SONET - Synchronous Optical Networking / SDH - Synchronous Digital Hierarchy

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 highly coherent light from light-emitting diodes (LEDs). At low transmission rates data can also be transferred via an electrical interface. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large amounts of telephone calls and data traffic over the same fiber 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, which are essentially the same, were originally designed to transport circuit mode communications (e.g., DS1, DS3) from a variety of different sources, but they were primarily designed to support real-time, uncompressed, circuit-switched voice encoded in PCM format. The primary difficulty in doing this prior to SONET/SDH was that the synchronization sources of these various circuits were different. This meant that each circuit was actually operating at a slightly different rate and with different phase. SONET/SDH allowed for the simultaneous transport of many different circuits of differing origin within a single framing protocol.



Both SDH and SONET are widely used today: SONET in the United States and Canada, and SDH in the rest of the world. Although the SONET standards were developed before SDH, it is considered a variation of SDH because of SDH's greater worldwide market penetration.



The SDH standard was originally defined by the European Telecommunications Standards Institute (ETSI), and is formalized as International Telecommunications Union (ITU) standards G.707, G.783, G.784, and G.803. The SONET standard was defined by Telcordia and American National Standards Institute (ANSI) standard T1.105.



Advantages of SONET:

• Reduction in equipment requirements and an increase in network reliability.

• Allow transmission of data at higher speeds (50 Mbps+)

• Access to low-level signals directly

• SONET is synchronous. This requires a constant timing source for both the receiving and transmitting of traffic.

• It has a flexible architecture capable of accommodating future applications with a variety of transmission rates.

Reference:

http://en.wikipedia.org/wiki/Synchronous_optical_networking

http://www.fiberoptic.com/adt_sonet_sdh.htm