T.38 Fax Relay Mode

In Fax Relay mode, fax signals are transferred using the T.38 protocol. T.38 is the ITU standard for sending fax across IP networks in real-time mode. The device currently supports only the T.38 UDP syntax.

T.38 can be configured in the following ways:

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Switching to T.38 mode using SIP re-INVITE messages (see Switching to T.38 Mode using SIP Re-INVITE)

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Automatically switching to T.38 mode without using SIP re-INVITE messages (see Automatically Switching to T.38 Mode without SIP Re-INVITE)

When fax transmission ends, the reverse switching from fax relay to voice is automatically performed at both the local and remote endpoints.

You can change the fax rate declared in the SDP, using the 'Fax Relay Max Rate' parameter [FaxRelayMaxRate]. The parameter doesn't affect the actual transmission rate. You can also enable or disable Error Correction Mode (ECM) fax mode using the 'Fax Relay ECM Enable' parameter [FaxRelayECMEnable].

When using T.38 mode, you can define a redundancy feature to improve fax transmission over congested IP networks. This feature is activated using the 'Fax Relay Redundancy Depth' parameter [FaxRelayRedundancyDepth] and the 'Fax Relay Enhanced Redundancy Depth' parameter [FaxRelayEnhancedRedundancyDepth]. Although this is a proprietary redundancy scheme, it should not create problems when working with other T.38 decoders.