Optical Networks - _6_1 SONET_SDH_74

The path layer in sonet and sdh is responsible for

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The path layer in SONET (and SDH) is responsible for end-to-end connections between nodes and is terminated only at the ends of a SONET connection. It is possible that intermediate nodes may do performance monitoring of the path layer
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6.1 SONET/SDH 379 Regenerator TM ADM TM Section Section Section Line Line Path Connection Figure 6.4 SONET/SDH layers showing terminations of the path, line, and section layers for a sample connection passing through terminal multiplexers (TMs) and add/drop multiplexers (ADMs). The physical layer is not shown. signals, but the path overhead itself is inserted at the source node of the connection and terminated at the destination node. Each connection traverses a set of links and intermediate nodes in the network. The line layer ( multiplex section layer in SDH) multiplexes a number of path-layer connections onto a single link between two nodes. Thus the line layer is terminated at each intermediate line terminal multiplexer (TM) or add/drop multiplexer (ADM) along the route of a SONET connection. The line layer is also responsible for per- forming certain types of protection switching to restore service in the event of a line failure. Each link consists of a number of sections, corresponding to link segments be- tween regenerators. The section layer ( regenerator-section layer in SDH) is terminated at each regenerator in the network. Finally, the physical layer is responsible for actual transmission of bits across the fiber. 6.1.4 SONET Frame Structure Figure 6.5 shows the structure of an STS-1 frame. A frame is 125 μ s in duration (which corresponds to a rate of 8000 frames/s), regardless of the bit rate of the SONET signal. This time is set by the 8 kHz sampling rate of a voice circuit. The frame is a specific sequence of 810 bytes, including specific bytes allocated to carry overhead information and other bytes carrying the payload. We can visualize this frame as consisting of 9 rows and 90 columns, with each cell holding an 8-bit byte.
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380 Client Layers of the Optical Layer B B B 87B 9 rows 90 columns Transport overhead STS-1 envelope capacity 125 s m Figure 6.5 Structure of an STS-1 frame. B denotes an 8-bit byte. The bytes are transmitted row by row, from left to right, with the most significant bit in each byte being transmitted first. The first three columns are reserved for section and line overhead bytes. The remaining bytes carry the STS-1 SPE. The STS-1 SPE itself includes one column of overhead bytes for carrying the path overhead. An STS- N frame is obtained by byte-interleaving N STS-1 frames, as shown in Figure 6.6. The transport overheads are in the first 3 N columns, and the remaining 87 N columns contain the payload. The transport overheads need to be frame aligned before they are interleaved. However, because each STS-1 has an associated payload pointer to indicate the location of its SPE, the payloads do not have to be frame aligned. An STS- N c frame looks like an STS- N frame, except that the payload cannot be broken up into lower-speed signals in the SONET layer. The same 87 N columns contain the payload, and special values in the STS-payload pointers are used to indicate that the payload is concatenated.
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  • Spring '09
  • Boussert
  • Bit rate, Synchronous optical networking, SLM SLM SLM

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