A&B
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The communications term A&B refers to a bit signaling procedure used in SF (D4) framed T1 transmission facilities where one bit from the sixth and twelfth frame of each of 24 T1 subchannels is used for carrying supervisory signaling.
On T1 lines that use ESF (Extended SuperFrame) framing, the signaling bits are robbed from the 6th, 12th, 18th, and 24th frame, resulting in "ABCD" signaling bits.
- This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.