Chapter 1. The T.38 real-time FAX over IP (FoIP) protocol

There are two ITU recommendations which address sending FAXes over IP networks. T.37 specifies a method of encapsulating FAX images in e-mails, and transporting them to the recipient (an e-mail box, or another FAX machine) in a store-and-forward manner. T.38 specifies a protocol for transmitting a FAX across an IP network in real time. This document discusses the implementation of the T.38, real time, FAX over IP (FoIP) protocol.

FAXing between two PSTN FAX machines provides the illusion of real-time communication, with confirmed delivery. This is, of course, no more than an illusion. The sender has no idea what is receiving the FAX. It might be a FAX mailbox, where the FAX might only be retrieved much later, or might never be retrieved at all. It might be a FAX machine which is out of paper, and is storing the FAX in memory. If nobody adds paper before the FAX machine is switched off, the FAX might never be printed. With increasing amounts of FAX spam, many people don't ever bother to collect FAXes from the FAX machine in their office. Still, with all these reasons why the real-time confirmed delivery nature of FAXing is an illusion, large numbers of people still insist on using direct FAXing.

The FAX protocols - in particular T.30 - were designed for the PSTN. The PSTN is very different from a packet network like the Internet. It offers very strict timing; latency is rock steady throughout a call; and latency is seldom very high. The lack of these features in packet networks tends to spoil the quality of voice over IP, compared to voice over the PSTN. However, it can totally destroy modem data, like that used for FAX. Jitter and packet loss can cause modem reception to fail, and excessive delays can cause timers designed for a low latency environment to expire. T.38 tries to mitigate these factors, and can greatly improve the reliability of FAXing across the internet. It can also send FAXes using less bandwidth than using VoIP protocols. There are limits to what can be achieved on a congested network, though, and T.38 can never offer the reliability of a store and forward protocol, like T.37. Sadly, the average office worker's love affair with real-time FAX ensures a bright future for the T.38 protocol, even though the T.37 protocol makes far more sense.

The T.38 protocol primarily operates between:

T.38 is the only standardised protocol which exists for real-time FoIP. Reliably transporting a FAX between PSTN FAX terminals, through an IP network, requires use of the T.38 protocol at FAX gateways. VoIP connections are not robust for modem use, including FAX modem use. Most use low bit rate codecs, which cannot convey the modem signals accurately. Even when high bit rate codecs, such as G.711, are used, VoIP connections suffer dropouts and timing adjustments, which modems cannot tolerate. In a LAN environment the dropout rate may be very low, but the timing adjustments which occur in VoIP connections still make modem operation unreliable. T.38 FAX gateways deal with the delays, timing jitter, and packet loss experienced in packet networks, and isolate the PSTN FAX terminals from these as far as possible. In addition, by sending FAXes as image data, rather than digitised audio, the required bandwidth of the IP network might be reduced. However, the redundant transmission needed to make T.38 work acceptably over an unreliable network tends to offset much of the potential bandwidth gain.

The original T.38 specification provides for operation up to 14,400bps, using a V.17 modem. The latest version of the T.38 specification adds features for FAXing at up to 33,600bps, using a V.34 modem. However, it appears most current T.38 implementations only support operation up to 14,400bps.