3Com® VCX V7111 VoIP Gateway User Guide 255is supported and associated RTP payload types ('gpmd' SDP attribute), supported codecs,and packetization periods for all codec payload types ('ptime' SDP attribute). After this initialnegotiation, no Re-INVITE messages are necessary as both endpoints are synchronized interms of the other side's capabilities. If negotiation fails (i.e., no match was achieved for anyof the transport capabilities), fallback to existing logic occurs (according to the parameterIsFaxUsed).Below is an example of media descriptions of an SDP indicating support for V.152.v=0o=- 0 0 IN IPV4 s=-t=0 0p=+1c=IN IP4 m=audio RTP/AVP 18 0a=ptime:10a=rtpmap:96 PCMU/8000a=gpmd: 96 vbd=yesIn the example above, V.152 implementation is supported (using the dynamic payload type96 and G.711 mu-law as the VBD codec) as well as the voice codecs G.711 mu-law andG.729.Instead of using VBD transport mode, the V.152 implementation can use alternative relay faxtransport methods (e.g., fax relay over IP using T.38). The preferred V.152 transport methodis indicated by the SDP ‘pmft’ attribute. Omission of this attribute in the SDP content meansthat VBD mode is the preferred transport mechanism for voice-band data.FXO Operating ModesThis section provides a description of the FXO operating modes and gateway configurationsfor Tel-to-IP and IP-to-Tel calls.IP-to-Telephone CallsThe FXO gateway provides the following FXO operating modes for IP-to-Tel calls: One-stage dialing Waiting for dial tone Time to wait before dialing Answer supervision Two-stage dialing Dialing time Disconnect supervision
