Switching to T.38 Mode using SIP re-INVITE
In the Switching to T.38 mode using the SIP re-INVITE, upon detection of a fax signal, the terminating device negotiates T.38 capabilities using a re-INVITE message. If the far-end device doesn't support T.38, the fax fails. In this mode, the 'Fax Transport Mode' parameter [FaxTransportMode] is ignored.
| ➢ | To configure T.38 mode using SIP re-INVITE messages: |
| 1. | On the Fax/Modem/CID Settings page, configure the following optional parameters: |
| ● | 'Fax Relay Redundancy Depth' [FaxRelayRedundancyDepth] |
| ● | 'Fax Relay Enhanced Redundancy Depth' [FaxRelayEnhancedRedundancyDepth] |
| ● | 'Fax Relay ECM Enable' [FaxRelayECMEnable] |
| ● | 'Fax Relay Max Rate' [FaxRelayMaxRate] |
The terminating gateway sends T.38 packets immediately after the T.38 capabilities are negotiated in SIP. However, the originating device by default, sends T.38 (assuming the T.38 capabilities are negotiated in SIP) only after it receives T.38 packets from the remote device. This default behavior cannot be used when the originating device is located behind a firewall that blocks incoming T.38 packets on ports that have not yet received T.38 packets from the internal network. To resolve this problem, you should configure the device to send CNG packets in T.38 upon CNG signal detection [CNGDetectorMode = 1].