Bellcore developed SONET in 1985 for a set of interfaces used in LEC/IXC optical networks in the US.

ITU (CCITT) recommendation I.121 says that ATM can be supported with an appropriate digital transmission system, and lists SONET G.707, 708, 709.

The line rate, that is, the speed at which the bits are transferred over a SONET line, is higher than the nett speed of the incoming digital flow. For example, for incoming digital flows of 140 Mb/s and 565 Mb/s, the speed on the optical carrier is 155.52 Mb/s and 622.08 Mb/s, resp. Additional information that exists in the header (payload pointers) enables direct access to lower levels (channels) without the need for sequential demultiplexing. Such operations result in considerable transport, technological and economical advantages.

Common SONET rates are:

  • OC-1 (51.84 Mbps),
  • OC-3 (155.52 Mbps),
  • OC-12 (622.08 Mbps),
  • OC-24 (1244.16 Mbps) and
  • OC-48 (2488.32 Mbps, 2.5 Gbps).

A bit less common are:

  • OC-192 (9953 Mbps, 10 Gbps),
  • OC-768 (39813 Mbps, 40 Gbps),
  • OC-1536 (79626 Mbps, 80 Gbps),
  • OC-3072 (159 Gbps),
  • etc.

In SDH, these rates are referred to as STS rates.

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