SONET, used in North America, and SDH, used in Europe, are almost identical standards for the transport of data over optical media between two fixed points. They use 810-byte frames as a container for the transport of data at speeds of up to OC–192 (9.6 Gbps).

SONET/SDH is used as the bearer layer for higher-layer protocols such as ATM or IP/PPP, employed on devices that switch or route traffic to a particular end point. The functions of SONET/SDH in the broadband arena are roughly analogous to those of T1/E1 in the narrowband world. The SONET/SDH standards define the packaging of data within SONET/SDH frames, the encoding of signals on a fiber-optic cable, and the management of the SONET/SDH link.

The advantages of SONET/SDH include the following:

• rapid point-to-point transport of data with little overhead

• standards-based multiplexing of SONET/SDH datastreams

• transport independent from the services and applications that it supports

• self-healing ring structure to reroute traffic around faults within a particular link

• widely deployed transmission infrastructure within carrier networks

• TDM grooming and aggregation from the DS0 level

Figure 2. A SONET/SDH Pipe Carrying ATM