MGCP T.38 Fax Relay
The MGCP T.38 fax relay feature conforms to ITU-T T.38, “Procedures for real-time
Group 3 (G3) facsimile communication over IP networks,” which determines procedures
for real-time facsimile communication in various External Gateway Control Protocol
(XGCP) applications.
MGCP T.38 fax relay provides two modes of implementation:
■ Gateway-controlled mode: Gateways negotiate fax relay transmission by exchanging
capability information in SDP messages. Transmission of SDP messages is
transparent to the call agent. Gateway-controlled mode allows the use of a MGCPbased
T.38 fax without the necessity of upgrading the call agent software to support
the feature.
■ Call agent-controlled mode: Call agents use MGCP messaging to instruct gateways
to process fax traffic. For MGCP T.38 fax relay, call agents can also instruct gateways
to revert to gateway-controlled mode if the call agent is unable to handle the
fax control messaging traffic, as is the case in overloaded or congested networks.
MGCP-based T.38 fax relay enables interworking between the T.38 application that
already exists on Cisco gateways and the MGCP applications on call agents.
Following is the call flow for an MGCP-based T.38 fax relay:
1. A call is initially established as a voice call.
2. The gateways advertise capabilities in an SDP exchange during connection
establishment.
3. If both gateways do not support T.38 fax relay, fax pass-through is used for fax
transmission. If both gateways support T.38, they attempt to switch to T.38 upon fax
tone detection. The existing audio channel is used for T.38 fax relay, and the existing
connection port is reused to minimize delay. If failure occurs at some point during
the switch to T.38, the call reverts to the original settings it had as a voice call. If this
failure occurs, a fallback to fax pass-through is not supported.
4. Upon completion of the fax image transfer, the connection remains established and
reverts to a voice call using the previously designated codec, unless the call agent
instructs the gateways to do otherwise.
A fax relay MGCP event allows the gateway to notify the call agent of the status (start,
stop, or failure) of T.38 processing for the connection. This event is sent in both call
agent-controlled and gateway-controlled mode.