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Tele-Communication (Telecom) Terms Glossary and Dictionary
ABCD Signaling Bits
ABCD Signaling Bits, also refered as ABCD Bits, are 4-bits telephony line signaling coding in which each letter represents 1 of the 4 bits. The implementations of ABCD signaling are different in E1 and T1 digital trunks. On E1, ABCD bit values may be used to signal an incoming call, disconnect, seize and so on, They are closely analogous to the A and Bits commonly used on T-1 digital trunks. On E1, there are 32 channels (time-slots) in contrast to 24 channels on T-1, hence the faster bit rate of 2.048 MHz versus 1.544 MHz for T-1. On E1, channels 0 and 16 are used to carry the ABCD bits and synchronization (framing) bits; hence only 30 channels are available for audio conversations. On T-1 digital trunks, the ABCD Signaling Bits implements the robbed bit signaling.
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