SDH Telecommunications)(97

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A

REPOPRT

ON

PRACTICAL TRAINING

AT

VODAFONE ESSAR DIGILINK LIMITED

SESSION: 2010-2011

Submitted By:

VINAY SHARMA

( VII semester)

Department of Electronics & communication

Engineering

Maharishi Arvind Institute Of Engineering

& Technology, Jaipur

(AFFILIATED TO RAJASTHAN TECHNICAL UNIVERSITY,

KOTA)


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Acknowledgements

No report is created by an individual. Many people

have helped to create this report and each of their

contribution has been valuable.

I have no words to express my gratitude towards

the Vodafone employees namely Mr. Ajay Mishra ,

Mr. Gurbindra, Mr Avnish and Mr. Roy who were

the key behind the making of this report as well

as helped me throughout my training period at the

Vodafone office.

Finally, I would like to thanks my college professor

Mr. Pushpendra for his guidance to create this

report.

I do not claim the originality of the subject matter

presented in this report and help has been taken

from various text-books and internet, so I m also

thankful to those authors and the one who

provided the matter over the internet.

Vinay Sharma

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Contents

Introduction....................................................... 4

Overview........................................................... 6

Plesiochronous Transmission.................................8

SDH concept and Principle.................................. 11

SDH evolution...................................................13

SDH standards................................................. 14

Basic Definitions............................................... 16

Multiplexing Principle......................................... 20

Section overhead brief description.......................23

Network element in SDH....................................25

Terminal multiplexer..........................................25 Add/drop multiplexer(ADM)................................ 26

Digital cross connect(DXS).......................... ...... 26

Regenerator..................................................... 27

Synchronization................................................ 28

Advantage of SDH.............................................32

Conclusion....................................................... 36

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Introduction

The introduction of any new technology is usuallypreceded by much hyperbole and rhetoric. Inmany cases, the revolution predicted never getsbeyond this. In many more, it never achieves thewildly over optimistic growth forecasted by marketspecialists - home computing and the paperlessoffice to name but two. It is fair to say, however,by whatever method you use to evaluate a new

technology that synchronous digital transmissiondoes not fall into this category. The fundamentalbenefits to be gained from its deployment by PTOsseem to be so overwhelming that, bar acatastrophe, the bulk of today's plesiochronoustransmission systems used for high speedbackbone links will be pushed aside in the next

few years. To quote Dataquest:, "It has beenclaimed by many industry experts that the impactof synchronous technology will equal that of thetransition from analogue to digital technology orfrom copper to fiber optic based transmission."

For the first time in telecommunications historythere will be a world-wide, uniform and seamlesstransmission standard for service delivery.

Synchronous digital hierarchy (SDH) provides thecapability to send data at multi-gigabit rates overtoday's single-mode fiber-optics links. This firstissue of Technology Watch looks at synchronousdigital transmission and evaluates its potentialimpact. Following issues of TW will look atcustomer oriented broad-band services that will

ride on the back of SDH deployment by PTOs.These will include:

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Frame relay

SMDS (Switched Multi-Megabit Data Service)

ATM (asynchronous transfer mode)

High speed LAN services such as FDDI

Figure 1 shows the relationship between thesetechnologies and services.

Figure 1 - The Relationship between Services

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Overview

The use of synchronous digital transmission byPTOs in their backbone fiber-optic and radionetwork will put in place the enabling technologythat will support many new broad-band dataservices demanded by the new breed of computeruser. However, the deployment of synchronousdigital transmission is not only concerned with theprovision of high-speed gigabit networks. It has asmuch to do with simplifying access to links and

with bringing the full benefits of software controlin the form of flexibility and introduction ofnetwork management.

In many respects, the benefits to the PTO will bethe same as those brought to the electronicsindustry when hard wired logic was replaced bythe microprocessor. As with that revolution,synchronous digital transmission will not take holdovernight, but deployment will be spread over adecade, with the technology first appearing onnew backbone links. The first to feel the benefitswill be the PTOs themselves, as demonstrated bythe technology's early uptake by many operatorsincluding BT. only later will customers directlybenefit with the introduction of new services such

as connectionless LAN-to-LAN transmissioncapability.

