!!! Overview
[{$pagename}] ([SONET]) and [Synchronous Digital Hierarchy] ([SDH]) are [standardized|Standard] [protocols] that transfer multiple digital bit streams synchronously over optical [fiber] using lasers or highly coherent light from light-emitting diodes (LEDs). At low transmission rates [data] can also be transferred via an electrical interface. The method was developed to replace the [Plesiochronous Digital Hierarchy] ([PDH]) system for transporting large amounts of telephone calls and data traffic over the same [fiber] without synchronization problems.[SONET] and [SDH], which are essentially the same, were originally designed to transport circuit mode communications (e.g., [DS1], [DS3]) from a variety of different sources, but they were primarily designed to support real-time, uncompressed, circuit-switched voice encoded in [PCM] format. 
The primary difficulty in doing this prior to [SONET]/[SDH] was that the synchronization sources of these various circuits were different. This meant that each circuit was actually operating at a slightly different rate and with different phase. [SONET]/[SDH] allowed for the simultaneous transport of many different circuits of differing origin within a single framing [protocol]. [SONET]/[SDH] is not a [communications] [protocol] in itself, but a transport [protocol].[Asynchronous Transfer Mode] ([ATM]) [frames] also known as cells. It quickly evolved mapping structures and concatenated [payload] containers to transport [ATM] connections. In other words, for ATM (and eventually other protocols such as Ethernet), the internal complex structure previously used to transport circuit-oriented connections was removed and replaced with a large and concatenated frame (such as STS-3c) into which [ATM] cells, IP packets, or [Ethernet] [frames] are placed.[{$pagename}] 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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