A/B signalling

A/B signaling is a method used in most TransmissionSystem 1( T1) transmission equipment.

In A/B signaling, one bit from every 6th frame of the 24 auxiliary channels contributes to the transmission of the signaling information. This is inband signaling in the T1 transmission.

On T1 lines that use an Extended Superframe( ESF) for framing, the signaling bits are taken from the 6th, 12th, 18th, and 24th frames, which is called ABCD signaling.