Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standard multiplexing protocols used to transfer multiple digital bit streams over optical networks. This 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 without synchronization problems.

SONET and SDH, were designed to transport circuit mode communications from a variety of different sources, to support real-time, uncompressed, circuit-switched voice encoded in PCM format. Prior to SONET/SDH this was not possible because the synchronization sources for these 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.

Due to SONET/SDH’s essential protocol neutrality and transport-oriented features, SONET/SDH was the obvious choice for transporting the fixed length Asynchronous Transfer Mode (ATM) frames also known as cells. It quickly evolved mapping structures and concatenated payload containers to transport ATM connections. In other words, for ATM (and eventually other protocols such as Ethernet), the internal complex structure previously used to transport circuit-oriented connections was removed and replaced with a large and concatenated frame (such as STS-3c) into which ATM cells, IP packets, or Ethernet frames are placed.

SONET is a set of transport containers that allow for delivery of a variety of protocols, including traditional telephony, ATM, Ethernet, and TCP/IP traffic. SONET is not a separate communication protocol in itself. The basic unit of transmission over the SONET is STM-1 with a bandwidth of 155.52 Mb/s, also referred as STS-3.

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