T.38 is a protocol for sending faxes over a voice over IP (VoIP) network or the Internet in real time.
The protocol defines the transport of data (a fax) between PSTN fax terminals through a fax gateway, between two Internet-aware fax terminals, or from a PSTN fax terminal through a fax gateway to an Internet-aware fax terminal. A T.38 stream is sometimes referred to as Fax over IP (FoIP).
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PSTN fax terminals traditionally use the T.30 protocol to send analog data. To exchange analog fax data with a PSTN terminal over the Internet, the T.38 protocol first converts analog data into digital data. The protocol then converts the data back to analog on the receiving end if the receiver is a PSTN fax terminal.
T.38 most often uses UDP but can use TCP and RTP as well. UDP is preferred to avoid delays, since UDP sends redundant data packets to deal with possible packet loss.
See also: fax server, fax polling