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Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network (SONET) The synchronous hierarchy digital (SDH) is emergingthe as universal technology for transmission telecommunications in networks. the publication Since first of international standards by ITU-T in 1989, SDH equipment has been rapidly developedand deployed across the world,andisrapidlytakingoverfromitspredecessor,thePlesiochronousDigitalHierarchy (PDH).ThischapterdescribesSDHandtheNorthAmericanequivalentofSDH,SONET (SynchronousOpticalNetwork),fromwhichitgrew...

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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) 13 Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network (SONET) Thesynchronous hierarchy digital (SDH) is emergingthe as universal technology for transmission telecommunications in networks. the publication Since first of international standards by ITU-T in 1989, SDH equipment has been rapidly developedand deployed across the world,andisrapidlytakingoverfromitspredecessor,thePlesiochronousDigitalHierarchy (PDH).ThischapterdescribesSDHandtheNorthAmericanequivalentofSDH,SONET (SynchronousOpticalNetwork),fromwhichitgrew.Inparticular,thechapterdescribesthe features of SDH which characterize its advantages over PDH. 13.1 HISTORY OF THESYNCHRONOUS DIGITAL HIERARCHY (SDH) The synchronousdigitalhierarchy ( S D H ) was developedfromits 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. 13.2 THEPROBLEMS OF PDHTRANSMISSION Historically, digital telephone networks, modern data networks and the transmission infrastructures serving themhave been based on atechnology 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
268 SYNCHRONOUS HIERARCHY DIGITAL AND SYNCHRONOUS NETWORK OPTICAL (a) Europe 992 564 264 139 368 34 a448 2048 64 kbit/s (El1 (E31 (E21 = X32 - X4 - X4 = X4 X4 - MUX - MUX - MUX - MUX - MUX b L hierarchy level 0 1 2 3 4 5 (b) North America 176 274 736 44 6312 64 1544 kbiffs (DSO) . (DS1 .I 4 x 7 3 x 2 4 13 x 4 or T1) (DS2 or T2) (DS3 1 or T3) H -I X 6 , MUX MUX MUX MUX U U U U hierarchy level 0 1 2 3 4 (c) Japan 728 97 064 32 2 631 64 1544 kbiffs X 24 X4 X5 X3 MUX MUX MUX MUX b hierarchy level 0 1 2 3 4 Figure 13.1 The various plesiochronous multiplexing hierarchies (ITU-T/G.571) They are all based on the needs of telephone networks, i.e. offering integral multiples of 64 kbit/schannels,synchronized to someextent at thefirstmultiplexing level (1.5 Mbit/s or 2 Mbit/s). Theyrequiremultiplemultiplexingstages to reachthehigherbitrates, andare 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 is not locked synchronously in step (it free- but is 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 three bits or more or less than another. If the system running slightly fasteris 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). Extraframing bits areaddedat eachstageof thePDH multiplexingprocess. Unfortunatelythismeansthatthe efficiency of thehigherorder line systems (e.g. 139264kbit/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 PDH 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 amultiplexingtechnique 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). A B C Note 3xE3 1 =2 a= 3x E3 3x E2 3x E2 Note 2 - 4xEl 4xEl IX B-C El t - - - - - - - - ) ElEl t W 6 3 ~ A-to-C El 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 Figure 13.2 Breaking-out2Mbit/s (El) from140Mbit/s a line system at intermediate an exchange
270 SYNCHRONOUS HIERARCHY DIGITAL AND SYNCHRONOUS NETWORK OPTICAL A B C STM-l STM-1 drop and Insert - - multiplexor El El 62 X El Figure 13.3 Drop and insertmultiplexorused to break-out 2 Mbit/s (El) from a 155 Mbit/s (STM-1) line at an intermediate exchange Figure 13.4 Optical fibre back-up using m standby PDH system components Before SDH, networks had to built up from separatemultiplex and line terminating be equipment (LTE),as the optical equipment interfaces in particular were manufacturer- + specific (i.e. proprietary). Back-uptended 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. 13.3 THEMULTIPLEXING STRUCTURE OF SDH 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
STRUCTURE THE MULTIPLEXING OF SDH 271 XN X 1 pointer processing c- multiplexing Figure 13.5 Synchronous digital hierarchy (SDH) multiplexing structure (ITU-T/G.709) so abbreviated), tributary units ( T U ) , tributary groups unit ( T U G ) , administrative units ( A U ) and finally administrativeunitgroups ( A U G ) into synchronoustransport modules ( S T M ) . The basicbuilding block of the SDH hierarchy is the administrativeunitgroup ( 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, theframe 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 transporta PDH 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 STM-1 frame (of19 440 bits). The SOHis added toprovide forblock framing and for the maintenance and performance information carried on a transmission section line
