Editing Synchronous optical networking (section)

{{Short description|Standardized protocol}}
{{Use dmy dates|date=September 2022}}
{{More citations needed|date=July 2007}}
'''Synchronous Optical Networking''' ('''SONET''') and '''Synchronous Digital Hierarchy''' ('''SDH''') are standardized protocols that transfer multiple [[Digital data|digital]] [[bit stream]]s [[synchronous]]ly over [[optical fiber]] using [[laser]]s or highly [[Coherence (physics)|coherent]] light from [[light-emitting diode]]s (LEDs). At low [[Bit rate|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 the problems of synchronization.

SONET and SDH, which are essentially the same, were originally designed to transport [[circuit mode]] communications, e.g. [[Digital Signal 1|DS1]], [[Digital Signal 3|DS3]], from a variety of different sources. However, they were primarily designed to support real-time, uncompressed, circuit-switched voice encoded in [[Pulse-code modulation|PCM]] format.<ref name="td-hdbk"/> 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 complete communications protocol in itself, but a transport protocol (not a "transport" in the [[OSI Model]] sense).

Due to SONET/SDH's essential protocol neutrality and transport-oriented features, SONET/SDH was the 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.
[[File:SDH Racks.jpg|thumb|Racks of [[Alcatel]] STM-16 SDH [[add-drop multiplexer]]s]]

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.
SONET is subdivided into four sublayers with some factor such as the path, line, section and physical layer.

The SDH standard was originally defined by the [[European Telecommunications Standards Institute]] (ETSI), and is formalised as [[International Telecommunication Union]] (ITU) standards G.707,<ref name="G.707"/> [[G.783]],<ref name="G.783"/> G.784,<ref name="G.784"/> and G.803.<ref name="G.803"/><ref name="techfest"/> The SONET standard was defined by [[Telcordia]]<ref name="GR-253-CORE"/> and [[American National Standards Institute]] (ANSI) standard T1.105.<ref name="techfest"/><ref name="ANSI-T1.105"/> which define the set of transmission formats and transmission rates in the range above 51.840&nbsp;Mbit/s.