Overview#
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 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.
SONET/
SDH is not a
communications protocol in itself, but a transport
protocol.
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.Synchronous Optical NETworking standard was defined by Telcordia and
American National Standards Institute (
ANSI) standard T1.105. which define the set of transmission formats and transmission rates in the range above 51.840 Mbit/s.
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