210-060 · Question #275
Which signaling method uses robbed bit signaling?
The correct answer is A. CAS. Channel Associated Signaling (CAS) uses robbed bit signaling, a technique used on T1 circuits where the least-significant bit of every 6th frame in each DS0 channel is 'robbed' (overwritten) to carry in-band signaling information such as on-hook/off-hook states. Because the…
Question
Which signaling method uses robbed bit signaling?
Options
- ACAS
- BFXS
- CFXO
- DCCS
How the community answered
(54 responses)- A94% (51)
- C4% (2)
- D2% (1)
Explanation
Channel Associated Signaling (CAS) uses robbed bit signaling, a technique used on T1 circuits where the least-significant bit of every 6th frame in each DS0 channel is 'robbed' (overwritten) to carry in-band signaling information such as on-hook/off-hook states. Because the signaling bits travel within the same channel as the voice data, it is called 'channel associated.' In contrast, Common Channel Signaling (CCS/D) - used by ISDN PRI - dedicates a separate D-channel for all signaling, keeping it entirely out of the voice payload. FXS (Foreign Exchange Station) and FXO (Foreign Exchange Office) are analog interface types that describe the physical endpoints of an analog loop, not a digital signaling method, so they do not use robbed bit signaling.
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