The T38 to G711 Fallback Procedure

If the SIP External Gateway configuration parameter is “T38 to G711 Fallback” and if the IP Couplers in front of FAX are INTIP3/MG3 couplers and if the initial call is established with G711, OXE will try to establish the FAX in T38 mode.

If the remote SIP Party is not able to support FAX in T38 mode, it will send Error message. This will result in OXE to switch the FAX to G711 Mode.

Outgoing call
If OXE receives a RE-INVITE with T38 parameters, the call will proceed in T38. If OXE receives FAX call in G711, it will directly detect and handle it.

Incoming call 
Step1: When OXE detects a T38 FAX call, it sends Re-INVITE with T38 parameters as usual.
Step2: If the SIP Carrier accepts it and 200 OK is received with T38 parameters, then call proceeds in T38 mode.

Else if the SIP Carrier does not accept it and sends an Error response, the following cases are envisaged: -

• Case 1: If the negotiated codec is G711 and the IP couplers are INTIP3/MG3 couplers, then OXE will switch to G711 mode. 

• Case 2: If the coupler in front of FAX is other than INTIP3/MG3 coupler, or if the negotiated codec is G729/G723, the call is disconnected. 

Remark: If OXE is in transit position, the Error response will be relayed transparently

Overview of G711 Transparent Fax and T38 fallback G711

In a FAX over IP communication, when a SIP External Gatway is involved, the transmission is done through T38 Procedure. From OXE R11, the G711 procedure for fax communication is implemented, as well as a “Fallback” procedure from T38 to G711.

With this feature, OXE will support two more procedures. For SIP calls, FAS support will be done in 3 modes:

• The T38 only procedure

• The G711 transparent procedure

• The T38 to G711 Fallback procedure (In a first step, fax will try to establish with T38, if remote side doesn’t support it, it will fallback to G711 mode) The configuration of the above options is made in the corresponding External Gateway parameter (Fax procedure type).

Remark: this feature is applicable for the INTIP3/MG3 couplers only