1 .

2.2 The amplitude of the analogue waveform has an infinite number of possible values, both positive and negative, and to limit the binary numbers required to transmit the instantaneous values of the waveform, the sample voltages are compared with a "quantizing" scale having a discrete number of steps arranged symmetrically about zero volts. See Figure 2. All samples falling within a particular interval between steps ( "quantum level") are allocated the same binary number. Thus, the re-constituted waveform at the analogue output of the transmission system will not in general be an exact replica of the waveform at the input to the system. The effect of this difference is equivalent to the injection of noise, and is referred to as "quantizing distortion". The greater the number of steps on the quantizing scale, the less the amplitude of the quantizing distortion, but the greater the number of bits required to indicate the mstantaneous level of each sample. Thus, the choice of the number of levels in the quantizing scale is a compromise between the number of bits required to transmit the information and the amount of quantizing distortion which can be tolerated.

2.3 The analogue waveform at the input to a PCM system must be sampled at least once during each half cycle of the highest frequency it is required to transmit. During the intervals between the transmission of the bits corresponding to successive samples on one speech circuit (channel) , bits corresponding to the samples on other channels may be interleaved, so that information relating to a number of channels may be carried over the same line conductors. This is one example of a process referred to as "Time Division Multiplexing". The principle is illustrated in Figure 3.

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TIME

ANALOGUE WAVEFORM / TO BE � TRANSMITTED

ASSUME:- Al = 8 units IB inary Number = 1 000) A2 = 6 units I Binary Number = 0 1 1 0) A3 = 3 units IBinary Number = 001 1 )

NOTE: I n this example only 4 d igit binary numbers are used. In Monarch 120B Compact 8 digit bi nary numbers are used, and hence 8 pulse positions are used to transmit each sample on the l ine.

f.��t�y� ____ t_o �+ l 1 -SAMPLING J 0 I 0 I 1 I 1 1 1 0 I 1 I 1 I 0 1 1 1 I 0 I 0 I 0 1 •

QUANTUM LEVELS 6

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4 --

3 ---

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INSTANTS f3 � f2 '-v-' fl TO SAMPLE A3 SAM PLE A2 SAMPLE Al LINE INTERVAL INTERVAL BETWEEN BETWEEN SAMPLES SAMPLES

\�------------___ � ______________ -JI V LINE SIGNALS

" 1 " represents a pulse; "0" represents absence of pulse.

PART 1, APPENDIX 3, F IGURE 1. PRINCIPLE OF PCM ENCODING

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ANALOGUE WAVEFORM

TO BE TRANSMITTED

NOTES: 1 The samples at fl and fa and fl l a l l fa l l in quantum band 0 and wi l l be transmitted by the binary N o = 0

2 The samples at f4 and f5 and fa a l l fal l i n quantum band 4 and wi l l be transmitted by the binary No = 4.

o --/ --� -I- -� \ 3 The effects referred to in Notes 1 and 2 indicate the orig in of quantizing distortion. When a large number of quantum leve ls is used in conjunction with a non- l inear

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scale, as in Monarch 120B Compact. the quantizing distortion becomes negligible.

PART 1 , APPENDIX 3, FIGURE 2. QUANTIZIN G AN ANALOGUE WAVEFORM

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I' POSITIONS

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ON THIS EXAMPLE = 8 CHANNELS IN MONARCH 120B COMPACT = 32 CHANN ELS)

PART 1, APPENDIX 3. F IGURE 3. MULTlPLEXING PCM CHANNELS.

The greater the number of bits required for each sample, the smaller the number of channels which can be accommodated at any given bit rate, and hence the compromise referred to at the end of Paragraph 2 .2 becomes a compromise between quantizing distortion and system capacity. The total number of bits corresponding to all channels in one sampling period, together with any other bits which may be transmitted during the period for signalling and lor internal control purposes, are referred to as a "frame", and the period within a frame allocated to a particular channel is referred to as a "time slot".

2.4 The linear quantizing scale used for explanatory purposes in Figure 3 is not suitable for use in a practical network, where the level of the analogue signals at the input to the PCM system may vary over a wide range, since a level of quantizing which would give an acceptable signal-to-noise ratio with high amplitude speech signals could be unacceptable with very low amplitude speech signals. To overcome this problem, a non-linear quantizing scale is used, the effect of which is to reduce the quantizing noise produced at low input speech levels at the expense of an increase in the quantizing noise produced at high input speech levels, thus ensuring an acceptable signal-to-noise ratio over the complete input level range. The non-linear scale is so arranged that the quantum levels for low amplitude input speech are small, but progressively increase until for high amplitude input speech they are relatively large. At the receiving end, where the digital signals are converted to analogue, the decoding equipment must, of course, incorporate an equal and opposite non-linear capability to ensure an accurate reconstitution of the analogue wave form. The equipment which introduces and subsequently removes this non-linearity is referred to as a "compandor".

2.5 Signalling information for call control and supervisory purposes may be transmitted by providing an additional bit in each channel time slot, the presence or absence of the bit corresponding to the presence or absence of a DC signalling condition on an analogue circuit using physical conductors. Alternatively, one channel in the PCM SysteII). may be used exclusively for signalling purposes and carry the signals relating to all the speech channels in the system. In either case, the number of signalling conditions available may be increased by assembling successive signalling bits relating to the same speech channel into groups, and allocating different meanings to different combinations of bits within the groups.

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3 Characteristics of the PCM System used in the Monarch 120B Compact System

3.1 The speech transmission characteristics of the Monarch 120B Compact system conform to agreed international standards. As in the systems recommended by CCITT, each channel is sampled 8000 times a second, and 8 bits are used to convey the value of each sample . This sampling rate ensures satisfactory speech transmission for a speech bandwidth in excess of 300-3400 Hz and the use of8 bits to indicate the value of each sample gives 28 (ie 256) different quantum levels, sufficient to reduce the quantizing distortion to a very low level indeed. The encoding technique used employs one bit to indicate whether the sampled voltage was positive or negative and the remaining 7 bits to indicate its amplitude. As in the systems recommended by CCITT, 32 channels are multiplexed to form a frame, and this results in the system architecture being based on modules of 32 channels (timeslots) .

3.2 The characteristics of the compandors used b y British Telecom in the line coding and decoding equipment conform to those of the "A LAW" compandors specified by the Conference of European Postal and Telegraph Administrations (CEPT). Alternative codecs using " f! LAW" coding are available when required.

3.3 On the non-concentrating shelf positions, signalling between the line encoding equipment and the multiplexing equipment is effected by using an additional (ninth) bit within the channel time slot on the speech paths, giving a channel bit rate of72 kbit/s (9 X 8000) in this part of the system; at the multiplexing equipment the signalling bits are extracted and passed over separate highways to the signalling equipment. On the concentrating shelf positions signalling is effected over a separate signalling bus, and the bitratein each time slot on the speech highways is 64 kbit/s (the standard CCITT rate) . Within the more central part of the system bit rates of2.048 Mbit/s (64,000 X 32) are used on the speech highways (the standard CCITT rate) and 256 kbit/sec (8,000 X 32) on the signalling highways.

4 PCM Switching Principles

4.1 A digital exchange used to interconnect channels in PCM systems must be capable of establishing a connection between any channel time slot in one system and any channel time slot in another system, or any other channel time slot in the same system. In large exchanges it is usual to provide a combination of space switching and time division switching. Typically a Space-Time-Space configuration may be used in which the space switching function, using high speed electronic gates to interconnect PCM highways, enables a connection to be made between any PCM system and a selected time switch, for a short period during each frame ( the channel time slot period) , and the time switch provides for the mutual transfer of information between any two time slots in the systems so connected to it. Alternatively, a Time-Space-Time configuration may be used, depending on the economics of the system concerned.

4.2 In the Monarch 120B Compact exchange, which is relatively small, it has been found economically advantageous to dispense with the space switching stages and use only time division switching. A description of the ways in which the time switches function is given in the main text of Part 1 .

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APPENDIX 4

The Hexadecimal Numbering System

General

1 . 1 The binary numbering system is extensively used in electronic circuit design because each column in a binary number can only have the values ° or 1 , and these two conditions are highly compatible with the states of electronic devices (toggles, gates, memory cells, etc) .However, for certain purposes, the relatively large number of digits in a binary number can be a disadvantage typically when it is required to give a simple indication of a number which may have any one of a large number of possible values. To overcome this disadvantage, the hexadecimal system is sometimes used since it is more compatible with binary techniques than the decimal system, and for any given number of columns can indicate a greater number of different values than that system. In the Monarch 120B Compact PABX it is used for a variety of purposes such as the indication of fault conditions and the identification of the units on which faults have occurred.

The Hexadecimal System

2 .1 The base of the hexadecimal numbering scheme is 16 and the significances of the col umns in a hexadecimal number are accordingly as follows:

COLUMN 4 COLUMN 3 COLUMN 2 COLUMN 1 (Four thousand (Two hundred

and ninety and fifty (sixteens) (units) sixes) sixes)

ie x 163 ie x 162 ie x 161 ie x 16°

It will be seen that each of the columns in a hexadecimal number must be capable of accommodating any o£16 possible values and these must be represented by single symbols. The symbols used are:

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F.

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Table 1 shows the relationship between the first 32 decimal numbers and their binary and hexidecimal equivalents.

DECIMAL BINARY HEXADECIMAL 0-9 0-1 O-F

0 0 0 1 1 1 2 10 2 3 1 1 3 4 100 4 5 101 5 6 1 10 6 7 1 1 1 7 8 1000 8 9 1001 9 10 1010 A 11 1011 B 12 1100 C 13 1 101 D 14 1 1 10 E 15 1 1 1 1 F 16 10000 10 17 10001 1 1 18 10010 12 19 10011 13 20 10100 14 21 10101 15 22 10110 16 23 101 1 1 1 7 24 11000 18 25 11001 19 26 1 1010 lA 27 11011 IB 28 1 1 100 lC 29 11 101 ID 30 1 1 1 10 lE 31 1 1 1 1 1 IF

and so on

Part 1, Appendix 4, Table 1 . Relationship between the first 32 decimal numbers and their binary and hexadecimal equivalents.

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2.2 The compatibility between the binary system and the hexadecimal system may be seen from the example below, which shows how a four digit hexadecimal number may be readily represented by four 4 digit binary numbers:

Hexadecimal Number 4 C 7 E (=4X4096) (=12X256) (=7X16) (=14X1)

§ ~ ~ § (=4) (=12) (=7) (=14) Binary Groups

Thus 4C7E (hex) = 19582 (decimal) = 0100110001111110 (binary).

2.3 The relationships between certain commonly used numbers in computer/microprocessor technology are shown below:

DECIMAL BINARY HEXA-DECIMAL

4096 (4K) 0001 '0000'0000'0000 1000

8192 (8K) 0010' 0000' 0000'0000 2000

16384 (16K) 0100'0000'0000'0000 4000

49152 (48K) 1100'0000'0000'0000 COOO

65536 (64K) 1 '0000'0000'0000'0000 10000

2.4 A two digit hexadecimal number is capable of identifying any of 256 (ie 16 X 16) different

values, whereas a two digit decimal number can only indicate any of 100 (ie 10 X 10) different

values and a two digit binary number any of 4 (ie 2 X 2) different values.

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