Editing T-carrier (section)

== Bit robbing ==
Twelve DS1 frames make up a single T1 [[Superframe]] (T1 SF). Each T1 Superframe is composed of two signaling frames. All T1 DS0 channels that employ in-band signaling will have its eighth bit over written, or "robbed" from the full 64&nbsp;kbit/s DS0 payload, by either a logical ZERO or ONE bit to signify a circuit signaling state or condition. Hence robbed bit signaling will restrict a DS0 channel to a rate of only 56&nbsp;kbit/s during two of the twelve DS1 frames that make up a T1 SF framed circuit. T1 SF framed circuits yield two independent signaling channels (A and B) T1 ESF framed circuits four signaling frames in a twenty four frame extended frame format that yield four independent signaling channels (A, B, C, and D).

Fifty-six kbit/s DS0 channels are associated with digital data service (DDS) services typically do not utilize the eighth bit of the DS0 as voice circuits that employ A&B out of band signaling. One exception is Switched 56&nbsp;kbit/s DDS. In DDS, bit eight is used to identify [[Data terminal equipment|DTE]] request to send (RTS) condition. With Switched 56 DDS, bit eight is pulsed (alternately set to logical ZERO and ONE) to transmit two state dial pulse signaling information between a SW56 DDS [[CSU/DSU]], and a digital end office switch.

The use of [[robbed-bit signaling]] in America has decreased significantly as a result of [[Signaling System No 7]] (SS7) on inter-office dial trunks. With SS7, the full 64&nbsp;kbit/s DS0 channel is available for use on a connection, and allows 64&nbsp;kbit/s, and 128&nbsp;kbit/s [[Integrated Services Digital Network|ISDN]] data calls to exist over a switched trunk network connection if the supporting T1 carrier entity is optioned [[B8ZS]] (Clear Channel Capable).<ref name="ANSI T1.403"/><ref>The Book On ESF, Verilink Corporation, 1986</ref><ref>D4 Digital Channel Bank Family, Bell System Technical Journal, November 1982</ref>