Optical Networks - _6_1 SONET_SDH_74

Figure 67 shows the overhead bytes in an sts 1 frame

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used to indicate that the payload is concatenated. Figure 6.7 shows the overhead bytes in an STS-1 frame or an STS- N c frame. In an STS- N frame, there are N sets of overhead bytes, one for each STS-1. Each STS-1 has its own set of section and line overheads. An STS- N c, on the other hand, has only a single set of overhead bytes, due to the fact that its payload has to be carried intact from its source to its destination with the SONET network. We cover the overhead bytes here because they provide some key management functions that make SONET so attractive for network operators. In the following discussion, the actual locations and formatting of the bytes are not as important as understanding the functions they perform. We will look at these functions in more
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6.1 SONET/SDH 381 9 rows Transport overhead STS- envelope capacity N 125 s m N + 90 columns N + 3 columns Figure 6.6 Structure of an STS- N frame, which is obtained by byte-interleaving N STS-1 frames. detail in the context of the optical layer in Chapter 8. The section and line overheads in particular are of great interest to the optical layer. Some if not all these bytes are monitored by optical layer equipment. In addition, some of the overhead bytes are currently undefined, and these bytes are now being considered as possible candidates to carry optical layer overhead information. We will discuss this aspect in more detail in Chapter 8. For a more detailed description of the overhead bytes, see [Tel99]. Section Overhead Framing (A1/A2). These two bytes are used for delineating the frame and are set to prespecified values in each STS-1 within an STS- N . Network elements use these bytes to determine the start of a new frame. Section Trace(J0)/Section Growth(Z0). The J0 byte is present in the first STS-1 in an STS- N and is used to carry an identifier, which can be monitored to verify connectivity between adjacent section-terminating nodes in the network. The Z0 byte is present in the remaining STS-1s, and its use is still to be determined. Section BIP-8 (B1). This byte is located in the first STS-1 in an STS- N and is used to monitor the bit error rate performance of each section. The byte locations in the remaining frames within an STS- N are currently undefined. The transmitter computes a bit interleaved parity (BIP) computed over all bytes in the previous STS- N frame after scrambling and places it in the B1 byte of the current frame
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382 Client Layers of the Optical Layer Framing A2 Orderwire E1/undefined Datacom D2/undefined Pointer H2 APS K1/undefined Datacom D5/undefined Datacom D8/undefined Datacom D11/undefined REI-L/Growth M0 or M1/Z2 Trace/Growth J0/Z0 User F1/undefined Datacom D3/undefined Pointer H3 APS K2/undefined Datacom D6/undefined Datacom D9/undefined Datacom D12/undefined E2/undefined Orderwire Framing A1 BIP-8 B1/undefined Datacom D1/undefined Pointer H1 BIP-8 B2 Datacom D7/undefined Datacom D4/undefined Datacom D10/undefined Sync status/Growth S1/Z1 Trace J1 BIP-8 B3 Signal label C2 Path status G1 User channel F2 Indicator H4 Growth Z3 Tandem connection Z5 Z4 Growth Section overhead Line overhead Path overhead Figure 6.7 SONET overhead bytes. Entries of the form X/Y indicate that the first label
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  • Spring '09
  • Boussert
  • Bit rate, Synchronous optical networking, SLM SLM SLM

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