Complete Communications Engineering

V.152 fax passthrough and T.38 fax relay are two general methods for facsimile transmission over IP networks. Fax passthough transmits the fax signals as audio compressed with a suitable codec, such as G.711. Fax relay demodulates the incoming fax data at the gateway and transmits the pertinent information over the IP network to the remote gateway so that it may be modulated and sent to the remote fax machine. Methods for performing fax passthough and relay have been standardized in ITU-T Recommendations V.152 and T.38, respectively.

The fast-paced migration to IP-based communications has generated a need for transmitting voice-band data (VBD), such as fax and data modem signals, over the IP network. This can be accomplished through the use of gateways to interface between the PSTN and IP networks. While facsimile transmission using T.38 fax relay and V.152 fax pass-through allow existing PSTN fax machines to transmit images over an IP network, each one has its advantages and drawbacks.

The most significant advantage of V.152 fax passthrough is its simplicity. This allows V.152 to be implemented quicker and with less potential for interoperability issues than T.38. A V.152 implementation also consumes less MIPS and memory than a T.38 implementation. V.34 fax is supported natively by V.152 as well, because the fax signals are simply sent as audio to the remote end. Since V.152 only uses RTP for transport, it is easier to add support for additional RTP-based features, such as Secure RTP (SRTP).

The primary advantage of T.38 fax relay is it’s resistance to packet loss.  V.152 transmissions are easily disrupted by even a small amount of packet loss – usually resulting in a failed transmission.  T.38 is much more resistant to packet loss and can usually recover and continue.  The next big advantage to T.38 is its lower bandwidth consumption. Since the fax signals are transmitted after demodulation, the full bandwidth of an audio call is not necessary. It is also simpler to implement redundancy for the transmitted data when using UDPTL with T.38 as compared to RTP.

Currently, T.38 fax relay is more commonly used for facsimile transmission than V.152 fax passthrough.