SIP Fax / Modem Support

The IMG 2020 first establishes a normal voice call using an audio codec. It then negotiates T.38 support once the voice path is established. The information below describes some of the supported scenarios.

IMG 2020 is terminating a fax call

Once a fax data mode has been detected at the IP bearer level, SIP will send a RE-INVITE message to the distant end. The IMG 2020 supplies T.38 port and parameter information in the SIP RE-INVITE SDP and then waits for the remote SIP gateway to accept the RE-INVITE with an OK including a responding SDP.

IMG 2020 is originating a fax call

Once a fax data mode has been detected at the IP bearer level, SIP will accept a RE-INVITE message from the distant end. The IMG 2020 will reply with a fax port in a SIP 200 OK SDP and will wait for the remote SIP gateway to accept with an ACK.

• T.38 or fax bypass modes are supported. The IMG 2020 allows G.711 A-Law or G.711 u-Law only for fax/modem bypass codecs.

• RTP redundancy levels will apply to fax bypass packets (RTP G.711 packets), but not to T.38 packets.  

• Fax Redundancy level setting applies only to T.38 fax relay packets (not to RTP voice or fax bypass packets).

Fallback to Fax Bypass

The Fax Fallback feature is a backup mechanism to transmit a fax using Fax Bypass mode when T.38 cannot be negotiated successfully. This feature allows you to configure T.38 Fax Relay as the preferred type, and also allow Bypass Fax when T.38 is not supported by the remote end. The added negotiation will therefore reduce the call setup failure rate by increasing the content of the media offer.

In the event neither a T.38 fax nor a Bypass fax can be established in a fax  fallback scenario, the IMG 2020 allows the voice call to proceed as if no negotiation had happened. 

Additional Information

For configuration information refer to the following topics

 IMG 1010 - IP Bearer Profile