272 SYNCHRONOUS HIERARCHY DIGITAL AND SYNCHRONOUS NETWORK OPTICAL basis. (Asection is an administratively defined point-to-point connectionin the network, typically an SDH-system between two major exchange sites, between two intermediate multiplexors or simply betweentwo 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 interpretedby 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) 260 1 bvte bytes . 1- Pot- C-4 container AUG frame (in this case one AU-4) 9 bytes bytes 261 4 b row 4 X 1 (X) (X; VC-4 virtual container pointers (up to ~ 3; here only one is used) STM-1 frame 9 bytes 261 bytes RSOH row 4 AUG (administrative unit group) MSOH Figure 13.6 Basic structure of an STM-I frame
THE TRIBUTARIES OF SDH 273 to make larger STM frames. Thus an STM-4 frame (4AUGs) has aframe size of 77760 bits, and a line rate of 622.08 Mbit/s. An STM-16 frame (16AUGs) has frame a size of 31 l 040 bits, and a line rate of 2488.32 Mbit/s. Tributary unit groups (TUGS) and tributary units (TUs) provide for further break- down of the VC-4 or VC-3 payload intolower speed tributaries, suitable for carriageof today’s T1, T3, El or E3 line rates(1.544Mbit/s, 44.736 Mbit/s, 2.048 Mbit/sor 34.368 Mbit/s). Figure 13.6 shows the gradual build up of a C-4 container intoan 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 topleft 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. 13.4 THE TRIBUTARIES OF SDH 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 indicate the start positions within the frameof the three separate to VCs. The various other U and VC formats follow similar patterns to the AUs and T VCs presented (TUs also includepointers like AUs). Table 13.1 presents various the -c row 41 AU-32 1 (1 3 ) AU-3 (2) AU-3 (3) / pointers 1 87 8% 261 175 174 AU-3 = 87 columns X 9 rows of bytes Figure 13.7 AUG frame arranged as 3 X AU-3 Table 13.1 Payloadrates of SDH containers Container Container Frame Capable of carrying tYPe frame size repetition rate PDH line type c-l 1 193 bits 8000 544 (1 T1 kbit/s) c-l2 256 bits 8000 E l (2048 kbit/s) c-21 789 bits 8000 T2 (63 12 kbit/s) C-22 1056 bits 8000 E2 (8448 kbit/s) C-3 1 4296 bits 8000 (34E3 368 kbit/s) C-32 5592 bits 8000 T3 (44 736 kbit/s) c-4 260 X 9 bytes 8000 139 264 kbit/s
274 SYNCHRONOUS HIERARCHY DIGITAL AND SYNCHRONOUS NETWORK OPTICAL ,! POH r ''Cl .1 .... 1 2 3.45 6 7 s il'.::..;..,.,. .......... ~~~ . . . . . . ,:'261 .... ....... .... ... .... .... ..,.... . . . . .. . . .. . . . .. ... ..... ... . . . . ... '.. : 1 . : 86 .,,. .',:; B 86 ~. ">'.... 1 86'.. ....... .... 86 columns X TUG-3 9 rows ofTUG-3 bytes TUG-3 Figure 13.8 VC-4 submultiplexing scheme as 3 X TUG-3 using byte interleaving container rates available within SDH. Note that the terminology C-l2 is intended to signify the hierachical structure and should not therefore called C-twelve, but instead be C-one-two. The relevant VC is VC-one-two, etc. Figure 13.8 shows an alternative demultiplexing scheme,based upon thesub- 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 TU-l 2 TU-2 123 1234 l23456789101112 1..N., .. . ... . . . . . . . . ... ... ... ... ... .. . .. .. . .. .. . 1 2 3 4 5 6 7 8......16......23......30......37...... ......51...... ......65...... ......79.................86 9 44 58 72 Figure 13.9 TUG-3 submultiplexing into 7 X TUG-2; TUG-2 submultiplexing
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-multiplexingofthe TUG-3s themselves may be continuedasshown 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 (synonymous 1 with VC-l 1) or TU-l2(synonymous with VC-12) in one of three manners, using either a noframing (i.e. asynchronously) a bitsynchronous framing a bytesynchronousframing VC-3 / VC-4 POH (9rows X 1 byte!8 bitsn Figure 13.10 Path overhead (POH) formats for VC-l, VC-2, VC-3 and VC-4
276 SYNCHRONOUS HIERARCHY DIGITAL AND SYNCHRONOUS NETWORK OPTICAL The asynchronous and bit synchronous framing methods allow a certain number of bits of for justzjication. This enables 1.5 Mbit/s or 2 Mbit/s tributaries an SDH transmission network to operate in conjunction with PDH 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 thepath 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. 13.6 SECTION OVERHEAD (SOH) Thediagramandtable of Figure 13.1 illustratetheconstitution 1 of thesection overhead. Table 13.2 Meaning and function of the fields in the SDH path overhead (POH) Field Name Function BIP-2 bit inserted parity error check function FEBE far end block error indication of received BIP error L1, L2, L3 signal label indication of VC payload type remote alarm remote alarm indication of receiving failure to transmitting end J1 path trace verification of VC-n connection B3 BIP-8 parity code error check function c2 signal label indication of VC payload type and composition G1 path status indication of received signal status to transmitting end F2 path user channel provides communication channel for network operating staff H4 multiframe indicator multiframe indication 2 3 , 24, z5 bytes reserved for national reserved network operator use
