STT Scalable Network Test Solution In higher OC rates, generally only the Section and Line overhead in the first STS is utilized. The rest is ignored. Each SPE within the OC-n signal has independent Path overhead. This figure shows the labels of the overhead, which will be gone into in more detail in the following sections. 4RANSPORT /VERHEAD
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Figure 124 SONET Overhead Bytes
Further Section Overhead Definitions • The framing bytes, A1 and A2, provide a frame alignment pattern (11110110 00101000, binary, F6 28 hex). • The B1 parity check byte provides Section error monitoring. It uses a bit-interleaved parity 8 code (BIP-8), with even parity. • The E1 Section orderwire byte provides for voice communications among regenerators, hubs, and remote terminal locations. • The F1 byte is the Section User Channel, for user’s purposes. It is terminated at all Section level equipment. • The last three Section OH bytes, D1-D3, proved a data communications channel for Operations, Administration, Maintenance, and Provisioning (OAM&P).
Line Overhead The three bytes H1-H3 facilitate the operation of the STS-1 payload pointer. The payload pointer is involved with synchronization of SONET. Ideally, all synchronous network elements should derive their timing signal from the same master network clock. However, current synchronized network timing schemes allow for the existence of more than one master clock. SONET uses pointers to compensate for frequency phase variations caused by multiple timing sources. Pointers enable the transparent transport of synchronous payload envelopes across plesiochronous boundaries. This means the SPE can be switched and transported though SONET without having to be examined an demultiplexed at intermediate nodes. The use of pointers avoids the delays and loss of data associated with the use of large (125 µs frame) slip buffers for synchronization. This permits the ease of dropping, inserting, and cross-connecting these payloads in the network. The pointer 170