Site Map
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. Two general methods for transmitting fax communication over the IP network are fax passthrough and fax relay. 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. While both V.152 and T.38 allow existing PSTN fax machines to transmit images over the IP network, each one has its advantages and drawbacks.
The most significant advantage of V.152 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 most significant advantage of 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 is more commonly used for facsimile transmission than V.152.