Editing Superframe (section)

{{Short description|T1 (DS1) Framing standard}}
{{More citations needed|date=December 2009}}

In [[telecommunication]]s, '''superframe''' ('''SF''') is a [[T-carrier|T1]] [[frame synchronization|framing]] standard.  In the 1970s it replaced the original T1/D1 framing scheme of the 1960s in which the framing bit simply alternated between 0 and 1.

Superframe is sometimes called '''D4 Framing''' to avoid confusion with [[single-frequency signaling]].  It was first supported by the D2 channel bank, but it was first widely deployed with the D4 channel bank.

In order to determine where each [[channel (communications)|channel]] is located in the stream of data being received, each set of 24 channels is aligned in a frame.  The frame is 192 [[bit]]s long (8 * 24), and is terminated with a 193rd bit, the framing bit, which is used to find the end of the frame.

In order for the framing bit to be located by receiving equipment, a predictable pattern is sent on this bit.  Equipment will search for a bit which has the correct pattern, and will align its framing based on that bit.  The pattern sent is 12 bits long, so every group of 12 frames is called a superframe.  The pattern used in the 193rd bit is 100011 011100.<ref name=ft100m>{{Cite book |title=FT100 M User's Guide |url=http://www.optimumdata.com/shop/files/motorola/ft100m/ |author=Motorola |date=September 1996 |chapter=Appendix D: T1 Overview |chapter-url=http://www.optimumdata.com/shop/files/motorola/ft100m/16apd.pdf |accessdate=2015-12-07}}</ref><ref name=Floyd>{{Cite newsgroup |url=http://yarchive.net/phone/t1_signaling.htm |title=Re: T1 signalling |first=Floyd |last=Davidson |newsgroup=comp.dcom.telecom.tech |date=5 Oct 1998}}</ref><ref>[http://www.techfest.com/networking/wan/t1.htm techfest.com] T1 Overview {{webarchive |url=https://web.archive.org/web/20110520180808/http://www.techfest.com/networking/wan/t1.htm |date=May 20, 2011 }}</ref>

Each channel sends two bits of call supervision data during each superframe using [[robbed-bit signaling]] during frames 6 and 12 of the superframe.

More specifically, after the 6th and 12th bit in the superframe pattern, the least significant data bit of each channel (bit 8; T1 data is sent [[big-endian]] and uses 1-origin numbering) is replaced by a "channel-associated signalling" bit (bits A and B, respectively).<ref name=ft100m/><ref name=Floyd/>

Superframe remained in service in many places through the turn of the century, replaced by the improved [[extended superframe]] (ESF) of the 1980s in applications where its additional features were desired.