W2 SDH Technology

Digital Transmission System(DTS) SDH Technology

EETP/BSNL Silver Certification Course/Ver. 02/June2014 Page 1 of 27

For Restricted Circulation

EETP/BSNL SILVER CERTIFICATION

COURSE DIGITAL TRANSMISIION

SYSTEM

VERSION-II JUNE 2014




Contents

Sl. No. Name of Topic Page No.

1 INTRODUCTION 3

2 S.D.H. EVOLUTION 4

3 SDH RATES 7

4 THE STM-1 FRAME FORMAT 8

5 PATH OVERHEAD 11

6 NETWORK ELEMENTS OF SDH 11

7 SDH MULTIPLEXING STRUCTURE AND PRINCIPLE 14

8 WHY SYNCHRONIZING NETWORKS? 20

9 THE PRINCIPLE OF NETWORK SYNCHRONISATION 21

10 SYNCHRONIZATION HIERARCHY 22

11 SDH APPLICATION AREAS 25

12 SUMMARY 25




2 SDH TECHNOLOGY

STRUCTURE

2 SDH TECHNOLOGY

2.1 INTRODUCTION

2.2 OBJECTIVE

2.3 S.D.H. EVOLUTION

2.4 SDH RATES

2.5 THE STM-1 FRAME FORMAT

2.6 PATH OVERHEAD

2.7 NETWORK ELEMENTS OF SDH

2.8 SDH MULTIPLEXING STRUCTURE AND PRINCIPLE

2.9 WHY SYNCHRONIZING NETWORKS?

2.10 THE PRINCIPLE OF NETWORK SYNCHRONISATION

2.11 SYNCHRONIZATION HIERARCHY

2.12 SDH APPLICATION AREAS

2.13 TYPICAL OPTICAL AGGREGATE INTERFACES USED IN SDH

2.14 SUMMARY

2.15 SELF ASSESSMENT QUESTIONS

2.16 REFERENCES AND SUGGESTED FURTHER READINGS

2.1 INTRODUCTION

Synchronous Digital Hierarchy (SDH) is standardized protocols that transfer

multiple digital bit streams over optical fiber using lasers or highly coherent light

from light-emitting diodes (LEDs). At low transmission rates data can also be

transferred via an electrical interface. The method was developed to replace the

Plesiochronous Digital Hierarchy (PDH) system for transporting large amounts of

telephone calls and data traffic over the same fiber without synchronization problems.

SDH, which is essentially the same, were originally designed to transport circuit mode

communications from a variety of different sources, but they were primarily designed

to support real-time, uncompressed, circuit-switched voice encoded in PCM format.

The primary difficulty in doing this prior to SDH was that the synchronization sources




of these various circuits were different. This meant that each circuit was actually

operating at a slightly different rate and with different phase. SDH allowed for the

simultaneous transport of many different circuits of differing origin within a single

framing protocol. SDH is not itself a communications protocol, but a transport protocol.

The SDH standard was originally defined by the European

Telecommunications Standards Institute (ETSI), and is formalized as International Telecommunication Union (ITU) standards G.707, G.783, G.784, and G.803.

2.2 OBJECTIVE

After reading this unit, you should be able to understand:

SDH Evolution

Advantages of SDH network

Different Network Elements of SDH

Synchronization

2.3 S.D.H. EVOLUTION

The hierarchy defined by the ITU was adopted and referred to as the

plesiochronous digital hierarchy (PDH). The PDH signals are the 2.048 Mbit/s signal

that carries 30 voice channels, the 8.488 Mbit/s signal that multiplexes four 2.048

Mbit/s signals, the 34.368 Mbit/s signal that multiplexes four 8.488 Mbit/s signals, and the 139.264 Mbit/s signal that multiplexes four 34.368 Mbit/s signals.

Fig : 1 PDH Bit Rates

Around the time of the Bell System break-up, fiber optic cables were being

deployed in transport networks. Fiber offered huge improvements in capacity and




signal quality relative to copper cable systems, and it was often easier to find right-of-

way for the cables due to the much smaller size of the optical cables. As the

technology improved, it became feasible to begin replacing the microwave radios in

the long distance network with optical fibers. Fiber brought some new challenges, but

it also offered some critical new opportunities. The early fiber optic systems were

built on the existing PDH multiplexing approach, with each vendor typically using its

own proprietary multiplexing frame format for the higher rate signals. Hence, there

were few economies of scale and almost no cases where different vendors equipment could inter-work. This meant that at a carrier-to-carrier interface, both carriers would

have to agree to a common equipment vendor if they wanted an optical interconnection.

