Synchronous optical networking

Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or light-emitting diodes (LEDs). Lower rates can also be transferred via an electrical interface. 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 fibre wire without synchronization problems. SONET generic criteria are detailed in Telcordia Technologies Generic Requirements document GR-253-CORE. Generic criteria applicable to SONET and other transmission systems (e.g., asynchronous fiber optic systems or digital radio systems) are found in Telcordia GR-499-CORE.
SONET and SDH were originally designed to transport circuit mode communications (e.g., T1, T3) from a variety of different sources. The primary difficulty in doing this prior to SONET was that the synchronization sources of these different circuits were different. This meant each circuit was actually operating at a slightly different rate and with different phase. SONET allowed for the simultaneous transport of many different 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 neutrality and transport-oriented features, SONET was the obvious choice for transporting asynchronous transfer mode (ATM) frames. It quickly evolved mapping structures and concatenated payload containers 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 are 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 is considered the variation.
The two protocols are standardized according to the following:
SDH or synchronous digital hierarchy standard was originally defined by the ETSI or European Telecommunications Standards Institute
SONET or synchronous optical networking standard as defined by GR-253-CORE from Telcordia and T1.105 from American National Standards Institute.