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

[Darren Nickerson]

On Feb 15, 2012, at 3:49 PM, Olivier wrote:

> 2012/2/15, Tim Nelson <tnelson at rockbochs.com>:
>> ----- Original Message -----
>>> Hi,
>>> 
>>> When someone says "T.38 is not reliable on a (normally loaded and
>>> managed) LAN", would you rather agree or disagree ?
>>> In this case, fax calls are coming in through an analog gateway,
>>> passing trough Asterisk and then going out to ISDN through a digital
>>> gateway.
>>> 
>> 
>> Is T.38 actually in use in this scenario? Or are you simply passing the fax
>> call through Asterisk as 'normal' audio (G.711u/a, etc)?
> 
> Yes, T.38 is in use between each gateway and Asterisk (I should have
> specified this more clearly) :
> Fax Machine <--> Analog Gw <--T.38--> Asterisk <--T.38 --> Digital Gw
> <--ISDN--> PSTN

Assuming you have Asterisk doing T.38 pass-through here, reinviting the T.38 payload to go directly between the analog GW and the Digital GW, and assuming that 'Digital Gw' has a good T.30 fax engine inside of it (because after all, the gateway is what's speaking convention audio-based fax to the remote sender/receiver, the above setup should work well independent of network conditions.

T.38 has ways of coping with extremely bad connections (via packet redundancy or FEC error correction) that you probably would not need on a LAN.

Note, however, the use of T.38 versus G.711 may limit the speed of your faxing to 14,400 and prevent the fax protocol from using its own error correction (many T.38 gateway implementations wrong-headedly disable ECM error correction). When it comes to faxing over a LAN, the choice of T.38 versus G.711 uLaw/aLaw is less than obvious. In your case, it will be highly dependent upon each piece of your call flow. The fax machine, the analog gateway, how Asterisk is setup, the digital gateway and the quality of the PSTN line. These days you cannot trust that your PSTN carrier is using TDM routes, sometimes they slip a little T.38 in the middle on you, and all bets are off.

No matter what scenario you go with though, you probably want to get Asterisk out of the media path and get a gateway-to-gateway conversation going eventually.

-Darren