Robbed-Bit Signaling

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The A and B bits can represent different signaling states or control features (on hook or off hook, idle, busy, ringing, and addressing). The robbed bit is the least significant bit from an 8-bit word.

ESF also uses RBS in frames 6, 12, 18, and 24 to yield four signaling bits, providing additional control and signaling information. These four bits are known as the A, B, C, and D bits.

Because the signaling occurs within each DS0, it is referred to as in band. Also, because the use of these bits is exclusively reserved for signaling each respective voice channel, it is referred to as CAS.

The robbed bits, depicted in Figure 2-33, are used to convey E&M status or FXS/FXO status and provide call supervision for both on hook and off hook.