The desire for a standard hierarchy for fiber optic signals was one of the

primary drivers for the development of the SONET (Synchronous Optical Network)

and SDH (Synchronous Digital Hierarchy) standards. Since this was a new standard,

one of the opportunities was to define a standard that was compatible between North

America and the PDH users. The other opportunity derived from the much higher

bandwidth capabilities of the optical fiber. With optical transmission, it was now

feasible to add a considerable amount of overhead bandwidth that could be used to

greatly reduce the cost and improved the capabilities of networks OAM&P (operations, administration, maintenance, and provisioning). The combination of the

SONET/SDH OAM&P overhead capabilities and the growing availability of

computer resources has revolutionized network management and opened the

possibility of more automated control of the network. At this point in history,

SONET/SDH forms the backbone of most of the worlds transport networks with

computer-based network management systems being common. A number of potential

future directions are being explored for next generation telecommunications networks.

One future direction will be the deployment of an increasing amount of wavelength-

division multiplexing (WDM). WDM is already seeing extensive deployment, and

interestingly, is essentially a return to an FDM technology (i.e., a wavelength can be

regarded as a carrier frequency). Another direction for transport networks is an

increasing capability for efficient, flexible data transport rather than transport that is

optimized only for voice traffic. At this time, the focus has been on adding transport

capabilities. Some carriers are promoting a migration to carrying and switching all

traffic as data traffic rather than using TDM. Multi-Protocol Label Switching (MPLS)

is expected to be the core technology in these packet based networks. Voice signals

can be packetized and carried as Voice over Internet Protocol (VoIP). Another

important future direction is an increase in the ability for automated or near-real-time

control of the transport network through the introduction of a control plane on top of

the management. The control plane has the potential to allow much faster and more

flexible initiation of new services and modification of services as they are being used.

SDH differs from Plesiochronous Digital Hierarchy (PDH) in that the exact

rates that are used to transport the data on SDH are tightly synchronized across the

entire network, using atomic clocks. This synchronization system allows entire inter-

country networks to operate synchronously, greatly reducing the amount of buffering

required between elements in the network.

SDH evolution is possible because of the following factors:




(i) Fibre Optic Bandwidth: The bandwidth in Optical Fiber can be increased and there is no limit for it. This gives a great advantage for using SDH.

(ii) Technical Sophistication: Although, SDH circuitry is highly complicated, it

is possible to have such circuitry because of VLSI technique which is also very cost effective.

(iii) Intelligence: The availability of cheaper memory opens new possibilities.

(iv) Customer Service Needs: The requirement of the customer with respect to

different bandwidth requirements could be easily met without much additional equipment.

The different services it supports are:

1. Low/High speed data.

2. Voice

3. Interconnection of LAN

4. Computer links

5. Broadband ISDN transport (ATM transport)

2.3.1 ADVANTAGES OF SDH

SDH brings the following advantages to network providers:

2.3.1.1 High transmission rates

Transmission rates of up to 40 Gbit/s can be achieved in modern SDH

systems. SDH is therefore the most suitable technology for backbones, which can be

considered as being the super highways in today's telecommunications networks.

2.3.1.2 Simplified add & drop function

Compared with the older PDH system, it is much easier to extract and insert

low-bit rate channels from or into the high-speed bit streams in SDH. It is no longer necessary to demultiplex and then remultiplex the plesiochronous structure.




Fig : 2 Simplified add & drop function

2.3.1.3 High availability and capacity matching

With SDH, network providers can react quickly and easily to the requirements

of their customers. For example, leased lines can be switched in a matter of minutes.

The network provider can use standardized network elements that can be controlled

and monitored from a central location by means of a telecommunications network management (TMN) system.

2.3.1.4 Reliability

Modern SDH networks include various automatic back-up and repair

mechanisms to cope with system faults. Failure of a link or a network element does

not lead to failure of the entire network which could be a financial disaster for the

network provider. These back-up circuits are also monitored by a management system.

2.3.1.5 Future-proof platform for new services

Right now, SDH is the ideal platform for services ranging from POTS, ISDN

and mobile radio through to data communications (LAN, WAN, etc.), and it is able to

handle the very latest services, such as video on demand and digital video

broadcasting via ATM that are gradually becoming established.

2.3.1.6 Interconnection

SDH makes it much easier to set up gateways between different network

providers and to SONET systems. The SDH interfaces are globally standardized,

making it possible to combine network elements from different manufacturers into a network. The result is a reduction in equipment costs as compared with PDH.

SDH supports the transmission of existing PDH payloads, other than 8Mbit/s.

