3Com VCX V7111 VoIP SIP Gateways User Manual 153A PPENDIX C: DTMF, FAX, AND MODEMMODESDTMF Relay SettingsYou can control the way DTMF digits are transported to the remote endpoint, using theDTMFTransport configuration parameter. The following four modes are supported: DTMFTransportType= 0 (MuteDTMF) – In this mode, DTMF digits are erased from theaudio stream and are not relayed to the remote side. Instead silence is sent in the RTPstream. DTMFTransportType= 1 (Proprietary DTMF Relay) – In this mode, DTMF digits areerased from the audio stream and are relayed to the remote side using a proprietary RTPsyntax. DTMFTransportType= 2 (Transparent DTMF) – In this mode, DTMF digits are left inthe audio stream and the DTMF relay is disabled. DTMFTransportType= 3 (RFC 2833 DTMF Relay) – In this mode, DTMF digits arerelayed to the remote side using the RFC 2833 Relay syntax.Fax/Modem SettingsYou can choose to use one of the following transport methods for Fax and for each modemtype (V.21/V.22/V.23/Bell/V.32/V.34): Fax relay – Demodulation / modulation Bypass – Using a high bit rate coder to pass the signal Transparent – Passing the signal in the current voice coderWhen the fax relay mode is enabled, distinction between fax and modem is not immediatelypossible at the beginning of a session. The channel is therefore in Answer Tone mode until adistinction is determined. The packets being sent to the network at this stage are fax relaypackets (The packets can be either T.38-complaint, or FRF.11-based proprietary syntax,selected by setting the channel’s configuration parameter UseT38orFRF11).Configuring Fax Relay ModeWhen FaxTransportType= 1 (relay mode), then on detection of fax, the channelautomatically switches from the current voice coder to answer tone mode, and then to faxrelay mode. The UseT38orFRF11 configuration parameter defines either T.38-compliant
