HANDOUT-5

SDH CONCEPT

Introduction

It is an international standard networking principle and a multiplexing

method. The name of hierarchy has been taken from the multiplexing method which is

synchronous by nature. The evolution of this system will assist in improving the

economy of operability and reliability of a digital network.

1. Historical Overview

In February 1988, an agreement was reached at CCITT (now ITU-TS) study

group XVIII in Seoul, on set of recommendations, for a synchronous digital hierarchy

representing a single world wide standard for transporting the digital signal. These

recommendations G-707, G-708, G-709 cover the functional characteristic of the

network node interface, i.e. the bit rates and format of the signal passing over the

Network Node Interface (NNI).

For smooth transformation from existing PDH, it has to accommodate the

three different country standards of PDH developed over a time period. The different

standards 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 inter-connection 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 layer network interface. The SONET hierarchy from 52 Mbit per second rate

onwards was accepted for SDH hierarchy (Fig.1).

2. Merits of SDH

(i) Simplified multiplexing/demultiplexing techniques.

(ii) Direct access to lower speed tributaries, without need to

multiplex/demultiplex the entire high speed signal.

(iii) Enhanced operations, Administration, Maintenance and provisioning

capabilities.

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(iv) Easy growth to higher bit rates in step with evolution of transmission

technology.

(v) Capable of transporting existing PDH signals.

(vi) Capable of transporting future broadband (ATM) channel bit rates.

(vii) Capable of operating in a multi-vendor and multi-operator

environment.

3. Advantages

(i) Multi-vendor environment (mid span meet) : Prior to 1988

international agreement on SDH all vendors used proprietary non-

standard techniques for transporting information on fibre. The only

way to interconnect was to convert to the copper transmission

standards (G702/703/704). The cost and complexity levels were very

high.

(ii) Synchronous networking : SDH supports multi-point or hub

configurations whereas, asynchronous networking only supports point-

to-point configurations.

(iii) Enhanced OAM&P : The telecoms need the ability to administer,

surveil, provision, and control the network from a central location.

(iv) Positioning the network for transport on new services : LAN to LAN,

HDTV, interactive multimedia, video conferencing.

(v) HUB : A hub is an intermediate site from which traffic is distributed to

3 or more spur. It allows the nodes to communicate as an angle

network, thus reducing the back-to-back multiplexing and

demultiplexing.

4. S.D.H. Evolution

S.D.H. evolution is possible because of the following factors :

(i) Fibre Optic Bandwidth : The bandwidth in Optical Fibre can be increased

and there is no limit for it. This gives a great advantage for using SDH.

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

is possible to have such circuitary 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. Feature services like H.D.T.V.

6. Broadband ISDN transport (ATM transport)

5. S.D.H. Standards

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 in

Figs. 2 and 3. 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 Fig.2.

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 :

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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.

A typical STM-1 frame is shown in Fig. 3. Earlier this was the basic rate but at

present STM-0 which is just 1/3rd of STM-1, i.e. 51.840 Mbit per second has been

accepted by CCITT. In STM-1 as in Fig.3 the first 9 rows and 9 columns

accommodate Section Overhead (SOH) and 9 rows x 261 columns accommodates the

main information called pay load. The interface speed of the STM-1 can be calculated

as follows :

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

The STM-0 contains just 1/3rd of the STM-1, i.e. 9 rows x 90 columns out of

that 9 rows x 3 columns consist of section overhead and 9 rows x 87 columns consist

of pay load. The STM-0 structure was accepted so that the radio and satellite can use

this bit rate, i.e. 51.840 Mbit/s across their section.

The different SDH level as per G-707 recommendations is as given in Fig.4.

6. Basic Definitions

(i) Synchronous Transport Module

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

head information field organised 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-I) the signal speed is increased from 32 bytes to 34 bytes in the case of

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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 seconds) is

increased to 756 bytes in 125 seconds adding 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 seconds frame. They are

further subdivided into 3 sub frames 3 x 84 (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 125 seconds) is

increased to 9 x 260 bytes. Details as follows :

9 x 260 bytes are partitioned into 20 blocks consisting of 13 bytes each. In

each row one justification opportunity bit(s) and five justification control bit(s) are

provided.

The first byte of each block consists of either

eight information 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

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Six information bits (I) plus one justification opportunity bit (s) plus one fixed

stuff bit (R).