Most importantly, because each type of payload is transmitted in containers

synchronous with the STM-1 frame, selected payloads may be inserted or extracted

from the STM-1 or STM-N aggregate without the need to fully hierarchically de-

multiplex as with PDH systems.

2.4 SDH RATES

SDH is a transport hierarchy based on multiples of 155.52 Mbit/s. The basic

unit of SDH is STM-1. Different SDH rates are given below:

STM-1 = 155.52 Mbit/s

STM-4 = 622.08 Mbit/s

STM-16 = 2588.32 Mbit/s

STM-64 = 9953.28 Mbit/s




Each rate is an exact multiple of the lower rate therefore the hierarchy is synchronous.

2.5 THE STM-1 FRAME FORMAT

The S.D.H. standards exploit one common characteristic of all PDH networks

namely 125 micro seconds duration, i.e. sampling rate of audio signals (time for 1

byte in 64 k bit per second). This is the time for one frame of SDH. The frame

structure of the SDH is represented using matrix of rows in byte units as shown. As

the speed increases, the number of bits increases and the single line is insufficient to

show the information on Frame structure. Therefore, this representation method is

adopted. How the bits are transmitted on the line is indicated on the top of the figure.

The Frame structure contains 9 rows and number of columns depending upon

synchronous transfer mode level (STM). In STM-1, there are 9 rows and 270 columns. The reason for 9 rows arranged in every 125 micro seconds is as follows:

For 1.544 Mbit PDH signal (North America and Japan Standard), there are 25

bytes in 125 micro second and for 2.048 Mbit per second signal, there are 32 bytes in

125 micro second. Taking some additional bytes for supervisory purposes, 27 bytes

can be allotted for holding 1.544 Mbit per second signal, i.e. 9 rows x 3 columns.

Similarly, for 2.048 Mbit per second signal, 36 bytes are allotted in 125 micro

seconds, i.e. 9 rows x 4 columns. Therefore, it could be said 9 rows are matched to

both hierarchies.

The standardized SDH transmission frames, called Synchronous Transport

Modules of Nth hierarchical level (STM-N). The STM-1 frame is the basic

transmission format for SDH. The frame lasts for 125 microseconds; therefore, there are 8000 frames per second.

A frame with a bit rate of 155.52 Mbit/s is defined in ITU-T Recommendation

G.707. This frame is called the synchronous transport module (STM). Since the frame

is the first level of the synchronous digital hierarchy, it is known as STM-1. Figure

shows the format of this frame. It is made up from a byte matrix of 9 rows and 270

columns. Transmission is row by row, starting with the byte in the upper left corner

and ending with the byte in the lower right corner. The frame repetition rate is 125

ms., each byte in the payload represents a 64 kbit/s channel. The STM-1 frame is capable of transporting any PDH tributary signal.




Fig : 3 Schematic diagram of STM-1 frame

The first 9 bytes in each of the 9 rows are called the overhead. G.707 makes a

distinction between the regenerator section overhead (RSOH) and the multiplex

section overhead (MSOH). The reason for this is to be able to couple the functions of

certain overhead bytes to the network architecture. The table below describes the

individual functions of the bytes.

Calculation of Bit Rate of STM-1

NO OF ROWS IN FRAME: 9

NO OF COLUMNS: 270

NO OF BYTES IN FRAME: 270*9

NO OF BITS IN A FRAME: 270*9*8

FRAME DURATION: 125us

NO OF BITS TRANSMITTED IN ONE SECOND: 270*9*8*1/125 s

=155.520Mb/S

2.5.1 SECTION OVERHEAD (SOH) AREA

The first 9 bytes in each of the 9 rows are called the overhead. SOH means the

additional bytes in the STM-N frame structure needed for normal and flexible

transmission of information payload and these bytes are mainly used for the running,

management and maintenance of the network. In the 1~ 9 N columns of the SDH

frame, 1~3 rows and 5~9 rows are allocated to the SOH. SOH can be further




categorized as RSOH (Regenerator Section Overhead) and MSOH (Multiplex Section

Overhead). 1~3 rows are allocated to RSOH and 5~9 rows to MSOH. RSOH can be

accessed either at the regenerator to at the terminal equipment. However, MSOH

passes a regenerator transparently and is terminated at the terminal equipment. Fig. 4 shows distinction between the regenerator section overhead (RSOH) and the

multiplex section overhead (MSOH).

Fig : 4 Section Overhead

The table below describes the individual functions of the bytes.

Table 1: Overhead bytes and their functions

2.5.2 PAYLOAD AREA

Information payload area is the place where information about various

services is stored in the SDH frame structure. Horizontal columns 10 N~270 N,

and vertical rows 1~9 belong to the information payload area. In it, there are still

some Path Overhead (POH) bytes transmitted as part of the payload in a network and

these bytes are mainly used for the monitor, management and control of the path

performance.




