This chapter discussed traditional fax over circuit-switched networks and described how
store and forward fax gateways can take calls from G3 fax machines, convert them into
e-mail messages, and transport them over the Internet as e-mail attachments. At the
terminating end of the call, another store and forward fax gateway receives the e-mail
message, converts it back into a fax message, and delivers it to a G3 fax machine. We
explained how the ITU developed the T.30 protocol and adopted the SMTP for fax called
MIME as part of the T.37 standard for store and forward fax.
We also described the ITU T.38 recommendation for fax relay and real-time fax with
spooﬁng. Using this standard, real-time fax gateway can deliver faxes to remote fax
machines while the sending fax machines are still processing fax pages. With fax relay, the
gateway receives an analog fax signal and demodulates it into its digital form using a fax
modem. The digital, demodulated fax is then packetized and transmitted over the IP
network. At the receiving end, the fax gateway remodulates the digital fax packets into T.30
analog fax signals to be transmitted to the destination fax machine through a gateway
We described in detail how Cisco implements T.37 store and forward fax, and we gave
conﬁguration guidelines and examples for both on-ramp and off-ramp fax gateways.
Finally, we described how Cisco implements T.38 real-time fax and fax rollover, or never-
busy fax, and we gave conﬁguration guidelines for those applications.