[asterisk-users] OT - "T.38 unreliable on a LAN" : truth or obscurantism ?

[Darren Nickerson]

On Feb 15, 2012, at 4:03 PM, Olivier wrote:

> 2012/2/15, Darren Nickerson <darren.nickerson at ifax.com>:
>> T.38 is tolerant of most network conditions, ... the challenges in getting
>> reliable performance are usually limited to getting the interop right once,
>> but the absolute success rate will depend on the quality of your T.38/PSTN
>> gateway's fax implementation. In general terms, T.38 is actually the right
>> way to cope with lossy or high jitter network conditions, and so it's
>> reliable over most networks.
> 
> Yes.
> 
> An other thing to factor in, is how Asterisk's load could influence
> its capability to let faxes passing through. To me, if Asterisk is
> installed on a modern CPU (dual core and more) and is configured in
> such a way that no transcoding happen, then passing faxes through is
> easy and works reliably.
> 
> Opinions ?

The devil is in the details, but in general it's nowhere near that simple. You don't clarify what "pass-through" role Asterisk is playing here. G.711? T.38? What are you passing through TO? A TDM card connected to the PSTN? Or some SIP trunking provider, who themselves may be using G.711 or T.38 ... 

Assuming you mean the specific case of one local LAN hop over SIP, connecting directly to a well-configured PSTN card on the same Asterisk server, it's possible to get reliable faxing over G.711 with careful network configuration, good and well configured ethernet interfaces, correct jitter buffer, gain and echo cancelation settings, etc etc.

-d