According to one market research company it willtake until the mid or late 1990s before 70% ofrevenue for network equipment manufacturers willbe derived from synchronous systems.Remembering that this is a multi-billion $ market,this constitutes a radical change by any standard(Figure 2).

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Users who extensively use PCs and workstationswith LANs, graphic layout, CAD and remotedatabase applications are now looking to the

telecommunication service suppliers to provide themeans of interlinking these now powerfulmachines at data rates commensurable with thoseachieved by their own in-house LANs. They alsowant to be able to transfer information to othermetropolitan and international sites as easily andas quickly as they can to a colleague sitting at thenext desk.

Figure 2 - European Revenue Growth of Transmission Equipment

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Plesiochronous Transmission.

Digital data and voice transmission is based on a2.048Mbit/s bearer consisting of 30 time divisionmultiplexed (TDM) voice channels, each running at64Kbps (known as E1 and described by the CCITTG.703 specification). At the E1 level, timing iscontrolled to an accuracy of 1 in 1011 bysynchronizing to a master Caesium clock.Increasing traffic over the past decade hasdemanded that more and more of these basic E1

bearers be multiplexed together to provideincreased capacity. During this time rates haveincreased through 8, 34, and 140Mbit/s. Thehighest capacity commonly encountered today forinter-city fiber optic links is 565Mbit/s, with eachlink carrying 7,680 base channels, and now eventhis is insufficient.

Unlike E1 2.048Mbit/s bearers, higher rate bearersin the hierarchy are operated plesiochronously,with tolerances on an absolute bit-rate rangingfrom 30ppm (parts per million) at 8Mbit/s to15ppm at 140Mbit/s. Multiplexing such bearers(known as tributaries in SDH speak) to a higheraggregate rate (e.g. 4 x 8Mbit/s to 1 x 34Mbit/s)requires the padding of each tributary by adding

bits such that their combined rate together withthe addition of control bits matches the finalaggregate rate. Plesiochronous transmission isnow often referred to as plesiochronous digitalhierarchy (PDH).

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Figure 3 - A typical Plesiochronous Drop & Insert

Because of the large investment in earliergenerations of plesiochronous transmissionequipment, each step increase in capacity hasnecessitated maintaining compatibility with whatwas already installed by adding yet another layerof multiplexing. This has created the situation

where each data link has a rigid physical andelectrical multiplexing hierarchy at either end.Once multiplexed, there is no simple way anindividual E1 bearer can be identified in a PDHhierarchy, let alone extracted, without fullydemultiplexing down to the E1 level again asshown in Figure 3.

The limitations of PDS multiplexing are:

A hierarchy of multiplexers at either end ofthe link can lead to reduced reliability andresilience, minimum flexibility, longreconfiguration turn-around times, largeequipment volume, and high capital-equipment and maintenance costs.

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PDH links are generally limited to point-to-point configurations with full demultiplexing ateach switching or cross connect node.

Incompatibilities at the optical interfaces oftwo different suppliers can cause majorsystem integration problems.

To add or drop an individual channel or add alower rate branch to a backbone link acomplete hierarchy of MUXs is required asshown in figure 3.

Because of these limitations of PDH, theintroduction of an acceptable world-widesynchronous transmission standard called SDHis welcomed by all.

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SDH Concepts and Principle

Introduction

It is an international standard networking principle and amultiplexing method. The name of hierarchy has beentaken from the multiplexing method which issynchronous by nature. The evolution of this system willassist in improving the economy of operability andreliability of a digital network.

Historical OverviewIn February 1988, an agreement was reached at CCITT(now ITU-TS) study group XVIII in Seoul, on set ofrecommendations, for a synchronous digital hierarchyrepresenting a single worldwide standard for transportingthe digital signal. These recommendations G-707, G-708,G-709 cover the functional characteristic of the networknode interface, i.e. the bit rates and format of the signalpassing over the Network Node Interface (NNI).

