A Codec commonly used to transmit real-time fax data over Voice over Internet Protocol (VoIP) networks. Faxes can’t travel over a data network the same way they do across an analog connection. The T.38 protocol helps convert the fax from an analog signal to an image, and then back to an analog signal when it arrives at its destination. Most VoIP Gateway devices and Analog Telephone Adaptors (ATAs) support the T.38 protocol.