A & B (Robbed) Bit Signaling

Many T-1 connections use an in-band signaling method called robbed bit. In-band signaling is also known as Channel Associated Signaling (CAS), because line signaling information is carried within each channel s data stream. Robbed bit signaling literally robs one bit from the data path on each of the 24 DS-0 channels to indicate a hook condition. In the case of a T-1 using the D4 Super Frame (D4 SF) framing technique (based on 12 frame increments), every 6th and 12th frames have the Least Significant Bit (LSB) from every 8-bit byte overwritten to indicate the hook state. Because such bits are no longer available for the actual channels, the DS-0 s are now voice grade lines and only transmit at 56Kbps, as compared to the full 64Kbps in out-of-band or clear channel signaling. The bit robbed from the 6th frame is called the A-bit and the bit robbed from the 12th frame is called the B-bit. Note that in the case of T-1s using Extended Super Framing (ESF), which is based on 24 frames, the LSB of frames 6, 12, 18 and 24 are used for signaling purposes.

For straight voice transmissions, robbed bit signaling doesn t compromise quality very much. But it does limit the amount of call information that can be buried in the channels of a T-1 circuit. By robbing bits, a circuit can only indicate ringing, hang up, wink and pulse digit dialing. This does not translate to very much advanced intelligence of...