VoIP glossary
What is T.38?
T.38 is the protocol for sending fax over IP networks, converting fax signals into data packets rather than trying to carry them as audio.
T.38 explained
Fax machines are very sensitive to the small delays and losses that voice calls cope with easily. Sending fax tones over G.711 can work on a clean network, but it fails often, and it does not work at all over compressed codecs like G.729. T.38 avoids the problem by relaying the fax data itself, with redundancy to survive packet loss.
Usually a call starts as voice and switches to T.38 with a re-INVITE once fax tones are detected. Both the PBX or ATA and the trunk need T.38 enabled for that switch to succeed.
Example
A law firm’s fax machine connects to an ATA. When the far machine answers, the ATA sends a re-INVITE to T.38 and the 12-page document goes through cleanly.
Put it into practice
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