[asterisk-users] T.38 - Which endpoint shall reINVITE ? caller or callee ?

[Steve Underwood]

Vlasis Hatzistavrou (KTI) wrote:
> Olivier wrote:
>   
>> Hi,
>>
>> I've been playing with T.38.
>>
>> I observed that mostly but not always, it's the "calling endpoint" that 
>> reINVITE the other party to drop current SIP/G711 session and start a 
>> new T.38.
>> But sometimes, it's also the callee party that reINVITE the calling party.
>>
>> Which is the "standardized" or most common, way to start a T.38 session ?
>> Shall it come from callee or from caller ?
>>
>> Regards
>>     
>
> Fax transmission can be initiated from any one of the parties. AFAIK 
> T.38 as well as the PSTN fax standards do not show any preference 
> whether fax transmission is requested from a or b party.
>
> In practice, the caller usually initiates a fax transmission, but this 
> doesn't mean that the called party cannot initiate it, too.
>
> Best regards,
> Vlasis Hatzistavrou.
>   
Hey, why bother looking at a spec when its so much more fun to make it 
up as we go along?

T.38 says that if the call starts in audio mode it is the called end 
which should initiate a re-invite to change from audio to T.38. This 
makes sense, as that is the end which has the best chance of figuring 
out if a FAX machine answers the call. In practice many T.38 
implementations will send out a re-invite when they are the calling 
side, so any practical implementation has to allow for this. Clashes are 
possible, if both ends send re-invite, and this is not always handled 
properly  Also many implementations will only listen for a FAX machine 
at the beginning of a call, so if a human answers and later presses the 
start button on their FAX machine the T.38 gateway might miss this.

Regards,
Steve