The same reasons for running voice over data are the same reasons for faxing over data. Although faxing probably does not take as long to complete as a phone conversation, expenses can be high for long-distance faxing. There are a few ways to get that important fax across a voice network. Each of them was developed for specific situations.
This Cisco proprietary protocol was created to send faxes over a VoXX network. The DSP acts like a virtual fax machine, converts the signal back to its original form, and then sends it to the remote fax machine. To save bandwidth, it uses 9.6 kbps. A key point to know is why a fax cannot be treated like a normal call. When a fax comes into the GW, it cannot be treated like a normal voice call, because otherwise the GW will miss the fax signaling and continue to treat the call as a normal voice call.
This method uses a transparent IP connection to transport the fax. In this mode, no conversions take place. It treats the fax like a voice call with no special fax capabilities. The codec used is G.711, with no Voice Activity Detection (VAD), and supports H.323, MGCP, and SIP.
This method is like fax pass-through except there are modems at both ends of the connection. Modem pass-through uses the same transparent connection, and G.711, with no VAD. The supported protocols are H.323, MGCP, and SIP.
Modem relay is a method built to deal with overhead. It uses Simple Packet Relay Transport (SPRT) to transport modem signals over IP. The SPRT GW converts analog to digital. SPRT uses the UDP protocol, and is less sensitive to jitter and clocking issues than modem pass-through. The supported protocols are H.323, MGCP, and SIP.
T.37 is the way to send faxes across your network like email attachments. The document is scanned, converted to TIFF, and then sent to a receiver’s email address using Simple Mail Transport Protocol (SMTP). The Cisco way of implementing T.37 is by way of GWs. On-ramp GWs do the conversions and sending of faxes; the off-ramp GWs are where the messages are received or held until forwarded. T.37 uses Multi-Media over IP (MMoIP) dial peers. Interactive Voice Response (IVR) applications are used for on-ramp and off-ramp functions.
Like all things in life, there is an industry standard, and then there is the Cisco way. Well, T.38 is the industry standard for fax relay. The two ways are similar, but T.38 needs to have a T.38 GW on both ends of the connection. The main difference is that T.38 allows faxing to virtual fax machines compatible with T.38. This protocol uses VoIP dial peers. The supported protocols are H.323, MGCP, and SIP.