For smooth transformation from existing PDH, it has toaccommodate the three different country standards ofPDH developed over a time period. The differentstandards of PDH are given in Fig.1.

The first attempt to formulate standards for Optical

Transmission started in U.S.A. as SONET (Synchronous

Optical Network). The aim of these standards was to

simplify interconnection between network operators by

allowing interconnection of equipment from different

vendors to the extent that compatibility could be

achieved. It was achieved by SDH in 1990, when the

CCITT accepted the recommendations for physical layernetwork interface. The SONET hierarchy from 52 Mbit per

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second rate onwards was accepted for SDH hierarchy

(Fig.1).

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S.D.H. Evolution

Enhanced operation, administration,

Maintenance and provisioning capabilities.

Easy growth to higher bit rate in step width

evolution of transmission technology.

Capable of transporting existing PDH signals.

S.D.H.Evolution is possible because of the following

factors:

1. Fibre Optic Bandwidth: The bandwidth in OpticalFibre can be increased and there is no limit for it.This gives a great advantage for using SDH.

2. Technical Sophistication: Although, SDH circuitryis highly complicated, it is possible to have suchcircuitry because of VLSI technique which is alsovery cost effective.

3. Intelligence: The availability of cheaper memoryopens new possibilities.

4. Customer Service Needs: The requirement of thecustomer with respect to different bandwidthrequirements could be easily met without muchadditional equipment. The different services itsupports are:

o Low/High speed data.o Voice

o Interconnection of LAN

o Computer links

o Feature services like H.D.T.V.

o Broadband ISDN transport (ATM transport)

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S.D.H. Standards

The S.D.H. standards exploit one common characteristicof all PDH networks namely 125 micro seconds duration,i.e. sampling rate of audio signals (time for 1 byte in 64k bit per second). This is the time for one frame of SDH.The frame structure of the SDH is represented usingmatrix of rows in byte units as shown in Figs. 2 and 3. Asthe speed increases, the number of bits increases andthe single line is insufficient to show the information on

Frame structure. Therefore, this representation method isadopted. How the bits are transmitted on the line isindicated on the top of Fig.2. The Frame structurecontains 9 rows and number of columns depending uponsynchronous transfer mode level (STM). In STM-1, thereare 9 rows and 270 columns. The reason for 9 rowsarranged in every 125 micro seconds is as follows:

For 1.544 Mbit PDH signal (North America and JapanStandard), there are 25 bytes in 125 micro second andfor 2.048 Mbit per second signal, there are 32 bytes in125 micro second. Taking some additional bytes forsupervisory purposes, 27 bytes can be allotted forholding 1.544 Mbit per second signal, i.e. 9 rows x 3columns. Similarly, for 2.048 Mbit per second signal, 36bytes are allotted in 125 micro seconds, i.e. 9 rows x 4

columns. Therefore, it could be said 9 rows are matchedto both hierarchies.

A typical STM-1 frame is shown in Fig. 3. Earlier this wasthe basic rate but at present STM-0 which is just 1/3rd ofSTM-1, i.e. 51.840 Mbit per second has been accepted byCCITT. In STM-1 as in Fig.3 the first 9 rows and 9columns accommodate Section Overhead (SOH) and 9

rows x 261 columns accommodate the main informationcalled pay load. The interface speed of the STM-1 can be

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calculated as follows:

(270 columns x 9 rows x 8 bits x 1/125 s)= 155.52Mbps.

The STM-0 contains just 1/3rd of the STM-1, i.e. 9 rows x90 columns out of that 9 rows x 3 columns consist ofsection overhead and 9 rows x 87 columns consist of payload. The STM-0 structure was accepted so that the radioand satellite can use this bit rate, i.e. 51.840 Mbit/sacross their section.

