how to set iaxmodem receiving speed

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Asterisk Users 13 Comments

Hi all,

I’m trying to lower my iaxmodem speed but still I haven’t found any solution…I tried to add
Class1RMQueryCmd: “!24,48,72”
to config.IAXtty but does not work…Hylafax says it it running at 9600 (sometimes at 14400) baud..

Any ideas?

Thank you.


13 thoughts on - how to set iaxmodem receiving speed

  • I have iaxmodem version 1.2.0 installed on my system.

    I have set the following in the IAX configuration file, SIGHUP’d
    FaxGetty and submitted a single page outbound fax via Asterisk;

    Class1RMQueryCmd: “!24,48,72” # enable this to disable V.17
    Class1TMQueryCmd: “!24,48,72” # enable this to disable V.17

    The resulting output from my T.38 Gateway reports the following;

  • Read the subject line more closely.

    Tested receiving too,

    I set the Send & Receive speed of the receiving analogue modem to that
    below, the log file on the sending modem (iaxmodem) reported it capable
    of 9600.

    May 16 21:32:04.28: [ 2335]: REMOTE best rate 9600 bit/s
    May 16 21:32:04.28: [ 2335]: REMOTE max A3 page width (303 mm)
    May 16 21:32:04.28: [ 2335]: REMOTE max unlimited page length
    May 16 21:32:04.28: [ 2335]: REMOTE best vres R16 x 15.4 line/mm
    May 16 21:32:04.28: [ 2335]: REMOTE format support: MH, MR, MMR
    May 16 21:32:04.28: [ 2335]: REMOTE supports T.30 Annex A, 256-byte ECM
    May 16 21:32:04.28: [ 2335]: REMOTE best 0 ms/scanline
    May 16 21:32:04.28: [ 2335]: USE 9600 bit/s

    Perhaps the issue is with Hylafax.

    Setting the Transmit & Receive strings to “!24,48,72,96” seems to yield
    the most reliability in transmission



  • Hi Larry,

    thank you for your answer.

    This is same test I did. After this I lowered again to 4800…result:
    iaxmodem receives at 9600 b/s…..that’s why I cannot solve the puzzle.

    I put that line (Class1RMQueryCmd: “!24,48,72”) in config.IAXtty
    and tried other configuration files too but I always get faxes at 9600.

    Thank you


  • Hi Larry,

    I forgot to mention I tried to set ModemRate at 4800 as well but without


  • Did you restart iaxmodem and hylafax after you made those changes?


  • Hi,

    If you have an ATA in the path that is often the case. Many of them
    badly mess up a FAX signal. Without such a distortion machine V.17
    should be fine.
    This is correct behaviour. The sending side has fine control over the
    modem modes it uses. The receiving side can only specify that V.27ter,
    or V.27ter+V.29 or V.27ter+V.29+V.17 are OK. So, if you allow the
    7200bps mode of V.29 you are compelled to allows the 9600bps mode too.


  • The receiving analogue modem is directly connected to the PSTN network
    and was used to to determine if the reported issue could be reproduced
    on a non-iaxmodem.

    My outgoing connections were through an iaxmodem with T.38 gateway
    enabled and disabled, with most successful transmissions being when T.38
    gateway was used.


  • Hi Giorgio,

    You may want to try these settings to set the most basic form of
    transmission on your receiving modems, I would however have thought, ECM
    being on would be better for you as it could then deal with lost frames.

    Class1MRSupport: no
    Class1MMRSupport: no
    Class1ECMSupport: no

    To set the ECM frame size to a lower value than the default of 64, you
    would set the following

    Class1PersistentECM: yes
    Class1ECMFrameSize: 64

    Perhaps the corruption is occurring at the senders end before the data
    is pushed through the modem.



  • Hi guys, thanks for answers.

