Should voice over IP be disabled to fax?

Where an analogue machine has to stay in service, some carriers offer a dedicated mode that suspends compression for the call. That is a workaround; going through a gateway that speaks T.38 addresses the cause.

Not noticeably. Transmission time depends on the number of pages and the negotiated speed, not on the transport. In practice T.38 is often faster, because it avoids the renegotiations that signal degradation used to cause.

T.38 protocol

T.38: why analogue fax fails over IP, and what repairs it

Fax failures behind a router are not a technical inevitability. They come from a misunderstanding: voice over IP is optimised for speech, and a fax is not speech.

Short answer

T.38 carries T.30 protocol data inside IP packets instead of transmitting the modem's sound. Voice-over-IP codecs compress on the assumption of a human voice; applied to fax tones, they distort them enough to break the negotiation. T.38 sidesteps the problem by never carrying the sound at all.

What voice compression destroys

A voice-over-IP codec lowers the bit rate by discarding what the ear does not perceive. That reasoning is right for speech and wrong for a modem, whose entire information lies in the exact shape of the signal. Add packet loss and jitter, and the training sequence fails: the machines renegotiate, fail again, then give up.

Carry the protocol, not the sound

T.38 does the opposite of a voice gateway. The gateway demodulates the signal, extracts the T.30 messages, sends them as packets, and a symmetrical device remodulates them on arrival. The IP network never sees tones: it carries data, whose loss is offset by redundancy. The T.30 conversation runs normally end to end.

What this means for sending from a browser

An online service does not have this problem, since there is no analogue machine on the sending side: the gateway speaks the protocol directly. The fragile link moves to the receiving end — if the machine that receives is a fax wired to a poorly configured router, the failure originates there, and no setting on the sending side corrects it.

Frequently asked questions

Should voice over IP be disabled to fax?: Where an analogue machine has to stay in service, some carriers offer a dedicated mode that suspends compression for the call. That is a workaround; going through a gateway that speaks T.38 addresses the cause.
Is T.38 slower?: Not noticeably. Transmission time depends on the number of pages and the negotiated speed, not on the transport. In practice T.38 is often faster, because it avoids the renegotiations that signal degradation used to cause.

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