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Collaboration, Voice & AV · Foundational

Time-Division Multiplexing (TDM)

A technique that carries multiple channels on one circuit by giving each a fixed, repeating time slot.

Definition

In TDM voice networks each call is assigned a dedicated timeslot on a digital trunk for the life of the call, giving deterministic delay but wasting capacity during silence. T1, E1 and SONET are TDM hierarchies, and traditional PBXs and PSTN switches are TDM based. IP telephony replaces TDM with statistically multiplexed packets, so QoS is needed to give voice similar predictability.

Example

A legacy PBX connects to a voice gateway over a TDM PRI, and the gateway converts each timeslot into an RTP stream.

Exam tip

Contrast circuit-switched TDM (fixed bandwidth, no jitter) with packet-switched VoIP (shared bandwidth, needs QoS and jitter buffers).

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