[asterisk-users] Still on spandsp/app_fax and T.38

[Steve Underwood]

On 02/15/2010 11:27 PM, Kevin P. Fleming wrote:
> Steve Underwood wrote:
>
>    
>> FFA sends its repeating no-signal and preamble packets with incrementing
>> sequence numbers. While its not the only system which does that, it
>> confuses some T.38 implementations. The T.38 spec is too vague to say
>> whether the practice is right or wrong. In other applications of the FAX
>> machine in spandsp, the software has been set up to send multiple copies
>> on the key packets, with the same sequence number for all the copies.
>> This seems to be the most compatible way to send these repeats. I don't
>> know if the UDPTL infrastructure in Asterisk can do that.
>>      
> No, it can't without some modifications. The Asterisk UDPTL stack
> receives raw IFPs from the application, and generates sequence numbers
> itself. At this time internal Asterisk frames have no way of being
> marked as a 'repeat' frame, or marked as 'send this frame _x_ times',
> but that certainly could be done if it was deemed useful and worthwhile.
>    
In callweaver we gave the frames an extra parameter, which is the number 
of copies to send. It is the UDPTL code which creates the extra copies 
on the wire. They are not spaced in time, which would probably be a good 
enhancement. Packet loss tends to be bursty, so if you loose one packet 
sent right now you probably loose all of them.

Steve