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Basic Definitions

Synchronous Transport ModuleThis is the information structure used to supportinformation pay load and over head information fieldorganised in a block frame structure which repeats every125 micro seconds.

Container

The first entry point of the PDH signal is the container inwhich the signal is prepared so that it can enter into thenext stage, i.e. virtual container. In container (container-I) the signal speed is increased from 32 bytes to 34bytes in the case of 2.048 Mbit/s signal. The additionalbytes added are fixed stuff bytes (R), Justification ControlBytes (CC and C), Justification Opportunity bytes (s).

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

125 seconds) is increased to 756 bytes in 125 secondsadding fixed stuff bits(R). Justification control bits (C-1,C-2) and Justification opportunity bits (S-1, S-2).

Detail follows: 756 bytes are in 9 x 84 bytes/125 secondsframe. They are further subdivided into 3 sub frames 3 x84 (252 bytes or 2016 bits). Out of this

1431 information bits (I),

10 bits (two sets) (C-1, C-2)

2 Justification opportunity bits (S-1, S-2)

573 (fixed bits)

In container-4, 139.264 Mbit/s signal (2176 bytes in 125seconds) is increased to 9 x 260 bytes. Details as

follows:

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9 x 260 bytes are partitioned into 20 blocks consisting of13 bytes each. In each row one justification opportunitybit(s) and five justification control bit(s) are provided.

The first byte of each block consists of either eightinformation bit (I) or eight fixed stuff bits (R) or

One justification control bit (C) plus five fixed stuff bits(R) plus two overhead bits (o). Or six information bits (I)plus one justification opportunity bit (s) plus one fixedstuff bit (R).

The last 12 bytes of one block consists of information bits(I).

Virtual Container

In Virtual container the path over head (POH) fields areorganised in a block frame structure either 125 secondsor 500 seconds. The POH information consists of only 1byte in VC-1 for 125 seconds frame. In VC-3, POH is 1column of 9 bytes. In VC-4 also POH 1 column of 9bytes.The types of virtual container identified are lower ordersVCs VC-1 and VC-2 and higher order VC-3 and VC-4.

Tributary Unit

A tributary unit is a information structure which providesadaptation between the lower order path layer and thehigher order path layer. It consists of a information payload (lower order virtual container) and a tributary unitpointer which indicates the offset of the pay load framestart relating to the higher order VC frame start.Tributary unit 1 for VC-1 and Tributary unit 2 is for VC-2and Tributary unit 3 is for VC-3, when it is mapped for

VC-4 through tributary group-3. TU-3 pointer consists of3 bytes out of 9 bytes. Three bytes are H1, H2, H3 and

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remaining bytes are fixed bytes. TU-1 pointers are onebyte interleaved in the TUG-2.

Tributary Unit Group

One or more tributaries are contained in tributary unitgroup. A TUG-2 consists of homogenous assembly ofidentical TU-1s or TU-2. TUG-3 consists of a homogenousassembly 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 orone TU-3.

Network Node Interface (NNI)

The interface at a network node which is used tointerconnect with another network node.

Pointer

An indicator whose value defines frame offset of a VCwith respect to the frame reference of transport entity,

on which it is supported.Administrative Unit

It is the information structure which provides adaptationbetween the higher order path layer and the multiplexsection layer. It consists of information pay load and aA.U. pointer which indicates the offset of the pay loadframe start relating to the multiplex section frame start.

Two AUs are defined (i) AU-4 consisting VC-4 plus anA.U. pointer indicating phase alignment of VC-4 withrespect to STM-N frame, (ii) AU-3 consisting of VC-3 plusA.U. pointer indicating phase alignment of VC-3 withrespect to STM-N frame. A.U. location is fixed withrespect to STM-N frame.

Administrative Group

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

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Concatenation

The procedure with which the multiple virtual containerare associated with one another, with the result their

combined capacity could be used as a single containeracross which bit sequence integrity is maintained.

