Traditionally, there have been two approaches for sending FoIP networks: real-time methods and store-and-forward methods. The primary difference in service between these two approaches lies in the delivery and method of receipt confirmation. The Frame Relay Forum has defined a real-time protocol for the transmission of fax-over–frame relay networks. Likewise, the ITU and Internet Engineering Task Force (IETF) are working together to continue to evolve both the real-time FoIP network standard (T.38) as well as the store-and-forward FoIP network standard (T.37). Both T.37 and T.38 were approved by the ITU in June, 1998. Furthermore, T.38 is the fax transmission protocol selected for H.323.

There are tremendous opportunities for cost savings by transmitting fax calls over packet networks. Fax data in its original form is digital. However, it is modulated and converted to analog for transmission over the PSTN. This analog form uses 64 kbps of bandwidth in both directions.

The FoIP IWF reverses this analog conversion, transmitting digital data over the packet network and then reconverting the digital data to analog for the receiving fax machine. This conversion process reduces the overall bandwidth required to send the fax, because the digital form is much more efficient, and the fax transmission is half-duplex (i.e., only one direction is used at any time). The peak rate for a fax transmission is 14.4 kbps in one direction. A representation of this process is shown in Figure 7.

Figure 7. FoIP Conversion Process

An application for fax over packet, shown in Figure 8, is a network configuration of a company with numerous branch offices that wants to use the packet network, instead of the long-distance network, to provide fax access to the main office. The IWF is the physical implementation of the hardware and software that enables the transmission of fax over the packet network. The IWF must support analog interfaces that directly interface to fax machines at the branches and to a PBX at the central site. The IWF must emulate the functions of a PBX for the fax machines.

Figure 8. FoIP Application