Editing Robbed-bit signaling (section)

==Overview==
T1 is a protocol for digital transmission over telephone networks. A T1 circuit combines 24 [[DS0]] channels through [[time-division multiplexing]]. Data is transmitted in frames, where each frame contains an 8-bit sample of each of the 24 channels, plus one extra [[framing bit]] for a total of 193 bits. Each channel carries 8,000 samples per second. The frames are transmitted either in sequences of 12 frames, known as [[Superframe|superframes]] (SF), or sequences of 24 frames, known as [[Extended superframe|extended superframes]] (ESF). Robbed-bit signaling is a method to provide signaling information alongside data within a superframe. Robbed-bit signaling uses the least-significant bit of every channel in every sixth frame for signaling instead of data. These bits convey on or off hook, and busy signal status on telephone lines. Since a superframe consists of 12 frames, robbed-bit signaling provides two bits for signaling per channel in a superframe, or four bits per channel in an extended superframe. The first robbed bit in a superframe is called A bit, and the second bit is called the B bit. Extended superframes have two extra robbed bits, called the C and D bits.<ref>Tanenbaum, Andrew S.. Computer Networks. Upper Saddle River: Prentice Hall PTR, 2003</ref><ref name=":0">{{Cite book |last=Jardwick |first=Steve |title=ISDN Design: A Practical Approach |date=December 2, 2012 |publisher=[[Elsevier Science]] |isbn=9780323160865 |pages=104-105}}</ref>

Intuitively, 5 out of 6 frames have 8-bit resolution equal to {{nowrap|64 kbit/s}} (8 bits × 8,000 samples per second = {{nowrap|64 kbit/s}}) and 1 out of every 6 frames has a 7-bit resolution (7 bits × 8,000 samples per second = {{nowrap|56 kbit/s}}). The distortion effect on voice and data signals is negligible when a modem is used for modulation. However, for a {{nowrap|64 kbit/s}} digital signal, the data will render errors when a data signal is transmitted. To remove the errors, data can be transmitted 7 bits at a time instead of 8.<ref name=":0" /> Since robbed-bit signaling could impact voice quality and reduce data [[Bandwidth (computing)|bandwidth]], it was largely replaced by the [[Primary Rate Interface]] standard, which reserves one of the 24 voice channels for signaling information instead of using bits in every channel.<ref>{{Cite book |last=Bodin |first=Madeline |url=https://www.worldcat.org/title/ocm49300356 |title=The Call Center Dictionary: The Complete Guide to Call Center & Customer Support Technology Solutions |last2=Dawson |first2=Keith |date=January 3, 2002 |publisher=[[Taylor & Francis]] |isbn=978-1-57820-095-5 |edition=Revised |location= |pages=170 |oclc=ocm49300356}}</ref>