2.5.3 ADMINISTRATIVE UNIT POINTER (AU-PTR) AREA

AU PTR is a kind of indicator, mainly used to indicate the accurate position of

the first byte of information payload in the STM-N frame, so that the information can

be correctly decomposed at the receiving end. It is located at the fourth row of 1~9

N columns in the STM-N frame structure. The adoption of the pointer mode is an

innovation of SDH. It can perform multiplex synchronization and STM-N signal frame locating in the quasi-synchronization environment.

2.6 PATH OVERHEAD

Path Overhead (POH) bytes are mainly used for the monitor, management and

control of the path performance. A distinction is made between two different POH

types:

2.6.1 VC-11/12 POH

The VC-11/12 POH is used for the low-order path. ATM signals and bit rates

of 1.544 Mbit/s and 2.048 Mbit/s are transported within this path.

2.6.2 VC-3/4 POH

The VC-3/4 POH is the high-order path overhead. This path is for transporting

140 Mbit/s, 34 Mbit/s and ATM signals.

2.7 NETWORK ELEMENTS OF SDH

Figure 5 is a schematic diagram of a SDH ring structure with various

tributaries. The mixture of different applications is typical of the data transported by

SDH. Synchronous networks must be able to transmit plesiochronous signals and at

the same time be capable of handling future services such as ATM.




Fig : 5 Schematic diagram of hybrid communications networks

Current SDH networks are basically made up from four different types of network

element. The topology (i.e. ring or mesh structure) is governed by the requirements of the network provider.

2.7.1 TERMINAL MULTIPLEXER ( TM)

Terminal multiplexers are used to combine plesiochronous and synchronous

input signals into higher bit rate STMN signals as shown in Fig. 6 below. On the tributary side, all current plesiochronous bit rates can be accommodated. On the

aggregate, or line side we have higher bit rate STMN signals. Terminal multiplexers are used to combine plesiochronous and synchronous input signals into higher bit rate STM-N signals.

Fig : 6 TM

2.7.2 ADD/DROP MULTIPLEXERS(ADM)

Add/drop multiplexers (ADM) permits add and drop of lower order signals.

Lower bit rate synchronous signals can be extracted from or inserted into high speed

SDH bit streams by means of ADMs. This feature makes it possible to set up ring




structures, which have the advantage that automatic back-up path switching is possible using elements in the ring in the event of a fault.

Fig : 7 ADM

2.7.3 REGENERATORS

Regenerators as the name implies, have the job of regenerating the clock and

amplitude relationships of the incoming data signals that have been attenuated and

distorted by dispersion. They derive their clock signals from the incoming data

stream. Messages are received by extracting various 64 kbit/s channels (e.g. service

channels E1, F1) in the RSOH (regenerator section overhead). Messages can also be

output using these channels.

Fig : 8 Regenerator

2.7.4 DIGITAL CROSS-CONNECT (DXC)

This network element has the widest range of functions. It allows mapping of

PDH tributary signals into virtual containers as well as switching of various

containers up to and including VC-4. It permits switching of Transmission lines with different bit rates.

Fig : 9 DXC




2.7.5 NETWORK ELEMENT MANAGER

Telecommunications management network (TMN) is considered as a further

element in the synchronous network. All the SDH network elements mentioned so far

are software-controlled. This means that they can be monitored and remotely controlled, one of the most important features of SDH.

Fig : 10 Network Element Manager

2.8 SDH MULTIPLEXING STRUCTURE AND PRINCIPLE

The multiplexing principles of SDH follow, using these terms and definitions:

Mapping A process used when tributaries are adapted into Virtual Containers (VCs) by adding justification bits and Path Overhead (POH) information.

Aligning This process takes place when a pointer is included in a Tributary Unit

(TU) or an Administrative Unit (AU), to allow the first byte of the Virtual Container to be located.

Multiplexing This process is used when multiple lower-order path layer signals are

adapted into a higher-order path signal, or when the higher-order path signals are adapted into a Multiplex Section.

Stuffing As the tributary signals are multiplexed and aligned, some spare capacity

has been designed into the SDH frame to provide enough space for all the various

tributary rates. Therefore, at certain points in the multiplexing hierarchy, this space

capacity is filled with fixed stuffing bits that carry no information, but are required to fill up the particular frame.

