Re: [hylafax-users] hylafax as T.30 debugger?

[Steve Underwood <steveu@xxxxxxxxxxx>]

Klaus Darilion wrote:
> Hi!
>
> I currently have some problems when using T.38 over 2,5G data networks 
> (EDGE). The setup is: Fax<-->ATA<--SIP+T.38-->GW<------>any PSTN fax
>
> Decoding T.38 with wireshark did not showed me yet why faxing fails 
> (sometimes it works, sometimes it fails during the handshake, 
> sometimes it aborts after a few pages). Thus I want to go deeper and 
> analyze the data the fax machines are really seeing.
>
> Thus, my idea is to use Hylafax as the PSTN fax. Unfortunately I do 
> not have real fax hardware, but will use a Digium card and 
> Asterisk+Iaxmodem+Hylafax (via loopback). Now the question is - can I 
> use Hylafax as T.30 debugger? Will it report the reason why some 
> errors happen (e.g. timeouts ...). What part of the fax decoding will 
> happen in iaxmodem and what will happen in hylafax? Maybe there is a 
> tutorial you can point me to for low level fax debugging.
Where does the 2.5G network fit in that chain? Is it

   Fax<-->ATA<--SIP+T.38 over GPRS/EDGE-->GW<------>any PSTN fax

If so, it chances of working are very poor. T.38 is designed to be a lot 
more robust than using audio FAXing over an IP network. However, it has 
to live within the timing constraints of the T.30 protocol, and that 
makes it vulnerable to heavy packet loss. Bursts of heavy packet loss 
are pretty normal for a cellular network.

You said that decoding the T.38 didn't revel anything. Try capturing at 
both ends of the IP link, and compare the results. I expect you'll see 
significant differences due to packet loss. You really don't need to 
probe deep inside the terminals to see what is going on with this type 
of problem.

Regards,
Steve


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