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An Introduction to Synchronous Signals and Networks 292 Wide Area Networks
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type of system is often referred to as a Plesiochronous Digital Hierarchy (PDH) Figure 131 depicts a network built upon PDH, with multiplexers at each node PDH networks were developed at a time when point-to-point transmission was the predominant network requirement To support this requirement, the standard approach to network management and maintenance was to use manual distribution frames for access to individual signals By the late 1980s this scenario was out-ofdate In addition, the PDH networks then in place had been found to severely limit the ability of the network operators to respond to the demands of an evolving telecommunications market PDH networks are limited because:
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They are inflexible and expensive for telecommunications networking, They offer extremely limited network management and maintenance support capabilities, and Higher-rate line systems were proprietary
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For telecommunications networking purposes, flexibility is assessed in terms of how accessible an individual tributary signal on a particular line system is, so that it may be rerouted PDH high-capacity line systems are not viewed favorably in this respect because access to any tributary signal cannot be obtained without demultiplexing the whole line signal, step-by-step, down to the appropriate level From a cost perspective, gaining access to and rerouting a tributary signal covered only half of the equipment bill; the other half was incurred after rerouting, in remultiplexing
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Figure 131 Example PDH network
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An Introduction to Synchronous Signals and Networks An Introduction to Synchronous Signals and Networks 293
step-by-step back into the line signal for transmission This makes plesiochronous multiplexing technology an expensive solution for telecommunications networking When originally conceived, network management and maintenance practices in PDH high-capacity networks were based on manual signal crossconnection and outof-service testing techniques There was no need to add extra capacity to the frame structures of the multiplexed signals for management and maintenance As the complexity of the networks increased, however, and automatic computer management techniques became available, the lack of spare signal capacity in these signal frame structures severely limited the improvements that could be made A further limitation of PDH high-capacity line systems was that there was no common standard Individual manufacturers of network equipment had their own proprietary designs Both ends of the line system therefore had to be purchased from the same manufacturer There was no possibility of interworking among components supplied by different manufacturers 133 The Synchronous Network The arrival of optical fiber as a transmission medium led to a rapid rise in achievable data rates and consequent available bandwidth within the telecommunications network Coupled with the proliferation of automatic (microprocessor) control within the network, these developments opened the prospect of building extremely flexible and complex networks operating at high data rates The limitations of PDH systems meant that it would have been very expensive, if not impossible, to take full advantage of these changes using the existing techniques To answer this need, committees within ANSI and ITU-T in the mid-1980s started to define a new network standard The objective was to produce a worldwide standard for synchronous transmission systems that would provide network operators with a flexible and economical network In 1985 the ANSI-accredited T1X1 committee started work on the Synchronous Optical Network (SONET) standard for North America; in June of 1986, ITU-T s Study Group XVIII started work on the Synchronous Digital Hierarchy (SDH) standards SDH is the recognized international standard The two standards were first published in 1988 and are broadly similar, the major difference being the base data rate used for multiplexing (see section 136) Unless otherwise stated, the information in this chapter applies to both standards A list of documents relevant to each standard can be found in section 13131 near the end of the chapter The synchronous standards that were defined have the following advantages over the previous PDH standards:
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