Figure 11 illustrates the ITU-T SDH multiplexing structure defined in Rec. G.707




Fig : 11 Generic multiplexing structure

At the lowest level, containers (C) are input to virtual containers (VC). The

purpose of this function is to create a uniform VC payload by using bit-stuffing to

bring all inputs to a common bit-rate ready for synchronous multiplexing. Various

containers (ranging from VC-12 to VC-4) are covered by the SDH hierarchy. Next,

VCs are aligned into tributary units (TUs), where pointer processing operations are

implemented. These initial functions allow the payload to be multiplexed into TU

groups (TUGs). As Figure illustrates, the xN label indicates the multiplexing integer

used to multiplex the TUs to the TUGs. The next step is the multiplexing of the TUGs

to higher level VCs, i.e. VC-4. These VCs are multiplexed with fixed byte-stuffing to

form administration units (AUs) which are finally multiplexed into the AU group

(AUG). This payload then is multiplexed into the Synchronous Transport Module

(STM).




Fig : 12 Reduced SDH multiplexing structure

The KLM value represents the position of 63 VC12s into a VC4. K: is TUG-3 (1,2

or 3), L: is the TUG-2 (1,2,3,4,5,6,7) and M is the TU12 (1,2 or 3).

Table-1 Position of VC-12 in VC$

Timeslot TUG-3 TUG-2 VC-12 (> E1) KLM

1 1 1 1 1.1.1

2 1 1 2 1.1.2

3 1 1 3 1.1.3

4 1 2 1 1.2.1

5 1 2 2 1.2.2

6 1 2 3 1.2.3

7 1 3 1 1.3.1

8 1 3 2 1.3.2

9 1 3 3 1.3.3

10 1 4 1 1.4.1

11 1 4 2 1.4.2

12 1 4 3 1.4.3




13 1 5 1 1.5.1

14 1 5 2 1.5.2

15 1 5 3 1.5.3

16 1 6 1 1.6.1

17 1 6 2 1.6.2

18 1 6 3 1.6.3

19 1 7 1 1.7.1

20 1 7 2 1.7.2

21 1 7 3 1.7.3

22 2 1 1 2.1.1

23 2 1 2 2.1.2

24 2 1 3 2.1.3

25 2 2 1 2.2.1

26 2 2 2 2.2.2

27 2 2 3 2.2.3

28 2 3 1 2.3.1

29 2 3 2 2.3.2

30 2 3 3 2.3.3

31 2 4 1 2.4.1

32 2 4 2 2.4.2

33 2 4 3 2.4.3

34 2 5 1 2.5.1

35 2 5 2 2.5.2

36 2 5 3 2.5.3

37 2 6 1 2.6.1

38 2 6 2 2.6.2

39 2 6 3 2.6.3

40 2 7 1 2.7.1

41 2 7 2 2.7.2

42 2 7 3 2.7.3

43 3 1 1 3.1.1

44 3 1 2 3.1.2

45 3 1 3 3.1.3

46 3 2 1 3.2.1




47 3 2 2 3.2.2

48 3 2 3 3.2.3

49 3 3 1 3.3.1

50 3 3 2 3.3.2

51 3 3 3 3.3.3

52 3 4 1 3.4.1

53 3 4 2 3.4.2

54 3 4 3 3.4.3

55 3 5 1 3.5.1

56 3 5 2 3.5.2

57 3 5 3 3.5.3

58 3 6 1 3.6.1

59 3 6 2 3.6.2

60 3 6 3 3.6.3

61 3 7 1 3.7.1

62 3 7 2 3.7.2

63 3 7 3 3.7.3

Table-2: SDH/SONET transmission rates

PDH

(USA)

PDH

(Europe) SDH SONET

Bit

Rate

(Mbps)

Name Name Container Transport Container Transport

40000 STM-256 STS/OC-768

10000 STM-64 STS/OC-192

2500 STM-16 STS/OC-48

622 STM-4 STS/OC-12

155 STM-1 STS/OC-3

140 E4 VC4

51 STS/OC-1

45 DS-

3/T3 STS-1 SPE

34 E3 VC3

8 E2




6 DS-

2/T2

VT6 (not

really used)

2 E1 VC12

1.5 DS-

1/T1 VT1.5

2.8.1 BASIC DEFINITIONS

(i) Synchronous Transport Module

This is the information structure used to support information pay load and over

head information field organized in a block frame structure which repeats every 125

micro seconds.

(ii) Container

The first entry point of the PDH signal is the container in which the signal is

prepared so that it can enter into the next stage, i.e. virtual container. In container

(container-12) the signal speed is increased from 32 bytes to 34 bytes in the case of

2.048 Mbit/s signal. The additional bytes added are fixed stuff bytes (R), Justification Control Bytes (CC and C), Justification Opportunity bytes (s).

