Synchronous optical networking (SONET)

Synchronous optical networking (SONET) and Synchronous Digital Hierarchy (SDH), are two closely related multiplexing protocols for transferring multiple digital bit streams using lasers or light-emitting diodes (LEDs) over the same optical fiber. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting larger amounts of telephone calls and data traffic over the same fiber wire without synchronization problems.

SONET and SDH were originally designed to transport circuit mode communications (eg, T1, T3) from a variety of different sources. The primary difficulty in doing this prior to SONET was that the synchronization source of these different circuits were different, meaning each circuit was actually operating at a slightly different rate and with different phase. SONET allowed for the simultaneous transport of many differnet circuits of differing origin within one single framing protocol. In a sense, then, SONET is not itself a communications protocol per se, but a transport protocol.

Due to SONET's essential protocol neturality and transport-oriented features, SONET was the obvious choice for transporting ATM (Asynchronous Transfer Mode) frames, and so quickly evolved mapping structures and concatenated payload containers so as to transport ATM connections. In other words, for ATM (and eventually other protocols such as TCP/IP and ethernet), the internal complex structure previously used to transport circuit-oriented connections is removed, and replaced with a large and concatenated frame (such as STS-3c) into which ATM frames, IP packets, or ethernet is placed.

Both SDH and SONET are widely used today: SONET in the U.S. and Canada and SDH in the rest of the world. Although the SONET standards were developed before SDH, their relative penetrations in the worldwide market dictate that SONET now is considered the variation.
The two protocols are standardized according to the following:

SDH or Synchronous Digital Hierarchy standard developed by the International Telecommunication Union (ITU), documented in standard G.707 and its extension G.708

SONET or Synchronous Optical Networking standard as defined by GR-253-CORE from Telcordia and T1.105 from American National Standards Institute