NETWORK TOPOLOGY OF SDH NETWORKS 277 * Row 4 i used for the AUG .frame pointers s Field Function AI, A2 framing Bl, B2 parity check for error detection c1 identifies STM-1 in STM-n frame Dl-D12 data communications channel (DCC - for network management use) El, E2 orderwire channels (voice channels for technicians) F1 user channel K1, K2 automatic protection switchins channel (APS) z1,z2 reserved Figure 13.11 The SDH sectionoverhead (SOH) 13.7 NETWORK TOPOLOGY OF SDH NETWORKS SDH equipment is designed to be used in the construction of synchronous (in par- A ticular, optical fibre) transmission networks in redundant ring topologies. number of specific equipment types are foreseen by the standards as the building blocks of such networks. These are illustrated in Figure 12.12. SDH multiplexorsallow 2 Mbit/sandothersubSTM-1ratetributariesto be multiplexed for carriage by an SDH network. Drop and insert multiplexors (also called
278 SYNCHRONOUS HIERARCHY DIGITAL AND SYNCHRONOUS OPTICAL NETWORK ADM=add/drop multiplexor (drop and inserl multiplexor) STM-4 MUX=multiplexor Ring crossconnect DXGdigital Figure 13.12 Ring topology and generic equipment types used in SDH networks addldrop multiplexors ( A D M ) ) allowtributariesto be removedfromthe line at an intermediate station without complete demultiplexing (as we discussed in Figures 13.2 and 13.3). Crossconnectors (or DXC, digitalcrossconnectors) allow forthe flexible interconnection and reconfiguration of tributaries between separate sub-networks or rings. STM-4 and STM-16 multiplexors allow concentration of STM-1 signals onto high speed 622 Mbit/s (STM-4) or 2.5 Gbit/s (STM-16) backbone networks. Structuring of the network in interconnected rings allows for easy back-up (restora- tion) of failed connections in the network. In ahighly meshed network n : 1 (as opposed to 1 : 1) restoration is possible by choosing any alternative route to the destination. In simpler networks and single rings a 1 : 1 restoration may be possible, but leaves at least 50% of the capacity unused for most of the time. N : 1 restoration is useful in reducing the amount of normally unused plant (and thus costs) in cases where failures are rare (see Chapter 37). 13.8 OPTICAL INTERFACES FOR SDH ITU-T recommendation G.957 covers the optical interfaces defined for use with SDH, according to the light wavelength to be used and the application. A number of SDH system types are defined (Table 13.3). 13.9 MANAGEMENT OF SDH NETWORKS Compared with PDH networks, SDH networks more are efficient and easier to administrate (due to the availability of drop and insert (addldrop) multiplexors). Using
CHRONOUS SONET 279 Table 13.3 Classification of optical fibre interfaces for SDH equipment Application Inter-office Intra- office Shorthaul Longhaul Wavelength nominal/nm 1310 1550 1310 1550 1310 Fibre type G.653G.652 G.652G.652 G.652 (3.652 (3.654 Distance kilometres
280 SYNCHRONOUS HIERARCHY DIGITAL AND SYNCHRONOUS NETWORK OPTICAL Table 13.4 Comparison of SDH and SONET hierarchies North American SONET Carried Bitrate/Mbit/s SDH VT 1.5 1 S44 VC-I 1 VT 2.0 2.048 VC-l2 VT 3.0 3.152 - VT 6.0 6.312 VC-21 8.448 VC-22 34.368 VC-3 1 44.736 VC-32 - 149.76 VC-4 STS- 1(OC- 1) 51.84 STS-3 (OC-3) 155.52 STM- 1 STS-6 (OC-6) 31 1.04 - STS-9 (OC-9) 466.56 STS- 12 (OC- 12) 622.08 STM-4 STS- 18 (OC- 18) 933.12 STS-24 (OC-24) 1244.16 STS-36 (OC-36) 1866.24 STS-48 (OC-48) 2488.32 STM- 16 STS-96 (OC-96) 4976.64 - STS-192 (OC-192) 9953.28 STM-64 13.11 SDH AND ATM (ASYNCHRONOUS TRANSFER MODE) Finally, it is worth mentioning here the integration of SDH into the specifications for broadband-ISDN (B-ZSDN) and ATM (asynchronous transfer mode). These techniques will form the basis of future broadband networks. The C-4containermaybe used directly for carriage of ATM, and be one of the standard speeds at which ATM will will be used. ATM cells (of 53 bytes or octets) do notfit an integral numberof times into the C-4 frame (2340 bytes), but this is not important. The SDH standards require only that the ATM octets are aligned with the bytes of the SDH container.Individual ATM cells can be split between container frames when necessary. We return to B-ISDN and ATM in Chapters 25 and 26.