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Collaboration, Voice & AV · Intermediate

T.38

The ITU-T standard for real-time fax relay over IP that converts fax signals into data packets rather than audio.

Definition

T.38 demodulates T.30 fax tones at the originating gateway, sends the fax data in UDPTL (or RTP) packets with redundancy, and remodulates it at the far gateway. It is far more tolerant of jitter and loss than fax passthrough, where tones are sent as G.711 audio. Switching to T.38 mid-call uses a re-INVITE triggered by fax tone detection.

Example

fax protocol t38 version 0 ls-redundancy 0 hs-redundancy 0 fallback pass-through g711ulaw enables T.38 on a dial peer.

Exam tip

Fax over compressed codecs fails; use T.38 relay or G.711 passthrough with echo cancellation and VAD disabled.

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