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Multiplexing Principles

The basic multiplexing principles and processing stage by

stage, the information signal is shown in Fig.7. In C-11,1.544 Mbit per sec is mapped. In C12 container, theentry is 2.048 Mbit/sec. In C-2 container the entry, i.e.6.312 Mbit/sec which is of American standard. Thesethree containers passes through their respective virtualcontainers and tributary unit pointers. At TUG-2 it can beeither 4VC-11 with TU-11 or 3VC-12 with TU-12 or 1 VC-

2 with TU-2. The C-3 container takes the input 34 Mb/sor 44.7 Mb/s of the American Standard. These throughVC-3 container and with tributary unit-3 go to TributaryUnit Group3. 3 Nos. VC-3 with AU-3 can directly go toAUG and enter STM-frame. Similarly, 7 TUG-2 can bemapped into one VC-3. Otherwise one VC-3 with TU-3 or7 TUG2 can go to TUG-3 and 3 TUG-3 are mapped intoone VC-4. A 139.264 Mbit/sec signal can be mapped into

one VC-4 through C-4. VC-4 with AU-4 goes to AUG andthen to STM-frame. The different possibilities are shownin Fig.7.

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Section Overhead Brief Description

The section overhead portion of the STM-1 frame withtheir relevant bytes is indicated in Fig. 12. From the

figure, it is seen that 4th row 9 bytes are reserved for AUpointers and this will be discussed separately. The top 3rows x 9 columns of STM-1 frame reserved forRegenerator Section Overhead (R SOH). From the 5th rowto 9th row with 9 columns are reserved for MultiplexSection Overhead (M SOH). A brief idea of the differentbytes in regenerator section overhead and multiplexoverhead are given below:

A-1, A-2 are framing bytes. Their values are:

A1 : 11110110

A2 : 00101000

These two types of bytes form 16 bit Frame AlignmentWord (FAW). FAW formed by the last A-1 byte and theadjacent A-2 byte, in the transmitter sequence defines

the frame reference for each of signal rates. There are 3A-1 bytes in STM-1 and 3 A-2 bytes in STM-1. In higherorder STM their number increases with the STM order,i.e. in STM-4, there will be 12 A-1 bytes and 12 A-2bytes.

STM Identifier with C-1 Byte: In STM-1 there is a singleC-1 byte which is used to identify each of inter-leaved

STMs and in an STM-N signal. It takes binary equivalentto the position in the interleave.

D-1 or D-12: These bytes are for data communicationchannel. In this D-1, D-2 and D-3 are for regeneratorsection. It can support 192 kilo bit per section. D-4 to D-12 are for multiplex section. They can support 576 kilobit per second.

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E-1, E-2 for order wire purposes.E-1 is for regenerator section order wire.E-2 is for multiplex section order wire.

F-1 is used for fault control purposes.B-1 byte are called bit inter-leave parity-8. This is usedfor error monitoring in the regenerator section. There isonly 1 byte in STM1 or STM-4 or STM-16. On linemonitoring can be done in this case.

B-2 bytes. These are used for error monitoring in themultiplex section. There are 3 bytes for STM-1, STM-4

and 16 will have more number of B-2 bytes as per theirorder.

K-1, K-2 bytes. There are 2 bytes for STM-1, 4 or 16.These are used for co-ordinating the protection switchingacross a set of multiplex section organised as protectiongroup, they are used for automatic protection switching.

Z-1, Z-2 : These bytes are located for functions and yet

defined, as per CCITT recommendations.

Fig.12

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Network Elements in SDH

Before the evolution of the standards coveringsynchronous transmission systems, networks had to bebuilt up from separate multiplex and line terminalequipment. These are characterized by defined formatsand electrical interfaces at each level of the transmissionhierarchy; whereas optical interfaces were entirelyproprietary. This gave rise to large amounts of multiplexand separate optical line equipment.

