Fax Relay
Cisco Fax Relay is the oldest method of supporting fax on Cisco IOS gateways and has
been supported since Cisco IOS Release 11.3. Cisco Fax Relay uses RTP as the method of
transport. In Cisco Fax Relay mode, gateways terminate T.30 fax signaling by spoofing a
virtual fax machine to the locally attached fax machine. The gateways use a Ciscoproprietary
fax relay RTP-based protocol to communicate between themselves.
Unlike fax pass-through, fax relay, as depicted in Figure 2-7, demodulates the fax bits at
the local gateway, sends the information across the voice network using the fax relay protocol,
and then remodulates the bits back into tones at the far gateway. The fax machines
on either end are sending and receiving tones and are not aware that a demodulation/
modulation fax relay process is occurring.
70 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
IP Network
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Figure 2-7 Fax and Modem Relay Topology
The default method for fax transmission on Cisco IOS gateways is Cisco Fax Relay. This
is an RTP-based transmission method that uses proprietary signaling and encoding mechanisms.
The mechanism for Cisco Fax Relay is the same for calls that are controlled by
SIP, MGCP, and H.323 call control protocols.
Note Before T.38 standards-based fax relay was introduced, configuration was required
to enable Cisco Fax Relay.
Cisco provides two methods for fax relay:
■ Cisco Fax Relay: A Cisco-proprietary method, and the default on most platforms if
a fax method is not explicitly configured.
■ T.38 fax relay: A method based on the ITU-T T.38 standard. It is real-time fax transmission
(that is, two fax machines communicating with each other as if there were a
direct phone line between them). T.38 fax relay is configured by using a few additional
commands on gateway dial peers that have already been defined and configured
for VoIP calls.
The T.38 fax relay feature can be configured for H.323, SIP, and MGCP call control protocols.
For H.323 and SIP networks, the only configuration tasks that differ are those
involving the configuration of VoIP dial peers.
T.38 is an ITU-T standards-based method and protocol for fax relay. Data is packetized
and encapsulated according to the T.38 standard. T.38 fax relay has the following features:
■ Fax relay PLC
■ MGCP-based fax (T.38) and DTMF relay
■ SIP T.38 fax relay
■ T.38 fax relay for the T.37/T.38 fax gateway
■ T.38 fax relay for VoIP H.323