/mcpPrerequisites Requirements Be aware that several techniques are used to pass fax calls across a Packet Telephony network on Cisco IOS® gateways: Cisco proprietary Fax Relay • T.38 Fax Relay • Fax Passthrough • Fax Upspeed • T.37 Fax Store and Forward • In addition, three main Packet Telephony technologies are in use today, collectively referred to as Voice over "X" (VoX): Voice over IP (VoIP) • Voice over Frame Relay (VoFR) • Voice over ATM (VoATM) • The primary focus of this document is Cisco proprietary fax relay on Cisco IOS gateways, which operates across VoIP networks. T.38 Fax Relay and the other VoX technologies are also discussed. The technical intricacies of faxes and fax relay are not covered in detail, but you are able to troubleshoot for a majority of common fax relay issues. An overview of fax and Cisco fax relay is also provided. Components Used The information in this document is based primarily on Cisco IOS Software Release 12.2(5), although most of the information is also useful for other Cisco IOS Software releases. Some debug information was taken from a Cisco IOS gateway that ran Cisco IOS Software Release 12.2(7). This point is noted in the Debugging section of this document. The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command. Overview of Fax and Fax Relay Most modern fax devices are Group 3 compliant. Fax Group 3 is a standards-based technology that is made up primarily of the T.4 and T.30 ITU recommendations. T.4 pertains to how the fax image is encoded by a fax device, and T.30 details the facsimile negotiations and communication protocol. Group 3 fax devices were designed to be used over the public switched telephone network (PSTN). Because the PSTN was designed for human speech, Group 3 uses analog encoding or modulated signals as would an