Robbed Bit Signaling

24 Timeslots (DSO = 64 kbps)

DSO 1

DSo a

DSO 3

DSO 4

DSO 1

DSO 2

DSO 3

DSO 4

DSO 1

DSO 2

DSO 3

DSO 4

DSO 1 X

DSO 2 X

DSO 3 X

DSO 4 X

DSO 24

F

DSO 24

F

DSO 24

Frame No. 3

Framing Bit

X = Least significant bit in each DSO is "robbed" for signaling every sixth frame

Because each DS0 channel carries 64 kbps, and G.711 is 64 kbps, there is no room to carry signaling. Implemented for voice, the T1 uses every sixth frame to convey signaling information. In every sixth frame, the least significant bit (LSB) for each of the voice channels is used to convey the signaling. Although this implementation detracts from the overall voice quality (because only seven bits represent a sample for that frame), the impact is not significant. This method is called robbed-bit signaling (RBS). When SF employs this method, the signaling bits are conveyed in both the 6th (called the "A" bit) and 12th (called the "B" bit) frames. For control signaling, A and B bits provide both near- and far-end off-hook indication.

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, which yields ABCD signaling options, providing additional control and signaling information.

Channel Associated Signaling—T1

Cisco Public

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 are used to convey E&M status or FXS/FXO status and provide call supervision for both on hook and off hook.

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