Version 5.8 403 September 2009
SIP User's Manual 9. IP Telephony Capabilities
îš„ V23ModemTransportType = 2
îš„ V22ModemTransportType = 2
Configure the following parameters to use bypass mode for V.34 faxes and T.38 for T.30
faxes:
îš„ FaxTransportMode = 1 (Relay)
îš„ V34ModemTransportType = 2 (Modem bypass)
îš„ V32ModemTransportType = 2
îš„ V23ModemTransportType = 2
îš„ V22ModemTransportType = 2
9.5.3.2 Using Relay mode for both T.30 and V.34 faxes
In this scenario, V.34 fax machines are forced to use their backward compatibility with T.30
faxes and operate in the slower T.30 mode.
Use the following parameters to use T.38 mode for both V.34 faxes and T.30 faxes:
îš„ FaxTransportMode = 1 (Relay)
îš„ V34ModemTransportType = 0 (Transparent)
îš„ V32ModemTransportType = 0
îš„ V23ModemTransportType = 0
îš„ V22ModemTransportType = 0
9.5.4 V.152 Support
The device 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
appropriate for such signals.
For V.152 capability, the device supports T.38 as well as VBD codecs (i.e., G.711 A-law
and G.711 μ-law). The selection of capabilities is performed using the coders table (refer to
''Configuring Coders'' on page 105).
When in VBD mode for V.152 implementation, support is negotiated between the device
and the remote endpoint at the establishment of the call. During this time, initial exchange
of call capabilities is exchanged in the outgoing SDP. These capabilities include whether
VBD is supported and associated RTP payload types ('gpmd' SDP attribute), supported
codecs, and packetization periods for all codec payload types ('ptime' SDP attribute). After
this initial negotiation, no Re-INVITE messages are necessary as both endpoints are
synchronized in terms of the other side's capabilities. If negotiation fails (i.e., no match was
achieved for any of the transport capabilities), fallback to existing logic occurs (according to
the parameter IsFaxUsed).
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