McDewey

T.38 Fax Relay Fax Relay is a fax transport method where the TGWs and OGWs detect the T30 signals and page data. The gateways take those signals and convert them into relay messages, which are digital representations of the analog signals. Those relay messages are then sent through the IP network.
The T.38 Fax Relay switchover is also triggered by the detection of the V.21 preamble on the TGW. When the TGW operates with SIP, detection of the V.21 preamble triggers a T.38 ReINVITE (similar to what was previously described). G When the TGW operates with H323, detection of the V.21 preamble triggers a T.38 Request Mode. G When the TGW operates with MGCP, detection of the V.21 preamble triggers a notify (NTFY), which is sent to the Call Agent. The Call Agent then responds with a 200 OK, and sends either a Request Mode or a ReINVITE to CUBE, which depends on the VoIP protocol used. G