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13 Synchronous Digital Hierarchy S D H and Synchronous Optical Network SONET The synchronous digital hierarchy SDH is emerging as the universal technology for transmission in telecom

13 Synchronous Digital Hierarchy

( S D H ) and Synchronous Optical Network (SONET)

The synchronous digital hierarchy (SDH) is emerging as the universal technology for transmission in telecommunications networks Since the first publication of international standards by ITU-T in 1989, SDH equipment has been rapidly developed and deployed across the world, and is rapidly taking over from its predecessor, the Plesiochronous Digital Hierarchy

(PDH) This chapter describes SDH and the North American equivalent of SDH, SONET

(Synchronous Optical Network), from which it grew In particular, the chapter describes the

features of SDH which characterize its advantages over PDH

HIERARCHY (SDH)

The synchronous digital hierarchy ( S D H ) was developed from its North American

forerunner SONET (synchronous optical network) SDH is the most modern type of transmission technology, and as its name suggests it is based on a synchronous multi- plexing technology The fact that SDH is synchronous adds greatly to the efficiency of the transmission network, and makes the network much easier to manage

Historically, digital telephone networks, modern data networks and the transmission infrastructures serving them have been based on a technology called PDH (Plesio- chronous Digital Hierarchy) as we discussed in Chapter 5 As we also discussed, three

distinct PDH hierarchies evolved, as we summarize in Figure 13.1 They share three common attributes

267

Networks and Telecommunications: Design and Operation, Second Edition.

Martin P Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)

(a) Europe

64 2048 a448 34 368 139 264 564 992

kbit/s

( E l 1 (E21 (E31

b

L

(b) North America

6 4

1 5 4 4

6 3 1 2

4 4

(DSO) (DS1 or T1) (DS2 or T2) (DS3 or T3)

1 -

MUX MUX MUX MUX

hierarchy level

b

hierarchy level

They are all based on the needs of telephone networks, i.e offering integral multiples

of 64 kbit/s channels, synchronized to some extent at the first multiplexing level

(1.5 Mbit/s or 2 Mbit/s)

They require multiple multiplexing stages to reach the higher bitrates, and are therefore difficult to manage and to measure and monitor performance, and relatively expensive to operate

They are basically incompatible with one another

Each individual transmission line within a PDH network runs plesiochronously This means that it runs on a clock speed which is nominally identical to all the other line systems in the same operator’s network but is not locked synchronously in step (it is free- running as we discussed in Chapter 5) This results in certain practical problems Over a relatively long period of time (say one day) one line system may deliver two or three bits more or less than another If the system running slightly faster is delivering bits for the second (slightly slower) system then a problem arises with the accumulating extra bits Eventually, the number of accumulated bits becomes too great for the storage avail- able for them, and some must be thrown away The occurrence is termed slip To keep

this problem in hand, framing and stufJing (or justlJication) bits are added within the

THE PROBLEMS OF PDH TRANSMISSION 269

normal multiplexing process, and are used to compensate These bits help the two end systems to communicate with one another, speeding up or slowing down as necessary

to keep better in step with one another The extra framing bits account for the differ- ence, for example, between 4 X 2048(E1 bitrate) = 8192 kbit/s and the actual E2 bitrate (8448 kbit/s, see Figure 13.1)

Extra framing bits are added at each stage of the PDH multiplexing process Unfortunately this means that the efficiency of the higher order line systems (e.g

139264 kbit/s, usually termed 140Mbit/s systems) are relatively low (91%) More critically still, the framing bits added at each stage make it very difficult to break out a

single 2 Mbit/s tributary from a 140 Mbit/s line system without complete demultiplexing

(Figure 13.2) This makes PD H networks expensive, rather inflexible and difficult to manage

SDH, in contrast with PDH, requires the synchronization of all the links within

a network It uses a multiplexing technique which has been specifically designed to

allow for the drop and insert of the individual tributaries within a high speed bit rate Thus, for example, a single drop and insert multiplexor is required to break out a single

2 Mbit/s tributary from an STM-I (synchronous transport module) of 155 520 kbit/s (Figure 13.3)

Other major problems of the PDH are the lack of tools for network performance management and measurement now expected by most public and corporate network managers, the relatively poor availability and range of high speed bit rates and the inflexibility of options for line system back-up (Figure 13.4)

