254 3Com® VCX V7111 VoIP Gateway User GuideFaxTransportMode = 2 (Bypass)V34ModemTransportType = 2 (Modem bypass)V32ModemTransportType = 2V23ModemTransportType = 2V22ModemTransportType = 2In this configuration, both T.30 and V.34 faxes work in Bypass mode.OrFaxTransportMode = 1 (Relay)V34ModemTransportType = 2 (Modem bypass)V32ModemTransportType = 2V23ModemTransportType = 2V22ModemTransportType = 2In this configuration, T.30 fax uses T.38 Relay mode while V.34 fax uses Bypass mode.Using Relay mode for both T.30 and V.34 faxesIn this scenario, V.34 fax machines are compelled to use their backward compatibility withT.30 faxes; as a T.30 machine, the V.34 fax can use T.38 Relay mode.See the following configuration:FaxTransportMode = 1 (Relay)V34ModemTransportType = 0 (Transparent)V32ModemTransportType = 0V23ModemTransportType = 0V22ModemTransportType = 0Both T.30 and V.34 faxes use T.38 Relay mode. This configuration forces the V.34 faxmachine to operate in the slower T.30 mode.Supporting V.152 ImplementationThe gateway supports the ITU-T recommendation V.152 (Procedures for Supporting Voice-Band Data over IP Networks). Voice-band data (VBD) is the transport of modem, facsimile,and text telephony signals over a voice channel of a packet network with a codec appropriatefor such signals.For V.152 capability, the gateway supports T.38 as well as VBD codecs (that is, G.711 A-lawand G.711 μ-law). The selection of capabilities is performed using the coders table.When in VBD mode for V.152 implementation, support is negotiated between the gatewayand the remote endpoint at the establishment of the call. During this time, initial exchange ofcall capabilities is exchanged in the outgoing SDP. These capabilities include whether VBD
