Version 6.4 151 March 2012
SIP User's Manual 12. Media
Instead of using VBD transport mode, the V.152 implementation can use alternative relay
fax transport methods (e.g., fax relay over IP using T.38). The preferred V.152 transport
method is indicated by the SDP ‘pmft’ attribute. Omission of this attribute in the SDP
content means that VBD mode is the preferred transport mechanism for voice-band data.
To configure T.38 mode, use the CodersGroup parameter.
Note: You can also configure the device to handle G.711 coders received in INVITE
SDP offers as VBD coders, using the HandleG711asVBD parameter. For
example, if the device is configured with G.729 and G.711 VBD coders and it
receives an INVITE with an SDP offer containing G.729 and “regular” G.711
coders, it sends an SDP answer containing G.729 and G.711 VBD coders,
allowing subsequent bypass (passthrough) sessions if fax / modem signals
are detected during the call.
12.2.5 Fax Transmission behind NAT
The device supports transmission from fax machines (connected to the device) located
inside (behind) a Network Address Translation (NAT). Generally, the firewall blocks T.38
(and other) packets received from the WAN, unless the device behind the NAT sends at
least one IP packet from the LAN to the WAN through the firewall. If the firewall blocks T.38
packets sent from the termination IP fax, the fax fails.
To overcome this, the device sends No-Op (“no-signal”) packets to open a pinhole in the
NAT for the answering fax machine. The originating fax does not wait for an answer, but
immediately starts sending T.38 packets to the terminating fax machine.
This feature is enabled using the T38FaxSessionImmediateStart parameter. The No-Op
packets are enabled using the NoOpEnable and NoOpInterval parameters.
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