H.323 Fax-Modem Support

T.38 Fax

The IMG 2020 negotiates T.38 support at call establishment time. A T.38 port is supplied in the Fast Start component of the Setup, Call Proceeding, Alerting, and Connect message. Once the fax data mode has been detected at the IP bearer level, H.245 will send a RequestMode message to the distant end.  If the distant end had not supplied a T.38 port in the Fast Start component of the Setup, Call Proceeding, Alerting, or Connect message at call establishment time, this information must be supplied in the openLogicalChannelAck message. 

Fax Bypass

T.38 or fax bypass modes are supported (except for the AMR codec) in the IMG 2020.  Fax bypass codes are not negotiated by H.323 so they must be configured to match properly at each gateway. The IMG 2020 allows G.711 A-Law or G.711 u-Law only for fax/modem bypass codecs. Once the fax data mode has been detected at the IP bearer level, H.245 will send a RequestMode message to the distant end requesting to change the codec to the fax bypass codec.

Fallback to Fax Bypass

The Fax Fallback feature is a backup mechanism used to transmit a fax using Fax Bypass mode when T.38 cannot be negotiated successfully. The Fallback to Fax feature gives the user the ability 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.

Redundancy

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).

Port Number

For the VoIP Module SLM-IPM-0001, the IMG 2020 always assigns the T.38 port to be "RTP port + 2". RTCP port is always assigned as "RTP Port + 1". For the VoIP Module SLM-IPM-1001, the T.38 Port and the RTP port are the same.