truth or obscurantism ?

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Asterisk Users 8 Comments


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

Comments ?


8 thoughts on - truth or obscurantism ?

  • 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.

    The question people usually ask is whether fax over G.711 is unreliable on a LAN. To which the answer would be a definite ‘it depends’ 😉


  • 2012/2/15, Tim Nelson :

    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

  • 2012/2/15, Darren Nickerson :


    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.


  • 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.


  • While I can’t speak for Asterisk’s T.38 performance (it was barely past
    the point of “okay, it compiles” at the time of this datapoint), T.38 in
    general can handle nasty network conditions without a problem as long
    you enable some sort of error correction (either FEC or packet
    redundancy). Case in point, I ran several hundred SIP-based T.38 calls
    a month over VSAT links. The links ran anywhere from 550 to 750ms
    latency and would average around 1-2% packet loss (averaged over a 5
    minute period). Those were with a Quintum ASG400 at the far end and a
    Quintum CMS960 going into PRIs at the VSAT hub.

    So if T.38 can handle that, it can handle just about anything.

  • It seems like the bigger issue is interop between different gateways and
    fax machines. It seems to work 99% of the time, but that 1% is
    impossible to fix if you can’t control all the variables. If you
    *could* control all the variables you’d probably not be faxing. 🙂