In container-3, 34.368 Mbit/s signal (i.e., 534 bytes in 125 microseconds) is

increased to 756 bytes in 125 microseconds adding fixed stuff bits(R). Justification

control bits (C-1, C-2) and Justification opportunity bits (S-1, S-2).

In container-4, 139.264 Mbit/s signal (2176 bytes in 125 microseconds) is

increased to 9 x 260 bytes.

(iii) Virtual Container

In Virtual container the path over head (POH) fields are organized in a block

frame structure either 125 microseconds or 500 microseconds. The POH information

consists of only 1 byte in VC-12 for 125 microseconds frame. In VC-3, POH is 1

column of 9 bytes. In VC-4 also POH 1 column of 9 bytes. The types of virtual

container identified are lower orders VCs i.e. VC-12 and VC-2 and higher order VC-3

and VC-4.

(iv) Tributary Unit

A tributary unit is a information structure which provides adaptation, between

the lower order path layer and the higher order path layer. It consists of a information

pay load (lower order virtual container) and a tributary unit pointer which indicates

the offset of the pay load frame start relating to the higher order VC frame start.

Tributary unit-12 for VC-12 and Tributary Unit-2(TU-12) is for VC-2 and Tributary

unit-3(TU-3) is for VC-3, when it is mapped for VC-4 through tributary group-3.




(v) Tributary Unit Group

One or more tributaries are contained in tributary unit group. A TUG-2

consists of homogenous assembly of identical TU-12s or TU-2. TUG-3 consists of a

homogenous assembly of TUG-2s or TU-3. TUG-2 consists of 3 TU-12s (For 2.048 Mbit/sec). TUG-3 consists of either 7 TUG-2 or one TU-3.

(vi) Network Node Interface (NNI)

NNI is interface at a network node which is used to interconnect with another

network node.

(vii) Pointer

An indicator whose value defines frame offset of a VC with respect to the

frame reference of transport entity, on which it is supported.

(viii) Administrative Unit

It is the information structure which provides adaptation between the higher

order path layer and the multiplex section layer. It consists of information pay load

and a A.U. pointer which indicates the offset of the pay load frame start relating to the

multiplex section frame start. Two AUs are defined (i) AU-4 consisting VC-4 plus an

A.U. pointer indicating phase alignment of VC-4 with respect to STM-N frame, (ii)

AU-3 consisting of VC-3 plus AU pointer indicating phase alignment of VC-3 with respect to STM-N frame. A.U. location is fixed with respect to STM-N frame.

(ix) Administrative Group

AUG consists of a homogenous assembly of AU-3s or an AU-4.

(x) Concatenation

The procedure with which the multiple virtual container are associated with one

another, with the result their combined capacity could be used as a single container

across which bit sequence integrity is maintained.

2.9 WHY SYNCHRONIZING NETWORKS?

New digital technologies and value-added time sensitive services like real-time

Video on Demand, high speed Internet access, Videoconferencing, Bank to Bank

encrypted data exchange, multimedia applications, are based on reliable network

architectures (Internet, GSM, ATM, SDH, xDSL and many other network

technologies).

All those architectures and services underlie one basic principle: networks

must be synchronized.

Moreover, dramatic subscriber growth and consumer demand are driving

Telecommunication operators to place emphasis on quality, reliability and breadth of

services. Therefore, it is imperative that they address the serious timing and solve




synchronization problems that may degrade service quality. When timing

synchronization is off, quality issues range from distorted, unreadable faxes and

corrupted or lost data to frozen images on videoconferencing systems, requiring

retransmission as shown in Table-1.

Table-1: Loss of synchronization consequences on services

Service Consequences

Voice Traffic (PSTN) Clicks can be heard during conversation

Fax Transmission Loss of part or all of transmitted lines

Data Transfer on PSTN Data corruption

Video Conferencing Loss of image(s). Image at receiver frozen

Coded Data Loss of message (if coding key is lost)

SDH Pointer movements and faults at SDH/ PDH junction

This table clearly shows that the more services have a high value added, the

more they require to be supported by a very reliable synchronization function.

2.10 THE PRINCIPLE OF NETWORK SYNCHRONISATION

Synchronization is the means used in digital transmissions in order to ensure

that all network elements (NE) operate at the same frequency.

When a message has to be transmitted between two points (i.e. two cities), the

internal clock in the sender will control the frequency of information sent from this

node (f1). A second clock located on the receiver node will control the frequency at

which received data is read (f2). The basic principle of synchronization is that the two

frequencies must be exactly the same in order to allow the receiver to interpret the digital signal properly.

