13 Synchronous Digital Hierarchy (SDH) 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. 13.1 HISTORY OF THE SYNCHRONOUS DIGITAL HIERARCHY (SDH) The synchronous digital hierarchy (SDH) 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. 13.2 THE PROBLEMS OF PDH TRANSMISSION 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) 268 SYNCHRONOUS DIGITAL HIERARCHY AND SYNCHRONOUS OPTICAL NETWORK (a) Europe 64 2048 a448 34 368 139 264 564 992 kbit/s (El 1 (E21 (E31 = X4 X4 = X4 - X4 - X32 - MUX - MUX - MUX - MUX - MUX b L hierarchy level 0 1 2 3 4 5 (b) North America 64 1544 6312 44 736 274 176 kbiffs (DSO) (DS1 or T1) (DS2 or T2) (DS3 or T3) . . 1 - 3x241 3x4 I 4x7 HX6 I , MUX MUX MUX MUX U U U U hierarchy level 0 1 2 3 4 (c) Japan 64 1544 631 2 32 064 97 728 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/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 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 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). A B C 3xE3 Note 1 =2 3x E2 Note 2 4xEl IX El B-C t ) - El El t W 63~ El A-to-C Note 1 : 3 X E3 = 12 X E2 or 48 X El after demultiplexing Note 2: 3 X E2 = 12 X El after demultiplexing a= 3x E3 3x E2 4xEl Figure 13.2 Breaking-out 2Mbit/s (El) from a 140Mbit/s line system at an intermediate exchange 270 SYNCHRONOUS DIGITAL HIERARCHY AND SYNCHRONOUS OPTICAL NETWORK A B C STM-l STM-1 drop and Insert multiplexor - - El El 62 X El Figure 13.3 Drop and insert multiplexor used to break-out 2 Mbit/s (El) from a 155 Mbit/s (STM-1) line at an intermediate exchange m standby Figure 13.4 Optical fibre back-up using PDH system components Before SDH, networks had to be built up from separate multiplex and line terminating equipment (LTE), 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. 13.3 THE MULTIPLEXING 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 THE MULTIPLEXING STRUCTURE OF SDH 271 XN X1 pointer processing c- multiplexing Figure 13.5 Synchronous digital hierarchy (SDH) multiplexing structure (ITU-T/G.709) so abbreviated), tributary units (TU), tributary unit groups (TUG), administrative units (AU) and finally administrative unit groups (AUG) into synchronous transport modules (STM). The basic building block of the SDH hierarchy is the administrative unit group (AUG). 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 PDH link of 139.264 Mbit/s. To the container is added a path overhead (POH) of 9 bytes (72 bits). This makes a virtual container (VC). 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 (AU) (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 AU 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 (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 272 SYNCHRONOUS DIGITAL HIERARCHY AND SYNCHRONOUS OPTICAL NETWORK 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 bvte 260 bytes . 1- Pot- C-4 container AUG frame (in this case one AU-4) 9 bytes 261 bytes 4 b 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) 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 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 (TUGS) and tributary units (TUs) 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. 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 to indicate the start positions within the frame of the three separate VCs. The various other TU 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 Figure 13.7 AUG frame arranged as 3 X AU-3 Table 13.1 Payload rates of SDH containers Container Container Frame Capable of carrying tY Pe frame size repetition rate PDH line type c-l 1 193 bits 8000 T1 (1 544 kbit/s) c-l2 256 bits 8000 El (2048 kbit/s) c-2 1 789 bits 8000 T2 (63 12 kbit/s) C-22 1056 bits 8000 E2 (8448 kbit/s) C-3 1 4296 bits 8000 E3 (34 368 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 DIGITAL HIERARCHY AND SYNCHRONOUS OPTICAL NETWORK POH 1 2 3.45 67s il'.:: ; ,.,. ~~~ ,:'261 , r.1 1:. 86 .,,. .',:; ,! B 86 ~. ">' 1 ' : ''Cl TUG-3 TUG-3 86 columns X TUG-3 9 rows of bytes 86' 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 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 group3 (TUG-3s). In this case, the first three columns are used as path overhead, and each TUG occupies a total TU-l 1 123 TU-l 2 1234 1 N., TU-2 l23456789101112 1 2 3 4 5 6 7 8 9 16 23 30 37 44 51 58 65 72 79 86 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-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 El 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 VC-3 / VC-4 POH (9 rows X 1 byte!8 bitsn Figure 13.10 Path overhead (POH) formats for VC-l, VC-2, VC-3 and VC-4 276 SYNCHRONOUS DIGITAL HIERARCHY AND SYNCHRONOUS OPTICAL NETWORK 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 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 the path overhead (POH) 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) The diagram and table of Figure 13.1 1 illustrate the constitution of the section overhead. Table 13.2 Meaning and function of the fields in the SDH path overhead (POH) Field Name Function BIP-2 FEBE L1, L2, L3 remote alarm J1 B3 c2 G1 F2 H4 23, 24, z5 bit inserted parity far end block error signal label remote alarm path trace BIP-8 parity code 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