IMG 1010 - SIP Fax-Modem
Overview:
The IMG first establishes a normal voice call using an audio codec. It then negotiates T.38 support once the voice path is established. Below are some scenarios that the IMG supports.
As of software version 10.5.3 SP7.1, support for the AMR codec for fax/modem bypass was added.
IMG 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 supplies T.38 port and parameter information in the SIP RE-INVITE SDP and then waits for the remote SIP gateway to accept with an OK including a responding SDP.
IMG 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 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 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 allows the voice call to proceed as if no negotiation had happened.
Related Topics:
For configuration information see: