Manvel Telephone System

8/2/2019 Manvel Telephone System

1/13

INTRODUCTION

Alexander Graham Bell invented the telephone in the mid 1870s; a wired system for two way

voice communication between remote locations. You spoke into a unit at one location and you

voice was heard at the other location, immediately. This system was somewhat limited in that itonly allowed communication with one fixed location, so it was an obvious advance to have

lines going to other locations. Initially, this is what happened - each telephone had lines going

to many other telephones, which meant a lot of wires and there were practical limitations as to

the number of phones one could connect to.

Soon, the idea of central switching was developed. Each telephone connected to a central hub

(the exchange) and from there, the operator would connect your call to another subscriber.

Thus switchboards were developed where upon lifting your receiver, an operator was alerted

and you would tell her who you wished to speak to (telephone operators were almost

exclusively women in the early days). The operator would then take a wire from your socket on

her switchboard and plug you into the other persons socket. When you completed the call, youwould hang up your receiver and the operator would remove the plug from the called party's

socket.

As you might imagine, this was very labour intensive and as the popularity of the telephone

grew, the number of operators employed by the Post Office grew; large switching centres

(exchanges) could have many tens of operators, each with their own switchboard. Note that in

the early days, each telephone user was known as the 'subscriber' and that term is still used

today.


2/13

MANUAL TELEPHONE SYSTEM

The Manual Telephone System is one of the first full duplex (two way) communicationSystem, In this type of telephone Network the subscriber has a telephone set, this telephone set

consist of magnetic bell, Generator and hand Set, While the exchange consist on single board

which is known as F-36 Board, In F-36 Board type of telephone system, the subscriber cant

dial the required subscriber number directly, first he calls the operator and then the operator

connects them to the required subscriber. The F-36 Board has two ports, one is connected to the

subscriber, while the other one is connected to the exchange end, Each subscriber which is

connected to the exchange has a LED on the control penal of the exchange with a female

feeder. When a subscriber wants to call some one, first he generate a signal (voltage) through

the generator place in side the subscriber telephone set, this voltage turn ON LED placed

against each subscriber number, The operator in telephone exchange connect his hand set to the

calling subscriber and take from him the called party number, then the operator send AC

voltage towards the called subscriber, due to this voltage the called party telephone set ring up

and when he pickup the handset the operator connect his to the calling subscriber, this way a

connection establish between the subscriber.

LONG DISTANCE CALLING

If you wished to make a call to someone outside your own local exchange, say to the next

exchange, your operator would call an operator at the adjacent exchange and then ask her to

connect through to the desired subscriber. If you wanted to call someone much further away,


3/13

then the call would have to be set up with a whole chain of operators, each one calling the next.

As such, although long distance calls were possible, it was a very complicated process

involving a lot of operators.

TELEPHONE LINES

Early phone lines were single-wire - a steel or copper wire carried the signal, and it was earthed

(grounded) at both ends. This did not allow signals to be carried over long distances. It also

picked up interference from all the other electrical devices that were earthed in the

neighbourhood, like passing electric trams and other electrical motor devices. The background

electrical noise became so bad that in major cities the phone could be almost unusable.

The solution was to go to two wires in a long loop from the exchange, through the telephone,

and back to the exchange again. Unfortunately this meant twice as much wire and the city

skylines were soon black with thick masses of wire running over the rooflines. City telephone

exchanges had massive structures on their roof to support the thousands of wires needed. The

picture shows the cable frame on top of the KTAS exchange in Jorcks Passage, Copenhagen.

The wires to the customers were mounted on poles attached to roofs, chimneys, the front of

shops, and poles along streets. They were noisy in the wind, broke under the weight of snow

and ice,and were extremely ugly. The complaints from residents increased, and local Councils

and Corporations started exerting pressure on telephone companies to tidy them up and find

another way of reaching their customers. The problem was that for many years there simply

wasn't an alternative, and by now people were not going to do without a telephone. The

overhead wires were also fairly cheap to erect, which appealed to the telephone companies.


4/13

As an economy measure, party lines were introduced. This put a number of telephones across

the same wire or pair of wires. Each user had a "code ring" that corresponded to a letter in the

Morse code that identified a call to their phone. Operators had to learn these codes and crank

out the appropriate code. Since all the phones shared common wires, there was no secrecy on

party line calls. Many attempts were made to squeeze more phones onto a party line, butultimately the only answer was to provide more wires. Country areas sprouted lines of poles

carrying dozens of wires on cross arms.

Merits of the Manual Telephone Exchange

It is First full duplex Telephone System

It has a best sound quality.

Central battery System.

Easy to connect to the called party. Small & less in weight.

De-Merits of Manual Telephone Exchange

In this type of exchange there is less privacy.

24 hours Communication cant be possible. Without battery cant work.

This system is very costly.

The installation is much difficult.

AUTOMATIC TELEPHONE EXCHANGEStrowger developed a system of automatic switching using an electromechanicalcal switch

based around around electromagnets and pawls. With the help of his nephew (Walter S.

Strowger) he produced a working model in 1888 (US Patent No. 447918 10/6/1891). In this

selector, a moving wiper (with contacts on the end) moved up to and around a bank of many

other contacts, making a connection with any one of them.

Strowger did not invent the idea of automatic switching; it was first invented in 1879 by

Connolly & McTigthe but Strowger was the first to put it to effective use. Together with Joseph

B. Harris and Moses A. Meyer, Strowger formed his company 'Strowger Automatic TelephoneExchange' in October 1891.


5/13

Exchange is a central place through which one telephone can be connected to any other

telephone indirectly. A large number of telephone lines are interconnected with each other

through the exchange. An auto telephone exchange contains a number of electromechanical

switches, controlled by dialed pulses. The exchange is always located at the centre of

population.

Selector Theory

A selector starts in the 'home' position and with each 'impulse' the wiper contacts would

progress round the output bank to the next position. Each output would be connected to a

different subscriber, thus the caller could connect to any other subscriber who was connected to

that bank, without any manual assistance from an operator.

Diagram of a simple Selector

In Figure above, the selector has 10 outputs, so a caller can choose to connect to any of 10

different subscribers by dialling any digit from 1 to 0 (0=10). This sort of automatic selector is

known as a Uniselector, as it moves in just one plane (rotary).

By mounting several arcs of outlets on top of each other, the number of outlets can be increased

significantly but the wipers are then required to move both horizontally to select a bank and

then vertically to move around that bank to the required outlet. Such a selector is known as

aTwo-Motion Selector. Two-motion selectors typically have 10 rows of 10 outlets, thus 100

possible outlets altogether. A two-motion selector can therefore accept two dialled digits from a

subscriber and route the call to any of 100 numbers. The selector 'wipers' always start in their

resting 'home' position. The first digit moves the selector veritically up to the corresponding

level and then the second digit moves the wipers around the contacts of that level. This is

shown in figure 3, below.


6/13

A Two-Motion "Final" Selector

The type of selector shown above is known as a Final Selector as it takes the final two digits of

the number dialled. Most numbers dialled are several digits longer, and therefore pass through

a chain of selectors. Selectors previous to the Final Selectors are different; they are

called Group Selectors. Group selectors take only ONE digit from the caller, and step up the

number of levels according to the digit dialled. The rotary movement is then automatic; the

wipers search around that level to find a free outlet - i.e. the next free selector in the chain.

Exchange is a central place through which one telephone can be connected to any other

telephone indirectly. A large number of telephone lines are interconnected with each otherthrough the exchange. An auto telephone exchange contains a number of electromechanical

switches, controlled by dialed pulses. The exchange is always located at the centre of

population.

In the Strowger type of automatic exchange, there are two alternative methods, on the

answering side of the exchange:

1. Pre-selection

2. Line finding.

In the pre-selection type of automatic telephone exchange each subscriber's line is terminated

on the input side of the uniselector called pre-selector. The wipers of the uniselector are

connected to the subscriber line and the fixed contacts are connected to the contact arms of a

group of two-motion selectors, called first group selectors. The lsl group selector deal with the

first digit in the number dialed. The 1st group selector is connected to the 2nd group selector,

which deal with the second digit in the number. The 2nd group selector in turn is connected to

the final selector, which deal with the last two digits in the number. The final selector is

connected to the lines of the various subscribers, thus making the connection.

There are as many uni selectors as the number of subscribers. The respective fixed contacts of


7/13

all the uni selectors are connected together. So that all the subscribers have access to each of

the available 1st group selectors.

When the subscriber lifts the handset, his d.c. loop is completed and the wipers of the pre-

selector start moving to find a free 1st group selector. This is known as hunting of the

uniselector, for a free 1st group selector. When it find a free 1st group selector, it seizes it andthe subscriber hears a dial tone. This dial tone is the signal to start dialing.

When the subscriber dials the first digit, the 1st group selector's wipers step up to the desired

row of contacts, corresponding to the first digit dialed and then rotates on this row till it finds a

contact connected to a free 2nd group selector. When the second digit is dialed, the 2nd group

selector steps up to the desired row and then hunt for a contact connected to a free final

selector. The subscriber controls the vertical and rotary motion of the final selector, by dialing

the last two digits. When the second last digit is dialed, the wipers of the final selector steps up

to the row corresponding to the digit and when the last digit is dialed, the wipers of the final

selector rotates to the desired contact, without hunting. So making the connection to the wanted

four-digit number dialed.

Merits of an automatic telephone exchanges given below:

In the auto telephone exchange the subscriber himself is responsible for

making connection with other subscribers, without the help of human operator, by

dialing the number.

In this type of exchange the possibility of getting a wrong number is very much less. The quality of service is good. The service is uniform throughout the day and night.

The provision for privacy is high. As the conversation can not be overheard or

monitored without introducing additional circuits.

The traffic handling capacity in the auto exchange is high, so the connections

between subscriber are established much faster. In case the switching equipment is

inadequate or busy, busy tone is returned to the calling party, to give him the

impression that the called party is engaged.

The task of fault finding is very, much simplified due to the provision of automatic

routines.

Demerits of an auto telephone exchange are given below:

The cost of installation of an auto telephone exchange is very high.

The maintenance of an auto telephone exchange is costlier. As it requires more skilled

staff.

The switching equipment of an auto telephone exchange is more sensitive to dust.

The switching equipment of an auto telephone exchange is noisier.


8/13

Pulse Dialing

Pulse dialing originated in 1895 and is used extensively even today. In this form of

dialing, a train of pulses is used to represent a digit in the subscriber number. The

number of pulses in a train is equal to the digit value it represents except in the case ofzero which is represented by 10 pulses. Successive digits in a number are represented

by a series of pulse trains. Two successive trains are distinguished from one another by

a pause in between them, known as the interdigit gap. The pulses are generated

by alternately breaking and making the loop circuit between the subscriber and the

exchange.

three and two. The pulse rate is usually 10 pulses per second with a 10 per cent

tolerance. The interdigit gap is at least 200 ms although in some designs the minimum

gap requirement may be as much as 400-500 ms.

Rotary Dial Telephone

A rotary dial telephone uses the following for implementing pulse dialing:

Finger plate and spring; Shaft, gear and pinion wheel; Pawl and ratchet mechanism;

Impulsing cam and suppressor cam or a trigger mechanism; Impulsing contact;

Centrifugal governor and worm gear

Transmitter, receiver and bell-by-pass circuits.

The dial is operated by placing a finger in the hole appropriate to the digit to be dialled,

drawing the finger plate round in the clockwise direction to the finger stop position and

letting the dial free by withdrawing the finger. The finger plate and the associated

mechanism now return to the rest position under the influence of a spring. The dial

pulses are produced during the return travel of the finger plate, thus eliminating the

human element in pulse timings.

When the dial is in the rest position, the impulsing contacts are kept away from the

impulsing cam by the suppressor cam. When the dial is displaced from its rest position,

it is said to be in off-normal position. In this position, the impulsing contacts come nearthe impulsing cam. The rotation of the finger plate causes the rotation of the main shaft.
http://train-srv.manipalu.com/wpress/wp-content/uploads/2010/01/clip-image00278.jpg


9/13

The pawl slips over the ratchet during clockwise rotation. The ratchet, gear wheel,

pinion wheel and the governor are all stationary during the clockwise movement of the

dial. When the dial returns, the pawl engages and rotates the ratchet. The governor

helps to maintain a uniform speed of rotation. The impulsing cam which is attached to a

pinion shaft now breaks and makes the impulsing contacts which in turn cause thepulses in the circuit.

When the dial is about to reach the rest position, the suppressor cam moves the

impulsing contacts away from the impulsing cam. This action provides the required

interdigit gap timing independent of the pause that may occur between two successive

digits, due to human dialing habit.

Fig.: Rotary dial telephone parts and mechanism

Touch Tone Dial Telephone

In a rotary dial telephone, it takes about 12 seconds to dial a 7-digit number. From the

subscriber point of view, a faster dialing rate is desirable. The step-by-step switching elements

of Strowger systems cannot respond to rates higher than 10-12 pulses per second. With the

introduction of common control in crossbar systems, a higher dialing rate is feasible. It is also

of advantage as the common equipment, while not tied up for the duration of a call, is

nonetheless unavailable to respond to a new cal until it has received and processed all the digits

of an earlier call. Pulse dialing is limited to signaling between the exchange and the subscriber

and no signaling is a desirable feature and is possible only if the signaling is in the voice

frequency band so that the signaling information can be transmitted to any point in the

telephone network to which voice can be transmitted. Rotary dial signaling is limited to 10distinct signals, whereas a higher number would enhance signaling capability significantly.
http://train-srv.manipalu.com/wpress/wp-content/uploads/2010/01/clip-image00666.jpghttp://train-srv.manipalu.com/wpress/wp-content/uploads/2010/01/clip-image00474.jpg


10/13

Finally, a more convenient method of signaling than rotary dialing is preferable from the point

of view of human factors. These considerations led o the development of touch tone dial

telephones in the 1950s, which were introduced first in 1964 after field trails. They are

increasingly replacing rotary dial telephones all over the world.

Fig. The rotary dial is replaced by

a push button keyboard. Touching a button generates a tone which is a combination of two

frequencies, one form the lower band and the other from the upper band. For example, pressing

the pushbutton 9 transits 852 Hz and 1477 Hz. An extended design provides for an additional

frequency 1633 Hz in the upper band, and can produce 16 distinct signals. This design is useonly in military and other special applications. Another desing, known as decadic push button

type, uses a push button dial in place of rotary dial but gives out decadic pulses when a button

is pressed as in the case rotary dial telephone.

Principles of Crossbar Switching

The basic idea of crossbar switching is to provide a matrix ofn x m sets of contacts with only n

+ m activators or less to select one of the n x m sets of contact. This form of switching is also

known as coordinate switching as the switching contacts are arranged in a xy-plane. A

diagrammatic representation of a crosspoint switching matrix is shown in Fig. 2.9. There is an

array of horizontal and vertical wires shown by solid lines. A set of vertical and horizontalcontact points are connected to these wires. The contact points form pairs, each pair consisting

of a bank of three or four horizontal and a corresponding bank of vertical contact points. A

contact point pair acts as a crosspoint switch and remains separated or open when not is use.

The contact points are mechanically mounted (and electrically insulated) on a set of horizontal

and vertical bars shown as dotted lines. The bars, in turn, are attached to a set of

electromagnets.
http://train-srv.manipalu.com/wpress/wp-content/uploads/2010/01/clip-image02032.jpg


11/13

Fig. 2.9: 3 x 3 crossbar switching

When an electromagnet, say in the horizontal direction, is energised, the bar attached to it

slightly rotates in such a way that the contact points attached to the bar move closer to itsfacing contact points but do not actually make any contact. Now, if an electromagnet in the

vertical direction is energised, the corresponding bar rotates causing the contact points at the

intersection of the two bars to close. This happens because the contact points move towards

each other. As an example, if electromagnets M2 and M3 are energised, a contact is

established at the crosspoint 6 such that the subscriber B is connected to the subscriberC. In

order to fully understand the working of the crossbar switching, let us consider a 6 x 6 crossbar

schematic shown in Fig. 2.10. The schematic shows six subscribers with the horizontal bars

representing the inlets and the vertical bars the outlets. Now

consider the establishment of the following connections in sequence:A to CandB toE. First

