T.38 Fax Standard
The T.38 fax standard is an ITU (International Telecommunications Union) standard for real-time faxing over IP (Internet Protocol) networks. The T.38 fax recommendation was set in 1998 and supplemented the T.30 standard that was published in 1980, prior to the mass market acceptance of the Internet. Voice over IP (VoIP) installations started to become an alternative to analog voice in the 1990s. But VoIP systems are configured with compression that is characterized by delays in packet transmission and therefore was a poor fit for fax. Fax, in its T.30 form, had a handshake mechanism that was designed for analog phone lines. In this handshaking routine, a session could be dropped if there were delays in signals for as little as a fraction of a second. This worked for phone calls, because a phone call established a single circuit just for the one call and, given normal circumstances, there were no delays (electronic signals travel at the speed of light). But the same handshake would not work for IP networks, since they are packet-switched. In packet-switching, the data in a call (the pulses of a voice call, or the image data of a fax call) is broken into packets and may be sent out over different network links to get to the destination. This means that delays are commonplace in IP networks, as is jitter (the deviation in amplitude or phase timing of the signal) and packet losses. So, a way was needed so that fax could be sent over IP networks and not suffer huge increases in failed transmissions due to timing problems.
The T.38 fax standard defines the sending of faxes over IP (FoIP) networks in real-time. T.37 is a sepearte standard that defines the sending of faxes over store-and-forward networks, as occurs with email. Sending fax over store-and-forward networks is not optimal, as one of the hallmarks of fax is that, when the fax is fully received, the receiving fax sends the sending fax a confirmation in real-time. Many people, when they send important business faxes, wait for the confirmation to arrive. But in a store-and-forward configuration, the confirmation may be delayed. So a compelling need existed for a real-time fax standard that would still support confirmations.
In a voice call, if a packet is occasionally dropped or delayed, the human ear can compensate and still understand what is said. But if a fax receives information out of order, the resulting image will be deformed or incomplete. And if a fax receives delayed data during the handshake, the entire session may be dropped. So the T.38 fax protocol tricks the receiving fax into thinking that it’s communicating directly with the sending fax and receiving everything on time. Called spoofing, the T.38 fax standard adds a technique that corrects for delayed packets with special buffer-management routines. T.38 fax Spoofing also prevents network delays from causing the transaction to fail. One way this is done is by padding image lines to render network delays transparent to the receiving fax devices.
The T.38 fax standard also greatly reduces the bandwidth needed for FoIP calls. It is greatly desirable to send faxes via the T.38 fax protocol over the Internet. But it is not a requirement. Currently, many businesses get by using G.711 fax passthrough (which is not a standard), which takes fax calls and passes them over the Internet with no modification and no T.38 fax spoofing. G.711 may be a successful strategy if the network has no packet loss and very little jitter and delay.