Fig : 13 Information Sending/Receiving: Place of Synchronization Problem




Should f1>f2, the sender will transmit information faster than the receiver can

understand it, this will "miss" information ("slips of deletion").

Should f2>f1, the sender is slower than the receiver; this will duplicate a part of

received information ("slips of repetition).

2.11 SYNCHRONIZATION HIERARCHY

Digital switches and digital cross-connect systems are commonly employed in

the digital network synchronization hierarchy. The network is organized with a master

slave relationship with clocks of the higher-level nodes feeding timing signals to clocks

of the lower-level nodes. All nodes can be traced up to a Primary Reference Clock

(PRC).

2.11.1 SYNCHRONIZING SDH

The internal clock of an SDH terminal may derive its timing signal from a

Synchronization Supply Unit (SSU) used by switching systems and other equipment.

Thus, this terminal can serve as a master for other SDH nodes, providing timing on its

outgoing STM-N signal. Other SDH nodes will operate in a slave mode with their

internal clocks timed by the incoming STM-N signal. Present standards specify that an

SDH network must ultimately be able to derive its timing from a PRC.

2.11.2 PLANNING OF SYNCHRONIZATION NETWORKS

The planning of synchronization in the networks of the operators is fully

independent to each other with the exception of one operator providing another operator

with synchronization Signals. The architecture of the synchronization network of an

operator shall be in accordance with the ETSI standard. The clock reference signals are

distributed between levels of the hierarchy via distribution trails offered by normal SDH

or PDH transmission systems. No special transport network for the distribution of

synchronizing signals is used. It shall be noted that a 2.048 Mbit/s signal crossing SDH network shall not be used for timing distribution in the synchronization network.

To avoid this worst case scenario, all network elements are synchronized to a

central clock. This central clock is generated by a high-precision primary reference

clock (PRC) unit conforming to ITU-T Recommendation G.811. This specifies an

accuracy of 1X10-11

. This clock signal must be distributed throughout the entire

network. A hierarchical structure is used for this; the signal is passed on by the

subordinate synchronization supply units (SSU) and synchronous equipment clocks

(SEC). The synchronization signal paths can be the same as those used for SDH communications.

The clock signal is regenerated in the SSUs and SECs with the aid of phase-

locked loops. If the clock supply fails, the affected network element switches over to a

clock source with the same or lower quality, or if this is not possible, it switches to

hold-over mode. In this situation, the clock signal is kept relatively accurate by

controlling the oscillator by applying the stored frequency correction values for the

previous hours and taking the temperature of the oscillator into account. Clock islands

must be avoided at all costs, as these would drift out of synchronization with the




passage of time and the total failure disaster would be the result. Such islands are

prevented by signaling the network elements with the aid of synchronization status

messages (SSM, part of the S1 byte). The SSM informs the neighboring network

element about the status of the clock supply and is part of the overhead. Special

problems arise at gateways between networks with independent clock supplies. SDH

network elements can compensate for clock offsets within certain limits by means of

pointer operations. Pointer activity is thus a reliable indicator of problems with the clock supply.

Fig : 14 Clock supply hierarchy structure

The topology of the hierarchical synchronization network is tree-like as shown

in figure 14. The synchronization architecture requires that the timing of all network

element clocks are traceable to a PRC and hence the principal structure is the

synchronization network reference chain as shown in figure 15. Timing is distributed

via master-slave synchronization from the PRC to all clocks in the chain. To ensure the

correct operation of the synchronization network it is important that clocks of lower

hierarchical level only accept timing from clocks of the same or higher hierarchical

level and that timing loops are avoided. The distribution network shall be designed so

that the requirements for the hierarchical network reference chain (described below) will be met even under fault conditions.

In general, the quality of timing will deteriorate as the number of synchronized

clocks in tandem increases and hence for practical synchronization network design, the

number of network elements in tandem should be minimized. Based on theoretical

calculations it is recommended that the longest chain should not exceed 10 SSUs and 20

SECs interconnecting any SSUs with restriction that the total number of SECs is limited

to 60 (refer to figure 15). It is preferable that all SSUs and SECs are able to recover

timing from at least two synchronization trails. The slave clock shall reconfigure to

recover timing from an alternative trail if the original trail fails. Where possible

synchronization trails should be provided over diversely routed paths. In the event of a

failure of synchronization distribution, all network elements will seek to recover timing

from the highest hierarchical level clock source available. To effect this, both SSUs and

SECs may have to reconfigure and recover timing from one of their alternate synchronization trails.




Fig : 15 Synchronization network reference chain

SSM and squelching may be used on SDH trails for correct reference transfer

between the SSUs. The use of SSM also makes it possible to recover timing for the

SEC clocks in the chain from the opposite direction if the signal in the original direction

fails.

