IP Fax

Fax has traditionally been treated as a telephony application because it is sent over the telephone network. In reality, it is a data application that results in transmitting scanned document images from one facsimile terminal (fax machine) to another. The wide availability of the Internet has created an environment that allows the fax application to return to its true identity as a pure data service. If the real-time delivery of fax is not a requirement (and more often than not it is not), the image of a scanned document can be sent over the Internet in a way that is similar to sending e-mail.

For integrating the PSTN and the Internet, the following three scenarios are relevant:

1.  A traditional fax machine sends a document to another one by way of an IP network. (A configuration is depicted in Figure 1.)

Figure 1: Traditional fax–to–traditional fax.

2.  A traditional fax machine sends a document to an IP fax machine. (See Figure 2.)

Figure 2: Traditional fax–to–IP fax and vice versa.

3.  An IP fax machine sends a document to a traditional fax machine. (See Figure 2.)

Note that an IP fax machine is an appliance, typically a PC, that can be connected directly to an IP network. Because it is necessary to establish a phone call in order to interoperate with the traditional fax machine, the steps involved in making fax calls repeat those already discussed in the IP telephony-call scenarios.

The IP fax operation can take place in two modes: real-time and non-real-time. In the real-time mode, IP fax emulates the behavior of a traditional facsimile over the PSTN. The sequence of events, such as capability negotiation and confirmation of receipt by the endpoints in a traditional facsimile session, are largely preserved.

In the non-real-time mode, IP fax operates much like e-mail, relying on the store-and-forward process. The fax is stored at a staging point before being transmitted to the next staging point. The end-to-end transmission of the facsimile may involve several staging points with an unpredictable delay.