3x E2

Note 2 4xEl

IX E l B-C

6 3 ~ E l A-to-C

Note 1 : 3 X E3 = 12 X E2 or 48 X E l after demultiplexing

Note 2: 3 X E2 = 12 X E l after demultiplexing

3x E2

4 x E l

exchange

A B C

drop and Insert multiplexor

El El

62 X El

(STM-1) line at an intermediate exchange

m standby

Before SDH, networks had to be built up from separate multiplex and line terminating equipment ( L T E ) , as the optical equipment interfaces in particular were manufacturer- specific (i.e proprietary) Back-up tended to be on a I main + I standby protection basis,

making back-up schemes costly (Figure 13.4) and difficult to manage These problems have been eliminated in the design of SDH through in-built flexibility of the bitrate

hierarchy, integration of the optical units into the multiplexors, ring structure topologies and in-built performance management and diagnostic functions

As is shown in Figure 13.5, the containers (i.e available bitrates) of the synchronous digital hierarchy have been designed to correspond to the bit rates of the various PDH

hierarchies These containers are multiplexed together by means of virtual containers

(abbreviated to VCs but are not to be confused with virtual channels which are also

THE MULTIPLEXING STRUCTURE OF SDH 271

X N X 1

pointer processing

c- multiplexing

so abbreviated), tributary units ( T U ) , tributary unit groups ( T U G ) , administrative units ( A U ) and finally administrative unit groups ( A U G ) into synchronous transport modules ( S T M )

The basic building block of the SDH hierarchy is the administrative unit group ( A U G ) An AUG comprises one AU-4 or three AU-3s The AU-4 is the simplest form

of AUG, and for this reason we use it to explain the various terminology of SDH (con- tainers, virtual containers, mapping, aligning, tributary units, multiplexing, tributary unit groups)

The container comprises sufficient bits to carry a full frame (i.e one cycle) of user

information of a given bitrate In the case of container 4 (C-4 ) this is a field of 260 X 9 bytes (i.e 18 720 bits) In common with PDH, the frame repetition rate (i.e number of cycles per second) is 8000Hz Thus a C4-container can carry a maximum user throughput rate (information payload) of 149.76 Mbit/s (18 720 X 8000) This can either be used as a raw bandwidth or, say, could be used to transport a P DH link of 139.264 Mbit/s

To the container is added a path overhead ( P O H ) of 9 bytes (72 bits) This makes a

virtual container ( V C ) The process of adding the POH is called mapping The POH

information is communicated between the point of assembly (i.e entry to the SDH

network) and the point of disassembly It enables the management of the SDH system and the monitoring of its performance

The virtual container is aligned within an administrative unit ( A U ) (this is the key to

synchronization) Any spare bits within the AU are filled with a defined filler pattern

called fixed stuf In addition, a pointer field of 9 bytes (72 bits) is added The pointers

(3 bytes for each VC, up to three VCs in total (9 bytes maximum)) indicate the exact position of the virtual container(s) within the A U frame Thus in our example case, the AU-4 contains one 3 byte pointer indicating the position of the VC-4 The remaining

6 bytes of pointers are filled with an idle pattern One AU-4 (or three AU-3s containing

three pointers for the three VC-3s) are multiplexed to form an AUG

To a single AUG is added 9 X 8 bytes (576 bits) of section overhead(S0H) This makes

a single S T M - 1 frame (of 19 440 bits) The SOH is added to provide for block framing and

for the maintenance and performance information carried on a transmission line section

basis (A section is an administratively defined point-to-point connection in the network,

typically an SDH-system between two major exchange sites, between two intermediate multiplexors or simply between two regenerators) The SOH is split into 3 bytes of RSOH

(regenerator section overhead) and 5 bytes of MSOH (multiplex section overhead) The

RSOH is carried between, and interpreted by, SDH line system regenerators (devices appearing in the line to regenerate laser light or other signal, thereby avoiding signal

degeneration) The MSOH is carried between, and interpreted by the devices assembling and disassembling the AUGs The MOH ensures integrity of the AUG

As the frame repetition rate of an STM-1 frame is 8000Hz, the total line speed is

155.52 Mbit/s (19 440 X 8000) Alternatively, power of four (1, 4, 16, etc.) multiples of AUGs may be multiplexed together with a proportionately increased section overhead,

AUG frame (in this case one AU-4)

1-

Pot- C-4 container

AUG frame (in this case one AU-4)

9 bytes 261 bytes

row 41 X (X) (X; VC-4 virtual container pointers (up to ~

3; here only one is used)

9 bytes

STM-1 frame

261 bytes

RSOH

MSOH

row 4 AUG (administrative unit group)

THE TRIBUTARIES OF SDH 273

to make larger STM frames Thus an STM-4 frame (4AUGs) has a frame size of

77760 bits, and a line rate of 622.08 Mbit/s An STM-16 frame (16AUGs) has a frame size of 31 l 040 bits, and a line rate of 2488.32 Mbit/s

