T.38 Fax - Definition
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.
A typical scenario where T.38 is used is T.38 Fax relay - when 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 a 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 re-transmission 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
T.38 Fax - Synonyms
T.38; T.38 Fax; Fax over IP; Fax T.38 SIP