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Collaboration, Voice & AV · Intermediate

Q.931

The ITU-T ISDN Layer 3 call control protocol carried on the D channel to set up and tear down calls.

Definition

Q.931 defines messages such as SETUP, CALL PROCEEDING, ALERTING, CONNECT, DISCONNECT, RELEASE and RELEASE COMPLETE, along with information elements like called party number, calling party number and cause codes. It runs over Q.921 (LAPD) at Layer 2. H.323 reuses a variant of Q.931 in H.225.0 call signaling, and MGCP gateways can backhaul Q.931 to CUCM.

Example

debug isdn q931 shows a SETUP with called number 5551234 and a DISCONNECT with cause code 0x10 (normal call clearing).

Exam tip

Cause codes such as 1 (unallocated number), 16 (normal clearing), 17 (user busy) and 34 (no circuit available) are frequent troubleshooting clues.

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