the horizontal barA is energised. Then the vertical barCis energised. The crosspointACis

latched and the conversation betweenA and Ccan now proceed. Suppose we now energised thehorizontal bar ofB to establish the connectionB-E, the crosspointBCmay latch andB will be

brought into the circuit ofA-C. This is prevented by introducing and energising sequence for

latching the crosspoints.A crosspoint latches only if the horizontal bar is energised first and

then the vertical bar. (The sequence may well be that the vertical bar is energised first and then

the horizontal bar). Hence the crosspointBCwill not latch even though the vertical barCis

energised as the proper sequence is not maintained. In order to establish the connectionB-E,

the vertical barEneeds to be energised after the horizontal bar is energised. In this case, the

crosspointAEmay latch as the horizontal barA has already been energised for establishing the

connectionA-C. This should also be avoided and is done by de-energising the horizontal barA

after the crosspoint is latched and

making a suitable arrangement such that the latch is maintained even though the energisation inthe horizontal direction is withdrawn. The crosspoint remains latched as long as the vertical bar
http://train-srv.manipalu.com/wpress/wp-content/uploads/2010/01/clip-image02424.jpghttp://train-srv.manipalu.com/wpress/wp-content/uploads/2010/01/clip-image02234.jpg


12/13

Eremains energised. As the horizontal barA is de-energised immediately after the crosspoint

AC is latched, the crosspointAEdoes not latch when the vertical barEis energised.

Numbering Schemes

There were many different versions of Strowger type exchanges. The smallest ones, serving

small remote villages would handle just a few subscribers. The larger exchanges in urban

districts could handle thousands of subscribers. In a small village, there might be just 50

subscribers and so three digits would be plenty to identify them all. For example, subscribers

on a very small rural exchange might be allocated numbers in the range 200-299 - on the final

selector, level 2. Of course, numbers of just two digits would have been enough to cover 50

subscribers, but three digits were used to allow for special codes (Operator, Emergency,

Telegrams etc.) and also to separate payphones onto other 'levels'. On larger exchanges, four or

five digits were used, allowing a theoretical maximum of 10000 or 100000 subscribers. The

range was limited of course because there were no subscribers on levels '0' or '1' as they were

reserved for trunk and operator calls respectively.

The Director System

Calls made to other subscribers within the same exchange were obviously routed locally within

the exchange however many larger towns had many exchanges, each within the same area. To

connect to another exchange, its code could be dialed and the selectors would route

accordingly, but that dialing code would have to vary depending on where the call originated

from because obviously routes would vary. From the subscriber's point of view, it would beunacceptable to have to dial a different number depending on where you were. To get around

this, a set of uniform dialing codes was introduced so that a subscriber could dial the same

exchange from any other exchange always using the same dialing code. Because the actual

routing would vary depending on where the call originated from, a piece of equipment called a

Translator was introduced. This took the uniform dialing code as the subscriber dialed it and

translated it into the necessary impulse trains so that selectors could be routed accordingly.

The translator was electromechanical of course. The translator also includes 'digit absorption'

facilities so that if a subscriber dials someone on the same exchange, the exchange code is

ignored and the call routed locally. When the subscriber dials the exchange code, the translator

cannot start 'translating' until it has all three digits, then it can get to work, but in the meantime,the subscriber may dial the rest of the number. To allow for this, the translator must have Digit

Storage faculties so that it can store the rest of the digits dialed by the subscriber and repeat

them to the remote exchange once the connection has been established.

Local Phone Number Formats in India

In India, landline and mobile phones have similar, but different numbering schemes that affect

dialing for inbound international and domestic long distance calls. In fact, as compared to manycountries where landline and mobile numbers are intermingled, in India landline and mobilenumbers are set up on totally separate schemes. As a result, no landline number could ever be


13/13

assigned to a mobile phone - and vice versa. For inbound international callers (and domestic

long distance callers), all India phone numbers, both landline and mobile, are ultimately 10-

digits long - but how you get those 10-digits isn't the same. These differences affect how you

dial an inbound international call (or a domestic long distance call) - especially with respect to

whether you do or do not include an explicit area code in your dialing sequence.

Manvel Telephone System

Documents

Transcript of Manvel Telephone System