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

(iii) Virtual Container

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

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

consists of only 1 byte in VC-1 for 125 seconds 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 VC-1 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 1 for VC-1 and Tributary unit 2 is for VC-2 and Tributary unit 3 is for

VC-3, when it is mapped for VC-4 through tributary group-3. TU-3 pointer consists

of 3 bytes out of 9 bytes. Three bytes are H1, H2, H3 and remaining bytes are fixed

bytes. TU-1 pointers are one byte interleaved in the TUG-2.

(v) Tributary Unit Group

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

of homogenous assembly of identical TU-1s 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)

The 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.

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(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 A.U. 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 Unit 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.

7. S.D.H. Layer Structure

The S.D.H. can be based on layered concept as shown in Fig.5. The Fig.6

shows the layer interconnection in detail.

8. 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 C-12

container, the entry is 2.048 Mbit/sec. In C-2 container the entry, i.e. 6.312 Mbit/sec

which is of American standard. These three containers passes through their respective

virtual containers and tributary unit pointers. At TUG-2 it can be either 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/s or 44.7 Mb/s of the American Standard. These through VC-3 container

and with tributary unit-3 goes to Tributary Unit Group–3. 3 Nos. VC-3 with AU-3 can

directly go to AUG and enter STM-frame. Similarly, 7 TUG-2 can be mapped into

one VC-3. Otherwise one VC-3 with TU-3 or 7 TUG-2 can go to TUG-3 and 3 TUG-

3 are mapped into one VC-4. A 139.264 Mbit/sec signal can be mapped into one VC-

4 through C-4.

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VC-4 with AU-4 goes to AUG and then to STM-frame. The different

possibilities are shown in Fig.7.

9. Section Overhead Brief Description

The section overhead portion of the STM-1 frame with their relevant bytes are

indicated in Fig. 9. From the figure, it is seen that 4th row 9 bytes are reserved for AU

pointers and this will be discussed separately. The top 3 rows x 9 columns of STM-1

frame reserved for Regenerator Section Overhead (R SOH). From the 5 th row to 9th

row with 9 columns are reserved for Multiplex Section Overhead (M SOH). A brief

idea of the different bytes in regenerator section overhead and multiplex overhead are

given below :

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

A1 : 11110110

A2 : 00101000

(i) These two types of bytes form 16 bit Frame Alignment Word (FAW).

FAW formed by the last A-1 byte and the adjacent A-2 byte, in the

transmitter sequence defines the frame reference for each of signal

rates. There are 3 A-1 bytes in STM-1 and 3 A-2 bytes in STM-1. In

higher order STM their number increases with the STM order, i.e. in

STM-4, there will be 12 A-1 bytes and 12 A-2 bytes.

(ii) STM Identifier with C-1 Byte : In STM-1 there is a single C-1 byte

which is used to identify each of inter-leaved STM’s and in an STM-N

signal. It takes binary equivalent to the position in the inter-leave.

(iii) D-1 or D-12 : These bytes are for data communication channel. Inthis

D-1, D-2 and D-3 are for regenerator section. It can support 192 kilo

bit per section. D-4 to D-12 are for multiplex section. They can support

576 kilo bit per second.

(iv) E-1, E-2 for order wire purposes.

E-1 is for regenerator section order wire.

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E-2 is for multiplex section order wire.

(v) F-1 is used for fault control purposes.

(vi) B-1 byte are called bit inter-leave parity-8. This is used for error

monitoring in the regenerator section. There is only 1 byte in STM-1 or

STM-4 or STM-16. On line monitoring can be done in this case.

(vii) B-2 bytes. These are used for error monitoring in the multiplex section.

There are 3 bytes for STM-1, STM-4 and 16 will have more number of

B-2 bytes as per their order.

(viii) K-1, K-2 bytes. There are 2 bytes for STM-1, 4 or 16. These are used

for co-ordinating the protection switching across a set of multiplex

section organised as protection group, they are used for automatic

protection switching.

(ix) Z-1, Z-2 : These bytes are located for functions and yet defined, as per

CCITT recommendations.

S1 – Synchronous Status Message

M1 – MUX SECTION R.E.I. (Remote Error Indication).

