What is T38 fax?

Fax over T38

T.38 is a protocol that describes how to send a fax over a computer data network. It is needed because fax data cannot be sent over a computer data network in the same way as voice communications. Read our How does fax work in VoIP environments? article for more information. In essence, with T.38 a fax is converted to an image, sent to another T.38 fax device and then converted back to an analog fax signal. Most VoIP Gateways and ATA's now support T.38 reliably.

T.38 is described in RFC 3362, and defines how a device should communicate the fax data. In the diagram above both the gateway and the fax machine behind the gateway would have to be T.38-capable. For the G3 fax machine on an analog line, this process will be transparent. The analog fax machine does not need to know T.38.

3CX includes a full-featured T.38 fax server that allows faxes to be received from anywhere in the network, converted to PDF and forwarded via email.

When is T.38 used?

In practice, a T.38 fax call can have part of the call to be carried over PSTN (Public Switched Telephone Network), although this is not strictly required by the T.38 definition. Two T.38 devices can send faxes to each other directly without transmitting over PSTN. This particular type of device is called Internet-Aware Fax (IAF) device, capable of transmitting fax signals and data packed as IFP (Internet Fax Protocol) packets over the IP network. The T.38 recommendation defines both TCP and UDP to transport T.38 packets, even though implementations tend to use UDP. This is due to the TCP packet acknowledgement requirement, resulting to retransmission during packet loss that can cause delays. T.38 handles packet loss while using UDP with the use of redundant data packets.

A typical scenario where T.38 is used is a fax relay. In this scenario 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 a T.38 data stream. This is then re-transmitted either to a T.38-enabled end-point, such as a fax machine, a fax server or a subsequent T.38 gateway that can convert it back to analog PSTN PCM signals and deliver the fax on a T.30 device.

When using the 3CX fax server, connected fax devices or fax software transmit received faxes to 3CX for delivery via email as PDF documents.

Further reading