STS-3c/STM-1 which operates three times faster than STS-1

In packet-oriented data transmission such as Ethernet, a packet frame usually consists of a header and a payload. The header is transmitted first, followed by the payload (and possibly a trailer, such as a CRC). In synchronous optical networking, this is modified slightly. The header is termed the overhead and instead of being transmitted before the payload, is interleaved with it during transmission.

In the case of an STS-1, the frame is 810 octets in size while the STM-1/STS-3c frame is 2430 octets in size. For STS-1, the frame is transmitted as 3 octets of overhead, followed by 87 octets of payload. This is repeated nine times over until 810 octets have been transmitted, taking 125 microseconds. In the case of an STS-3c/STM-1 which operates three times faster than STS-1, 9 octets of overhead are transmitted, followed by 261 octets of payload.

This is also repeated nine times over until 2,430 octets have been transmitted, also taking 125 microseconds. For both SONET and SDH, this is normally represented by the frame being displayed graphically as a block: of 90 columns and 9 rows for STS-1; and 270 columns and 9 rows for STM1/STS-3c. This representation aligns all the overhead columns, so the overhead appears as a contiguous block, as does the payload.

The internal structure of the overhead and payload within the frame differs slightly between SONET and SDH, and different terms are used in the standards to describe these structures. Their standards are extremely similar in implementation making it easy to interoperate between SDH and SONET at particular bandwidths.In practice, the terms STS-1 and OC-1 are sometimes used interchangeably, though the OC-N format refers to the signal in its optical form. It is therefore incorrect to say that an OC-3 contains 3 OC-1s: an OC-3 can be said to contain 3 STS-1s.

September 20, 2011

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