On the other hand in SDH, multiplexers performs bothmultiplexing and line terminating functions. Synchronousmultiplexers can accept a wide range of tributaries andoffer a number of possible output data rates. Though theregeneration of signal at repeaters is similar to PDH,there are some additional equipment in SDH to performfunction like crossconnection and OA&M functions as

explained in following sections.Terminal Multiplexers

Terminal multiplexers are used to combineplesiochronous and synchronous input signals into higherbit rate STMN signals as shown in Fig.13 below. On thetributary side, all current plesiochronous bit rates can beaccommodated. On the aggregate, or line side we have

higher bit rate STMN signals.

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Digital CrossConnects (DXC)

Cross connection is a synchronous network involvessetting up semi permanent interconnections betweendifferent channels enabling routing to be performed downto a VC level. This network element can have widestrange of functions such as mapping of PDH tributarysignals into virtual containers and switching of variouscontainers up to and including VC4.

Regenerators

Regenerators, as the name implies, have the job ofregenerating the clock and amplitude of the incomingdata signals that have been attenuated and distorted bydispersion. They derive their clock signals from theincoming data stream. Messages are received byextracting various 64 Kbit/s channels (e.g. servicechannels E1, F1, etc. in RSOH) and also can be output

using these channels.

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Synchronisation

The role of synchronisation plan is to determine thedistribution of synchronisation in a network and to selectthe level of clocks and facilities to be used to time thenetwork. This involves the selection and location ofmaster clocks for a network, the distribution of primaryand secondary timing throughout the network and ananalysis of the network to ensure that acceptableperformance levels are achieved. Impropersynchronisation planning or the lack of planning can

cause severe performance problems resulting inexcessive slips, long periods of network downtime,elusive maintenance problems or high transmission errorrates. Hence, a proper synchronisation plan whichoptimises the performance, is a must for the entiredigital network. The status of synchronisation in theBSNL network is as follows:

3 nos. of cesium clocks at VSNL Bombay provide theMaster National Reference Clock (MNRC). The backupNRC is available at Delhi. The MNRC feeds the referencesignal to the VSNL GDS at Mumbai and from the GDSboth the new technology TAXs at Mumbai aresynchronised. From these two TAXs at Mumbai, all theother TAXs are to be synchronised. Part of this work hasalready been done. However, all the LevelI TAXs are yet

to be synchronised. A direct synchronisation link is alsoavailable between GDS Mumbai and Karol Bagh TAX atDelhi.

For synchronisation of the SDH network, it has beendecided to use the clock source available through theTAXs at the major stations. The synchronisation plan isbased upon provision of Synchronisation Supply Units(SSUs) which will be deployed as an essential componentof the synchronisation network which will support

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synchronised operation of the SDH network. Thearchitecture employed in the SDH requires that thetiming of all the network clocks be traceable to Primary

Reference Clock (PRC) specified in accordance with ITURec.G.811. The classical method of synchronisingnetwork element clocks is the hierarchical method(masterslave synchronisation) which is already adoptedin the BSNL network for the TAXs. This masterslavesynchronisation uses a hierarchy of clocks in which eachlevel of the hierarchy is synchronised with reference to ahigher level, the highest level being the PRC. The

hierarchical level of clocks is defined by ITU as follows:

P.R.C.

Slave Clock (Transit Node)

Slave Clock (Local Node)

SDH Network Element Clock.

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Each node is associated with a particular hierarchicallevel of clock prescribed above and is referred to as anodal clock. The SSU is an important component of this

hierarchical masterslave synchronisation networkscheme and of a slave clock belonging to the transit nodelevel or the local node level as defined in ITU Rec. G.812.

The BSNL, therefore, has decided to go in for 1020 nos.of SSUs to provide a clean reference primary source forother stations. These SSUs are basically high stabilityfilter clocks which eliminate phase transients, jitter and

wander and provide the exact sync. signal needed forevery network element.

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High Transmission rates

Transmission rates of up to 10Gbps can be achieved

in modern SDH systems making it the most suitable

technology for backbones-the superhighways in

todays telecommunication networks.

