/mcpA normal fax machine, after it generates a CED or hears a CNG, transmits a T.30 DIS message as part of a fax handshake. This process usually occurs at the termination fax machine. The DSP of the termination gateway then detects the HDLC flag sequence at the start of the DIS message and initiate fax relay switchover. This means that it unloads the voice codec and loads a fax codec to handle the fax call that takes place. Notification is also sent to the DSP on the other side of the VoX network so that the DSPs on each side of the fax call use the fax codec. Dependent on the fax relay protocol used, the notification mechanisms differ. With the fax codec loaded, the DSPs demodulate the T.30 HDLC frames, extract the fax information, and pass it between the routers with one of these fax relay protocols: Proprietary Cisco fax relay for VoIP – Fax relay is the default mode to pass faxes through a VoIP network, and Cisco fax relay is the default fax relay type. This capability has been supported in Cisco IOS Software Releases 11.3 and later, is widely available, and uses RTP to transport the fax data. • Standards-based T.38 fax for VoIP – T.38 has been available in Cisco IOS Software Releases 12.1(3)T and later on some platforms. It can be enabled with the fax relay protocol t38 command configured under the VoIP dial peer and uses UDP to transport fax data. • Standards-based FRF.11 Annex D for VoFR and VoATM. • Unlike inband faxes or fax passthrough, fax relay breaks down the T.30 fax tones into their specific HDLC frames (demodulation), transmits the information across the VoX network with the fax relay protocol, and then converts the bits back into tones at the far side (modulation). The fax machines on either end send and receive tones and are not aware of a demodulation/modulation fax relay process. Cisco fax relay and T.38 fax relay also differ from T.37 fax store and forward. T.37 provides a standards- based method to allow a VoIP gateway to receive this: Most Cisco voice gateways currently support two methods to transmit fax traffic across the IP network: Fax Pass-Through—In fax pass-through mode, the gateways do not distinguish a fax call from a voice call 1. Cisco Fax Relay— In fax relay mode, the gateways terminate the T.30 fax signaling 2. Cisco fax relay and T.38 fax relay also differ from T.37 fax store and forward. T.37 provides a standards- based method to allow a VoIP gateway to receive this: A fax from a fax machine and forward it to an SMTP-capable mail server. The mail server can then deliver the fax to a user as an email message. • An email message from a mail server and modulate it into a fax signal for receipt by a regular fax machine. • This diagram illustrates fax relay over a VoX network. The fax connection to the origination and termination gateways can be directly into FXS ports on the gateway, or can be via a PBX or the PSTN into an E1, Basic Rate Interface (BRI), FXO, or E&M port on the gateway.