bit robbing definition

Bit robbing is not the same as bit stuffing.

Bit robbing is a technique used in signaling on the T-carrier system, the widely-used system for transmitting both voice and data in digital form in the public switched telephone network (PSTN) and in private networks. In the basic T-1 system, a 193 bit frame, serving 24 channels, is transmitted in a sequence of 12 frames that are referred to as a superframe. Special signaling information, such as whether a voice channel is on-hook or off-hook, is included within the superframe by using a bit (called the  a bit) that is "robbed" from the sixth frame as a signaling bit and another bit (the b bit) that is robbed from the 12th frame.

Bit robbing is acceptable for voice conversations or data transmissions that are received by a modem but not for "pure" data transmission (where every bit is significant). Bit robbing is the reason that a 64 Kbps channel only carries 56 Kbps of usable data. Bit robbing is a form of in-band signalling.

This was first published in April 2005

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