Editing E-carrier (section)

==E1 frame structure==
An E1 link operates over two separate sets of wires, usually [[unshielded twisted pair]] (balanced cable) or using [[coaxial]] (unbalanced cable). A nominal 3 [[volt]] peak signal is encoded with pulses using a method avoiding long periods without polarity changes. The line data rate is 2.048&nbsp;[[Mbit/s]] ([[full duplex]], i.e. 2.048&nbsp;Mbit/s downstream and 2.048&nbsp;Mbit/s upstream) which is split into 32 timeslots, each being allocated 8&nbsp;[[bit]]s in turn. Thus each timeslot sends and receives an 8-bit [[PCM]] sample, usually encoded according to [[A-law algorithm]], 8,000 times per second (8 × 8,000 × 32 = 2,048,000). This is ideal for voice telephone calls where the voice is [[Sampling (signal processing)|sampled]] at that data rate and reconstructed at the other end. The timeslots are numbered from 0 to 31.

===Special timeslots===
One timeslot (TS0) is reserved for [[Frame synchronization|framing]] purposes, and alternately transmits a fixed pattern. This allows the receiver to lock onto the start of each frame and match up each channel in turn. The standards allow for a full [[cyclic redundancy check]] to be performed across all bits transmitted in each frame, to detect if the circuit is losing bits (information), but this is not always used. An alarm signal may also be transmitted using timeslot TS0. Finally, some bits are reserved for national use.<ref>[http://www.rad.com/Media/6966_E1backgrounder.pdf E1 Environment] {{Webarchive|url=https://web.archive.org/web/20131014084115/http://www.rad.com/Media/6966_E1backgrounder.pdf |date=2013-10-14 }}, RAD data communications University Tutorials</ref>

One timeslot (TS16) is often reserved for signalling purposes, to control call setup and teardown according to one of several standard telecommunications protocols. This includes [[channel-associated signaling]] (CAS) where a set of bits is used to replicate opening and closing the circuit (as if picking up the telephone receiver and pulsing digits on a rotary phone), or using tone signalling which is passed through on the voice circuits themselves. More recent systems use [[common-channel signaling]] (CCS) such [[Signalling System 7]] (SS7) where no particular timeslot is reserved for signalling purposes, the signalling protocol being transmitted on a freely chosen set of timeslots or on a different physical channel.<ref>{{Cite web |url=http://www.ss7-training.net/sigtran-training/ch08lev1sec5.html |title=Signaling System No. 7 (SS7/C7): Protocol, Architecture, and Services, Lee Dryburgh, Jeff Hewett, 2004 |access-date=2010-05-26 |archive-url=https://web.archive.org/web/20120101182748/http://www.ss7-training.net/sigtran-training/ch08lev1sec5.html |archive-date=2012-01-01 |url-status=dead }}</ref>

When using E1 frames for data communication, some systems use those timeslots slightly differently, either

* TS0: Framing, TS1–TS31: Data traffic — This is named Channelized E1, and is used where the framing is required, it allows any of the 32 timeslots to be identified and extracted.
* TS0–TS31: Data traffic — Often referred to as Clear Channel E1 or Unchannelized, it is used where no framing is required, timeslot extraction is not required and the full bandwidth (2&nbsp;Mb/s) is required.