Tributary unit groups ( T U G S ) and tributary units ( T U s ) provide for further break-

down of the VC-4 or VC-3 payload into lower speed tributaries, suitable for carriage of today’s T1, T3, El or E3 line rates (1.544Mbit/s, 44.736 Mbit/s, 2.048 Mbit/s or 34.368 Mbit/s)

Figure 13.6 shows the gradual build up of a C-4 container into an STM-1 frame The diagram conforms with the conventional diagrammatic representation of the STM-1 frame as a matrix of 270 columns by 9 rows of bytes The transmission of bytes, as defined by ITU-T standards is starting at the top left hand corner, working along each row from left to right in turn, from top to bottom row The structure is defined in ITU-T recommendations G.707 G.708 and G.709

The structure of an AUG comprising 3 AU-3s is similar to that for an AUG of one

AU-4, except that the area used in Figure 13.6 for VC-4 is instead broken into 3 separate areas of 87 columns, each area carrying one VC-3 (Figure 13.7) In this case all three pointers are required to indicate the start positions within the frame of the three separate VCs The various other T U and VC formats follow similar patterns to the AUs and VCs presented (TUs also include pointers like AUs) Table 13.1 presents the various

-c

row 41 1 2 3 AU-3 (1 ) AU-3 (2) AU-3 (3)

/

pointers

1 87 8% 174 175 261 AU-3 = 87 columns X 9 rows of bytes

t Y Pe frame size repetition rate PDH line type

c - l 1 193 bits 8000 T1 (1 544 kbit/s)

c - l 2 256 bits 8000 E l (2048 kbit/s)

c - 4 260 X 9 bytes 8000 139 264 kbit/s

POH

,

r1 : .86 1.,, .',:; ,! B 86 ~ ' ">' : 1

TUG-3 TUG-3 86 columns X

TUG-3 9 rows of bytes

86'

container rates available within SDH Note that the terminology C-l2 is intended to

signify the hierachical structure and should not therefore be called C-twelve, but instead

C-one-two The relevant VC is VC-one-two, etc

Figure 13.8 shows an alternative demultiplexing scheme, based upon the sub-

multiplexing of a VC-4 container into three tributary unit g r o u p 3 (TUG-3s) In this

case, the first three columns are used as path overhead, and each TUG occupies a total

TU-l 1

1 2 3

TU-l 2

1 2 3 4

1 . N., .

.

.

TU-2

l 2 3 4 5 6 7 8 9 1 0 1 1 1 2

THE TRIBUTARIES OF SDH 275

of 86 columns, but the individual TUGS are byte interleaved This sub-multiplexing

scheme lends itself better to the carriage of PDH signals

The sub-multiplexing of the TUG-3s themselves may be continued as shown in Figure 13.9, where each TUG-3 is sub-multiplexed into 7 X TUG-2, also using byte interleaving Finally, as Figure 13.9 also shows, the TUG-2s may be subdivided into

byte interleaved TU-l l tributaries (for T1 rate of 1.544Mbit/s) or TU-l2 tributaries (for E l rate of 2.048 Mbit/s)

The individual containers (C-l 1 or C-12) may be packed into the TU-l 1 (synonymous with VC-l 1) or TU-l2 (synonymous with VC-12) in one of three manners, using either

a no framing (i.e asynchronously)

a bit synchronous framing

a byte synchronous framing

The asynchronous and bit synchronous framing methods allow a certain number of bits for justzjication This enables 1.5 Mbit/s or 2 Mbit/s tributaries of an SDH transmission network to operate in conjunction with P DH or other networks running on separate clocks (i.e not running synchronously with the SDH network; we covered the subject of

justiJication in Chapter 5 ) Byte synchronous framing, in contrast, demands common clocking The advantage is the ability to directly access 64 kbit/s subchannels within the

1.5 Mbit/s or 2Mbit/s tributary using drop and insert methods (Figure 13.3) In addi-

tion, byte synchronous streams are simpler for the equipment to process

13.5 PATH OVERHEAD

Figure 13.10 illustrates the path overhead ( P O H ) formats used for creating VC-l, VC-2, VC-3 and VC-4 containers This information is added to the corresponding container

The meanings and functions of the various bits and fields are given in Table 13.2

The diagram and table of Figure 13.1 1 illustrate the constitution of the section overhead

BIP-2

FEBE

L1, L2, L3

remote alarm

J 1

B3

c 2

G1

F2

H4

2 3 , 24, z5

bit inserted parity far end block error signal label remote alarm path trace

signal label path status

path user channel

multiframe indicator bytes reserved for national network operator use

error check function indication of received BIP error indication of VC payload type indication of receiving failure to transmitting end

verification of VC-n connection error check function

indication of VC payload type and composition indication of received signal status to

transmitting end provides communication channel for network operating staff

multiframe indication reserved