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Collaboration, Voice & AV · Intermediate

Channel Associated Signaling (CAS)

Signaling carried within or tied to each voice channel, such as robbed-bit signaling on a T1.

Definition

With CAS, on-hook, off-hook and digit information for a call travels in the same channel as its voice. On a T1 this is done by robbing the least significant bit of selected frames (robbed-bit signaling), reducing usable bandwidth to 56 kbps for data; on E1, CAS uses timeslot 16 in a fixed per-channel pattern. CAS offers fewer features than ISDN, such as no reliable calling name delivery.

Example

ds0-group 0 timeslots 1-24 type e&m-wink-start configures a T1 CAS trunk on a Cisco gateway.

Exam tip

Robbed-bit signaling is the T1 CAS mechanism; a dedicated D channel means CCS/PRI, not CAS.

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