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** Synchronous optical networking **

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*Synchronous Optical Networking* (*SONET*) and *Synchronous Digital
Hierarchy* (*SDH*) are standardized protocols that transfer multiple
digital bit streams synchronously 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.^[1] 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.^[2]

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.^[3] 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.
SONET/SDH is not a communications protocol in itself, but a transport

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


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