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T.38 is an ITU recommendation for sending fax messages over IP networks in real time by encapsulating a standard T.30 fax data stream. T.38 is also used to describe the actual data and is is a registered mime sub-type in RFC 3362

• T.37 is a store and forward mechanism of sending faxes over IP using email as transport (SMTP and TIFF attachments).
• G.711 pass through - this is where the T.30 fax call is carried in a VOIP call encoded as audio. This is sensitive to network packet loss, jitter and clock synchronization.

In practical scenarios, a T.38 fax call has at least part of the call being carried over PSTN, although this is not required by the T.38 definition, and two T.38 devices can send faxes to each other.

The typical scenario where T.38 is used is - T.38 Fax relay - where a T.30 fax device sends a fax over PSTN to a T.38 Fax gateway which converts or encapsulates the T.30 protocol into T.38 data stream. This is then sent either to a T.38 enabled end point such as fax machine or fax server or another T.38 Gateway that converts it back to PSTN and terminates the fax on a T.30 device.

The T.38 recommendation defines the use of both TCP and UDP to transport T.38 packets. Implementations tend to use UDP, due to TCPs requirement for acknowledgement packets and resulting retransmission during packet loss, which introduces delays. When using UDP, T.38 copes with packet loss by using redundant data packets.

T.38 is not a call setup protocol, and the T.38 devices need to be using the same call setup protocols to negotiate the T.38 call. eg H.323, SIP & MGCP

• How T.38 works
• http://www.faqs.org/rfcs/rfc3362.html

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