STM-1 STM-4 STM-16 STM-64

Future-proof platform for new servicesSDH is the ideal platform for a wide range of

services including POTS, ISDN, mobile radio, and

data communications (LAN, WAN, etc.). It is also

able to handle more recent services such as video on

demand and digital video broadcasting via ATM.

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Provide built-in signal capacity for advanced

network management and maintenance capabilities

With SDH, network providers can react quickly andeasily to the requirements of their customers. Forexample, leased lines can be switched in a matter ofminutes. The network provider can use standardizednetwork elements (NE) that can be controlled andmonitored from a central location via atelecommunications management network (TMN)system.

InterconnectionSDH makes it much easier to set up gatewaysbetween different network providers and to SONETsystems. The SDH interfaces are globallystandardized, making it possible to combine NEsfrom different manufacturers into a single networkthus reducing equipment costs. -The trend in

transport networks is toward ever-higher bit rates,such as STM-256 (time division multiplex, TDM). Thecurrent high costs of such NEs however are arestricting factor. The alternative lies in densewavelength division multiplexing (DWDM), atechnology enabling the multiple uses of single modeoptical fibers. As a result, a number of wavelengthscan be used as carriers for the digital signals andtransmitted simultaneously through the fibers.

Software Control allows extensive use of intelligentnetwork management software for high flexibility,fast and easy re-configurability, and efficient networkmanagement.

Survivability. With SDH, ring networks becomepracticable and their use enables automaticreconfiguration and traffic rerouting when a link is

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damaged. End-to-end monitoring will allow fullmanagement and maintenance of the wholenetwork.

Efficient drop and insert. SDH allows simple andefficient cross-connect without full hierarchicalmultiplexing or de-multiplexing. A single E12.048Mbit/s tail can be dropped or inserted withrelative ease even on Gbit/s links.

Standardisation enables the interconnection ofequipment from different suppliers through supportof common digital and optical standards and

interfaces.

Robustness and resilience of installed networks isincreased.

Equipment size and operating costs are reducedby removing the need for banks of multiplexers andde-multiplexers. Follow-on maintenance costs arealso reduced.

Backwards compatibly will enable SDH links tosupport PDH traffic.

Future proof. SDH forms the basis, in partnership

with ATM (asynchronous transfer mode), of broad-

band transmission, otherwise known as B-ISDN or

the

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Conclusions

The introduction of synchronous digital transmission inthe form of SDH will eventually revolutionize all aspects

of public data communication from individual leased linesthrough to trunk networks. Because of the state-of-the-art nature of SDH and SONET technology, there areextensive field trials taking place in 1992 throughout theworld prior to introduction in the 1993 - 1995 time scale.

There is still a lack of understanding of the ramificationsof the introduction of SDH within telecommunicationsoperations. In practice, the use of extensive softwarecontrol will impact positively all parts of the business. Itis not so much a question ofwhetherthe technology willbe taken up, but when.

Introduction of SDH will lead to the availability of manynew broad-band data services providing users withincreased flexibility. It is in this area where confusionreigns with potential technologies vying for supremacy.

These will be discussed in future issues of TechnologyWatch.

Importantly for PTOs, SDH will bring about morecompetition between equipment suppliers designingessentially to a common standard. One practical effectcould be to force equipment prices down, brought aboutby the larger volumes engendered by access to world

rather than local markets. At least one manufacturer iscurrently stating that they will be spending up to 80% oftheir SDH development budgets on managementsoftware rather than hardware. Such was the situation inthe computer industry in the early 1980s. Not least, itwill have a great impact on such issues as staffing levelsand required personal skills of personnel within PTOs.

SDH deployment will take a great deal of investment andeffort since it replaces the very infrastructure of theworld's core communications networks. But it must not

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be forgotten that there are still many issues to beresolved.

The benefits to be gained in terms of improving operatorprofitability, and helping them to compete in the newmarkets of the 1990s, are so high that deployment ofSDH is just a question of time.

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