Synchronous Digital Hierarchy SDH

SDH defined by European Telecommunications standards Institute (ETSI) for Europe is now used everywhere outside of North America and Japan. SDH provides transmission networks with a vendor-independent and sophisticated signal structure that has a rich feature set. This has resulted in new network applications, the deployment of new equipment in new network topologies, and management by operations systems of much greater power than the earlier transmission networks. The original SDH standard defined the transport of 1.5/2/6/34/45/140 Mbps within a transmission rate of 155.52 Mbps and is being developed to carry other types of traffic such as ATM and IP, within the rates that are integer multiples of 155.52 Mbps.

SDH was designed to allow for flexibility in the creation of products for electronically routing telecommunication traffic. The key products are optical line system, radio-relay system, terminal multiplexers, ADM, hub multiplexers and digital cross-connect switches.

SDH Frame. The SDH forms a multiplexing rate based on the STM-N (synchronous transmission module) frame format. The frame has a repetitive structure with a period of 125 ^ sec. The basic STM-1 frame consists of 270 columns and 9 rows (2430 octets). 9 columns and 9 rows are for section overhead (81 octets). The remaining 2349 octets create the pay load.

Higher rate (STM-N) frames are derived from multiples of STM-1 according to the value of N. The first nine columns contain the section overhead (SOH) for transport-support features such as framing, management operations channels and error monitoring, with the first segment

Trans mi sgion Weiworits 257

Containing the frame word for demultiplexer aliment. The remain in^ columns can be assigned in many ways to carry lower bit rate signals. The administrative unit (AU> is the nnit of provision for bandwidth in the main networks. Its capacity can be used to carry a high bit rate signal lALi-2-15 Mbps, AU-4-140 Mbpsl. AU can be Further divided to carry lower rate signals.

Figure 10.28 shows the STM-1 frame formats.

T 9 10 27Q

T 9 10 27Q

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Repeater

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