252 3Com® VCX V7111 VoIP Gateway User Guidethe RFC 2833 payload type using local and remote SDP and sends packets using the PTfrom the received SDP. The gateway expects to receive RFC 2833 packets with thesame PT as configured by the RFC2833PayloadType parameter. If the remote side doesnot include telephony-event in its SDP, the gateway sends DTMF digits in transparentmode (as part of the voice stream).3 Sending DTMF digits (in RTP packets) as part of the audio stream (DTMF Relay isdisabled):Note that this method is normally used with G.711 coders; with other LBR coders thequality of the DTMF digits is reduced.To ser this mode: TxDTMFOption = 0 (1 st to 5 th DTMF Option = Disable) RxDTMFOption = 0 (Declare RFC 2833 in SDP = No) DTMFTransportType = 2 (DTMF Transport Type = Transparent DTMF)The gateway is always ready to receive DTMF packets over IP, in all possibletransport modes: INFO messages, NOTIFY and RFC 2833 (in proper payload type)or as part of the audio stream.To exclude RFC 2833 Telephony event parameter from the gateways SDP, setRxDTMFOption = 0 in the ini file.The following parameters affect the way the V7111 SIP handles the DTMF digits: TxDTMFOption, RxDTMFOption and RFC2833PayloadType (described in Table 22). MGCPDTMFDetectionPoint, DTMFDigitLength and DTMFInterDigitInterval (Table 59). DTMFVolume and DTMFTransportType (Table 56).Fax & Modem Transport ModesFax/Modem SettingsUsers may choose to use one of the following transport methods for fax and for each modemtype (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 T.38-compliant fax relay packets.