A general procedure in planning the synchronization network may be as follows.

If the synchronous method is used:

Find out the connections to the national PRC-system Plan the locations for SSUs

Plan the synchronization trails

If the pseudo-synchronous method is used:

Plan the PRC system Plan the locations for SSUs Plan the synchronization trails

When planning the placing for SSUs the importance of the node locations for the

traffic networks to be synchronized and the synchronization network itself is

considered. The maximum number of SEC clocks between two SSUs has also to be

taken into account. When planning the synchronization trails first the transmission




systems for the transfer of synchronization are selected. Secondly the timing configuration of the selected systems is planned in detail.

2.12 SDH APPLICATION AREAS

SDH systems are used in almost all areas of telecommunication network.

Some of the applications areas are given below.

Access Network

Aggregation Network

Metro Network

Long distance National as well as International

Wireless Backhauling

SCADA (Supervisory Control and Data Acquisition)

Fig : 16 A Typical application of SDH

2.13 TYPICAL OPTICAL AGGREGATE INTERFACES USED IN SDH

Sl.

No.

Type of Interface Configuration Bit Rate Operating

Wavelength

1 S 1.1 Short Haul STM-1 1310 nm

2 L 1.1 Long haul STM-1 1310 nm



5 100Base FX Long haul FE 1310 nm

6 1000Base-LH Long haul GE 1550 nm




2.14 SUMMARY

SDH (Synchronous Digital Hierarchy) is an international standard for high

speed telecommunication over optical/electrical networks which can transport digital

signals in variable capacities. It is a synchronous system which intends to provide a

more flexible, yet simple network infrastructure. SDH (and its American variant-

SONET) emerged from standard bodies somewhere around 1990. These two

standards create a revolution in the communication networks based on optical fibers,

in their cost and performance.

Traditionally, transmission systems have been asynchronous, with each

terminal in the network running on its own recovered clock timing. In digital

transmission, timing is one of the most fundamental operations. Since these clocks are not synchronized, large variations can occur in the clock rate and thus the signal

bit rate. For example, an E3 signal specified at 34 Mbit/s 20 ppm (parts per million)

can produce a timing difference of up to 1789 bit/s between one incoming E3 signal

and another.

In a synchronous system, such as SDH, the average frequency of all clocks in

the system is the same. Every slave clock can be traced back to a highly stable

reference clock. Thus, the STM-1 rate remains at a nominal 155.52 Mbit/s, allowing

many synchronous STM-1 signals to be multiplexed without any bit-stuffing. Thus,

the STM-1s are easily accessed at a higher STM-N rate.

2.15 SELF ASSESSMENT QUESTIONS

1. can be defined as the transfer of information from one point to another point

2. The . techniques are the process of translating individual speech circuits (300-3400 Hz) into pre-assigned frequency slots

3. Before SDH transmission networks were based on the .. 4. Filters are used to limit the speech signal to the frequency band .. Hz 5. SDH has provided transmission networks with a vendor-.

6. SDH supports the transmission of existing PDH payloads, other than ... Mbit/s 7. The VC-3/4 POH is the .. path overhead 8. SDH is a transport hierarchy based on multiples of . Mbit/s.

9. .is an important task to get excellent performance and and quality of service for the subscribers.

10. .. is the means used in digital transmissions in order to ensure that all

network elements (NE) operate at the same frequency.

11. The internal clock of an SDH terminal may derive its timing signal from a 12. Primary reference clock (PRC) specifies an accuracy of.

13. Based on theoretical calculations it is recommended that the longest chain should not exceed . SSUs and 20 SECs interconnecting any SSUs with restriction that the total number of SECs is limited to 60.

14. . is transferred by the 5th~8th bits of S1 byte in an SDH multiplex section overhead.

Answer: Communication, FDM, PDH hierarchy, 300-3400, independent, 8, high-

order, 155.52, Network synchronization, Synchronization, Synchronization Supply

Unit (SSU), 1X10-11,

10, SSM




2.16 REFERENCES AND SUGGESTED FURTHER READINGS

www.calyptech.com/

www.tek.com/Measurement/App_Notes/sdhprimer/intro.pdf

www.exfo.com/en/Applications/SONET-Overview.aspx

www.telecombasics.net/

www.cisco.com

www.telecom-sync.com/pdf/2005/jl_ferrant_sync_sdh_networks.pdf

www.home.agilent.com/agilent/redirector.jspx?ckey

home.dei.polimi.it/bregni/papers/SDH_course.pdf

www.tektronix.com/optical

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