OT – Is T.38 Possible On SPA8800 FXO Port ?

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Asterisk Users 8 Comments


If I’m not mistaken, it is not possible to get T.38 on a SPA3102 FXO
port (it is possible with the FXS port). Do you know, by experience preferably, if this is possible with an SPA8800 FXO port ?


8 thoughts on - OT – Is T.38 Possible On SPA8800 FXO Port ?

  • Perhaps page 50 (Using a Fax Machine) of this document http://www.cisco.com/c/dam/en/us/td/docs/voice_ip_comm/csbpvga/ata/administration/guide/ATA_AG_v3_NC-WEB.pdf will answer your question.

    There is also another document you may want to reference.


    In your sip.conf configuration you will need;


    For newer version of Asterisk I expect you would set;


    You may want to change the SPA8800 option ‘FAX T38 Redundancy’ to the value you have set for redundancy in udptl.conf or 3 if using the default settings.

    It’s been a while since I played with my SPA8800, with the test I
    performed it did work, that was inbound and outbound calls.


  • Reading your question again, no the FXO port will not perform T.38, you are limited to G711 pass-through, from memory my SPA8800 was on the LAN
    the Asterisk PBX was attached and faxes were pretty good though I would have preferred to see T.38.

    Grandstream ATA’s support T.38 on the FXO port.


  • 2014-11-17 12:55 GMT+01:00 Larry Moore :

    Thanks for sharing this here:
    while it’s not always easy to discover what a product can do, it’s harder to know what it CANNOT do.


    I had a lot of trouble to localize SPA3102 settings for Caller ID
    presentation, for instance.

    How does Grandstream localization works ?
    Do you also have hundreds of parameters for this or do you simply select a Country within a pre-configured list (like Patton smartnodes)

  • T.38 is a relay protocol for transmitting Group 3 facsimiles over a packet network. It has nothing to do with the analog FXS or FXO signaling, but it should not be too difficult to use Asterisk as a gateway. I never needed to do this myself, so I can’t tell whether there are any subtle points.


  • I was fortunate that there is plenty of information for configuring an SPA3000 & SPA3102 for my country.

    Perhaps page 53 of this document http://www.cisco.com/c/dam/en/us/td/docs/voice_ip_comm/csbpvga/ata/administration/guide/ATA_AG_v3_NC-WEB.pdf may help, the other parameter I needed to adjust to receive Caller ID
    was the ‘PSTN Answer Delay’ setting.

    I can’t recall any great pain setting up an HT-503, I suspect I based settings upon an SPA3102/SPA8800, the T.38 faxing on the FXO was successful after a Firmware Upgrade, it was a couple of years ago when I
    was setting this up.


  • 2014-11-17 12:16 GMT+01:00 jg :

    I know all this.

    My question came from the fact that as strange as it may seem, SPA3102
    and similar products do not offer the SIP features depending on terminating/originating port. More precisely, when a SIP fax call comes in through an FXS port, it triggers T.38 while it doesn’t trigger T.38 when an FXO port is used instead.

    Yes, sure.

  • So, if i understand the question correctly, you have:
    Asterisk SIP-> SPA3102 FXS -> Analog fax machine and the PBX to SPA3102 communications are T.38 before converting to analog to go to the fax machine. Then in the other situation you would have:
    Analog line -> SPA3102 FXO port -> Asterisk SIP
    and the SPA3102 to Asterisk communication isn’t doing T.38?

    If that is the case, the only thing I can think of is that maybe they were not thinking that many people would want to do the second situation with a low end device? I imagine the main use case it to keep the old analog device around after switching to SIP delivery. Probably didn’t expect to see analog delivery that gets translated to a sip fax endpoint.

    Though I do agree that once you have the transcoding option, I would think it would be trivial to apply it to both ports.

  • 2014-11-18 15:19 GMT+01:00 Kevin Larsen :

    I fully agree.

    Maybe a simple 3102 has memory limitations, but I would expect an
    8-ports device (SPA8800) to alsa support T.38 with communications passing through FXO ports. Thanks to Larry’s testimony, I’m aware this is not the case.