Fax Transmission Behind Nat - AudioCodes Mediant 800B User Manual

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.

14.2.7 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 upon receipt of a
re-INVITE with T.38 only in the SDP, or T.38 and audio media in the SDP. This feature is
configured using the T38FaxSessionImmediateStart parameter. The No-Op packets are
enabled using the NoOpEnable and NoOpInterval parameters.
User's Manual
Mediant 800B Gateway and E-SBC
182 Document #: LTRT-10620