Editing Time-division multiplexing (section)

==Telecommunications systems==
There are three types of synchronous TDM: T1, SONET/SDH, and ISDN.<ref name="White">{{cite book
| last = White
| first = Curt
| year = 2007
| title = Data Communications and Computer Networks
| publisher = Thomson Course Technology
| location = Boston, MA
| pages = [https://archive.org/details/datacommunicatio00whit_0/page/143 143–152]
| isbn = 978-1-4188-3610-8
| url-access = registration
| url = https://archive.org/details/datacommunicatio00whit_0/page/143
}}</ref>

[[Plesiochronous digital hierarchy]] (PDH) was developed as a standard for multiplexing higher order frames. PDH created larger numbers of channels by multiplexing the standard Europeans 30 channel TDM frames. This solution worked for a while; however PDH suffered from several inherent drawbacks which ultimately resulted in the development of the [[Synchronous Digital Hierarchy]] (SDH). The requirements which drove the development of SDH were these:<ref name="hanrahn" /><ref name="ericsson" />

*Be synchronous &ndash; All clocks in the system must align with a reference clock.
*Be service-oriented &ndash; SDH must route traffic from End Exchange to End Exchange without worrying about exchanges in between, where the bandwidth can be reserved at a fixed level for a fixed period of time.
*Allow frames of any size to be removed or inserted into an SDH frame of any size.
*Easily manageable with the capability of transferring management data across links.
*Provide high levels of recovery from faults.
*Provide high data rates by multiplexing any size frame, limited only by technology.
*Give reduced bit rate errors.

SDH has become the primary transmission protocol in most PSTN networks. It was developed to allow streams 1.544&nbsp;Mbit/s and above to be multiplexed, in order to create larger SDH frames known as Synchronous Transport Modules (STM). The STM-1 frame consists of smaller streams that are multiplexed to create a 155.52&nbsp;Mbit/s frame. SDH can also multiplex packet based frames e.g. [[Ethernet]], PPP and ATM.<ref name="hanrahn" /><ref name="ericsson" />

While SDH is considered to be a transmission protocol (Layer 1 in the [[OSI reference model|OSI Reference Model]]), it also performs some switching functions, as stated in the third bullet point requirement listed above.<ref name="hanrahn" /> The most common SDH Networking functions are these:

*''SDH Crossconnect'' &ndash; The SDH Crossconnect is the SDH version of a time–space–time crosspoint switch. It connects any channel on any of its inputs to any channel on any of its outputs. The SDH Crossconnect is used in Transit Exchanges, where all inputs and outputs are connected to other exchanges.<ref name="hanrahn" />
*''SDH Add–Drop Multiplexer'' &ndash; The SDH Add–Drop Multiplexer (ADM) can add or remove any multiplexed frame down to 1.544&nbsp;Mb. Below this level, standard TDM can be performed. SDH ADMs can also perform the task of an SDH Crossconnect and are used in End Exchanges where the channels from subscribers are connected to the core PSTN network.<ref name="hanrahn" />

SDH network functions are connected using high-speed optic fibre. Optic fibre uses light pulses to transmit data and is therefore extremely fast. Modern optic fibre transmission makes use of [[wavelength-division multiplexing]] (WDM) where signals transmitted across the fibre are transmitted at different wavelengths, creating additional channels for transmission. This increases the speed and capacity of the link, which in turn reduces both unit and total costs.<ref name="hanrahn" /><ref name="ericsson" />