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Collaboration, Voice & AV · Foundational

Primary Rate Interface

An ISDN digital trunk with 23 B channels plus one D channel on a T1, or 30 B channels plus one D channel on an E1.

Definition

PRI carries bearer channels of 64 kbps for voice or data and a single D channel for Q.931 common channel signaling. On T1 the D channel is timeslot 24 (23B+D); on E1 it is timeslot 16 (30B+D), with timeslot 0 used for framing. PRI provides fast call setup, calling number and name delivery, and clean disconnect supervision, and it is still widely used for PSTN connectivity alongside SIP trunks.

Example

isdn switch-type primary-ni and pri-group timeslots 1-24 configure a North American PRI on a Cisco voice gateway.

Exam tip

Know the channel math (T1 23B+D, E1 30B+D) and that the D channel uses Q.931, which is a form of common channel signaling.

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