Fax Transmission behind NAT

The device supports transmission from fax machines (connected to the device) located inside (behind) 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 problem, the device sends No-Op (“no-signal”) packets to open a pinhole in the NAT for the answering fax machine. The originating fax doesn't 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 packet feature is enabled using the [NoOpEnable] and [NoOpInterval] parameters.