    That could seem counter-intuitive but it is not. Not to mention the fact
    that information technology is not science, the solution to broken faxes
    is to lower down speed. This works even with normal telco lines even if
    you DO NOT have a pbx (telco technicians even say not to make faxes pass
    thru your PBX). I could ask my customer’s telco to lower the speed down
    but it depends on the guy working at the call-center…sometime you talk
    to dummy people who ARE sure it is impossible. But it is not. So, I do
    not want to spend days to convince people working at that telco
    call-center that what I’m asking is feasible and I do not want to tell
    my customer to tell their customer to lower their faxes speed (before
    installing our PBX they were able to send perfect faxes so, why should

    My idea was to tell iaxmodem not to accept fast speed rates so the fax
    machine on the other side should be forced to negotiate a slower speed
    as if my customer fax weren’t virtual as iaxmodem is but a real one.

    I suspect that the problem is about the primary lines because I tested
    iaxmodem many times on my LAN and it is (surprisingly 🙂 ) working fine
    (10 good received faxes out of 10 sent!!!) but, as you may know, talking
    to telco technician is a nightmare….they always say problems are
    always on the PBX side… 🙁

    Moreover, after sending a fax, the fax machine beeps correctly as the
    fax was correctly sent without corruption. 😮

    I hope I have made my point but I’ll try do dig deeper inside the
    problem as you suggested me.

    Thank you. 🙂


  • And as a matching data point, we use ActiveFax for sending (interfaced
    from an ERP package) and often get Comm Error 283 and incomplete
    faxes. If it’s just making a bad situation worse, how is it that our
    solution of turning off ECM mode fixes it 98% of the time? I’m
    curious. (We know it’s fixed as often we’re the receiver, and we can
    see the correct fax come through.)

    Sometimes we lower the speed too (to 9600) but often simply disabling
    ECM is the solution.


  • The DSP algorithms change slightly between bitrates and considerably
    between modulations. Changing the modulation/demodulation algorithms
    can many times avoid problems caused by some type of line audio
    disturbance. This is why fax machines and fax applications are
    programmed to try various bitrates and modulations when training is
    failing at the default settings.

    Because historically most fax machines have supported V.17 14400 bps as
    a default it became customary for support technicians to suggest
    “slowing” it down to 9600 bps. (I think that the real intent here was
    to switch to V.29 9600 bps, but in practice it often results in V.17
    9600 bps.) The purpose in this isn’t really so that the communication
    takes longer (you can imagine that stretching-out data over a longer
    period would increase the likelihood of some audio disturbance affecting
    the demodulation), but instead I believe that the purpose in this
    recommendation is to cause a change in the DSP algorithms.

    Now, disabling ECM (error correction) is just plain wrong as long as the
    ECM protocol is implemented properly on both ends. If ECM protocol is
    implemented properly on both ends (and most are implemented well-enough
    that this applies to them) then ECM should be left enabled. By
    disabling a well-implemented ECM feature you’re essentially making the
    claim that the remote-side ECM protocol is broken.

    If disabling ECM actually makes things work and you never get a
    corrupted fax image come through after that then it only means that one
    or both of the endpoints had faulty ECM protocol.

    Some technicians (including those working for fax machine manufacturers)
    will recommend disabling ECM if faxes aren’t getting through. While
    this may have originated with the purpose of avoiding problems in faulty
    ECM protocol I think that any regular use of this suggestion is simply
    to get the customer to go away. The customer will see a page come
    through with streaks and lines, but it will be “successful”, and so
    they’ll unfortunately be happy with that enough to let the technician
    off-the-hook with the “disable ECM” advice instead of actually fixing
    the real problem (either getting the line audio quality problem
    corrected or fixing the broken ECM protocol).

    And what you were doing with the HylaFAX modem config file for the
    iaxmodem should have worked to do this. Why it wasn’t working can only
    be determined by investigating your installation.



  • Because apparently the ECM protocol in ActiveFax is broken.

    If disabling a feature that is designed to *improve* fax reliability and
    performance actually does the opposite, then there’s no other
    explanation than to conclude that the implementation of that feature is
    broken in the product you’re using.