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Fig. 1Standardization of Digital Hierarchies

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Customer residence

Fig. 2SDH Interface Frame Representation Method

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Fig. 3STM-N Frame Structure

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Fig. 4SDH Standards – Bit Rates

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Fig. 5SDH–based Transport Network Layered Model

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Fig. 6SDH Layers

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Fig. 7Multiplexing Principles

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Fig. 8Layer Interaction

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A1 A1 A1 A2 A2 A2 C1

B1 E1 F1

D1 D2 D3

AU–n pointers

B2 B2 B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 F2 Z1 Z2 Z2 M1 E2

X – Byte reserved to national use – Media dependent

Blanks – Reserved for future use

Fig. 9Section Overhead

Table indicating different combinations of P.D.H. Tributaries

INPUT OUTPUT

2 Mb 34 Mb 140 Mb STM–1

– – 1 STM–1

– 3 – STM–1

21 2 – STM–1

42 1 – STM–1

63 – – STM–1

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RSOH

MSOH

9 ro

ws

H.O. POH : PATH OVER HEAD (VC3/VC4)

J1 J – PATH TRACE BYTE (LIKE FAW)

B3 B – BIT INTERLEAVED PARITY CODE (BIP–8) BYTE FOR PATH ERR. MON. CALCULATED OVER ALL BITS OF PREVIOUS VC BEFORE SCRAMBLING.

C2 C – PATH SIG. LABEL BYTE TO INDICATE, SPE EQPD (1) OR NOT (0) ATM – 00010011, MAN – 00010100, FDDI – 00010101, LOCKED TU – 00000011.

G1 G – PATH STATUS BYTE OR REMOTE STN. (BIT 1–4 FEBE, BIT 5 – FERF, BIT 6–8 NOT USED)

F2 F – E.O.W. BETWEEN PATH

H4 H – MULTIFRAME ALIGNMENT BYTE OR DENOTE STARTING POSITION OF ATM CELL

Z Z – FUTURE USE(F3)

Z K3 – APS FOR PROTN. SWG. (b1 …. b4) SPARE (b5 …. b8) TO INCREASE

(K3) N/W CAPABILITY

Z N1 – TANDOM CONN. MON AND PATH DATA BYTE(N1)

L.O. P.O.H (FOR VC–11, VC–12, VC–2)

V5 BIP–2 FEBE PT L1 L2 L3 FERF

FEBE – FAR END BLOCK ERROR.FERF – FAR END RECEIVE FAILUREPT – PATH TRACEL1 – MAPPING IS IN ASYNCH. MODEL2 – MAPPING IS IN BIT SYNCH. MODEL3 – MAPPING IS IN BYTE SYNCH. MODE

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MODE 1 – A 64 BYTE FIXED LENGTH STRING

MODE 2 – 15 BYTE STRING & 1 BYTE HEADER (CRC–7)

J2 PATH TRACE

K–4 APS

N–2 TANDOM CONNECTION

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Fig. 10

Multiplexing of STM–1 Signals

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Fig. 11 Synchronous Multiplexers

Optional Tributary Interfaces

Fig. 12Add Drop Multiplexer

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10.0 Network Elements in SDH

Before the evolution of the standards covering synchronous transmission

systems, networks had to be built up from separate multiplex and line terminal

equipment. These are characterized by defined formats and electrical interfaces at

each level of the transmission hierarchy; whereas optical interfaces were entirely

proprietary. This gave rise to large amounts of multiplex and separate optical line

equipment.

On the other hand in SDH, multiplexers performs both multiplexing and line

terminating functions. Synchronous multiplexers can accept a wide range of

tributaries and offer a number of possible output data rates. Though the regeneration

of signal at repeaters is similar to PDH, there are some additional equipment in SDH

to perform function like cross–connection and OA&M functions as explained in

following sections.

10.1 Terminal Multiplexers

Terminal multiplexers are used to combine plesiochronous and synchronous

input signals into higher bit rate STM–N signals as shown in Fig.13 below. On the

tributary side, all current plesiochronous bit rates can be accommodated. On the

aggregate, or line side we have higher bit rate STM–N signals.

Fig. 13Terminal Multiplexer

10.2 Add/Drop Multiplexer (ADM)

Plesiochronous and 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.

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Fig. 14Add/Drop Multiplexers

10.3 Digital Cross–Connects (DXC)

Cross connection is a synchronous network involves setting up semi–

permanent interconnections between different channels enabling routing to be

performed down to a VC level. This network element can have widest range of

functions such as mapping of PDH tributary signals into virtual containers and

switching of various containers up to and including VC–4.

Fig. 15Digital Cross–Connects

10.4 Regenerators

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

amplitude 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,

etc. in RSOH) and also can be output using these channels.

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