ANSI ratified the SONET standard in 1988 [1]. In 1989, CCITT (now ITU-T)
standardized the Synchronous Digital Hierarchy (SDH) [2], which is optimized to carry
E1 signals (2.048Mbps), but is in most other ways identical to SONET.
Synchronous Transport Signal-1 (STS-1) is the fundamental signal structure for
SONET. The bytes of the STS-1 may be represented by a 90-column?—9-row structure;
the first three columns (27 bytes) contain the transport overhead, whereas the remaining
87 columns (783 bytes) carry the STS payload. This structure is transmitted every 125 ?µs,
resulting in a bit rate of 51.840Mbps.
The Synchronous Payload Envelope (SPE) is an 87?—9-byte structure that occupies
the STS payload. The SPE has its own overhead, the Path OverHead (POH). An STS-1
SPE carries a single DS3 (44.736Mbps) or up to 28 DS1s. Generally, the SPE will not
align with STS-1 boundaries. A mechanism called a pointer (a byte in the STS-1 transport
overhead) indicates where the SPE begins inside the STS payload. The pointer
mechanism provides a simple, elegant way for SONET to map plesiochronous DS3
or DS1 signals into a synchronous SONET payload.
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