SONET is a high-speed fiber-optic wide area network. On the OSI model, it is part of the physical layer. Bell Labs developed the American version of SONET, which incorporated the ITU standard Synchronous Digital Hierarchy (SDH). In the United States, the system is called SDH-SONET, in Europe it ia called SDH-Europe, and in Japan it is called SDH-Japan. The differences arise out of the configuration and implementation of local telephone services.

SONET can work with mesh or ring network topologies. It provides point-to-point communications similar to FDDI. To increase the bandwidth of the system, Time Domain Multiplexing (TDM) is used.

Starting rates for SONET begin at 51.84Mbps and go up in multiples of this speed. Speed ratings for SONET are expressed on the OC scale. The higher the rating, the more optical channels are available. With such a wide bandwidth and high speed, large city-wide SONET rings can be constructed to transport video, movies, sound and imaging. SONET can carry data more efficiently than tradition broadband cable installations. The initial construction costs and equipment can be daunting, but institutions may save money in the future by expanding resources. An example of this is the SONET ring being developed for the colleges of Colorado. Full-motion video-on-demand can broadcast classes and on-line courses. Libraries can share multiple copies of books. Radio stations can broadcast perfect digital audio.

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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