Fax from FXS to PRI

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On Tue, 2011-09-20 at 20:57 -0500, Don Kelly wrote:
> On Tue, Sep 20, 2011 at 4:43 PM, Adam Moffett
> wrote:
> If I have a 4 port Digium FXS card and a single port PRI card on the
> same asterisk box, is it expected that I’d be able to plug a fax
> machine into the analog FXS port and have no problems sending or
> receiving faxes? Our connection to the Telco is on the PRI obviously.
> Nobody can say for sure. It is not a supported configuration. I can
> tell you that I have had great success and wasted days messing around
> with this configuration.

> Again, it has never been a supported configuration by Digium, and
> everyone that has dealt with faxing in Asterisk especially on
> different systems will tell you that you won’t know until you try.
> And even then, is it worth days of your time trying to get it as
> close to a POTS line as possible?

> Thanks,
> Steve T
> This is a scary answer—you’re saying that what should be simple “TDM”
> FXS to PRI does not work?
> Are you suggesting this is an Asterisk problem or a Digium hardware
> problem?

Like most faxing issues, at it’s root it is a timing problem IMO.
Sangoma makes a special timing cable to link their cards so you can do
exactly what you are asking to do. I’ve never purchased it, but last I
looked into the issue, that is what they suggested.


One thought on - Fax from FXS to PRI

  • There is no TDM connection on a PCI or PCI-Express bus. Transferring
    data between two cards in the system either requires a direct connection
    between them (there are older, more expensive systems out there that use
    H.100 for this purpose) or the data has to go through the computer and
    its software.

    This means the data has to be packetized (even DAHDI’s 1ms chunks are
    still packets), delivered to the host memory, an interrupt sent and
    acknowledged, and some code on the CPU then has to copy the data to
    another place in host memory for it to be sent out to the other card.
    Modern systems are of course fast enough to do this, but there are a lot
    of variables here, and it’s possible for data to be occasionally delayed
    on its way between cards. With a voice call, this will likely not be
    noticed at all, but with a modem call or FAX call, it can be catastrophic.

    It’s not a ‘problem’ with the hardware or the software; it’s the nature
    of the beast. If you want 100% guaranteed TDM reliability, you have to
    use TDM connections, not packetized connections. Packetized connections
    can strive for 100% reliability, but most of them will never achieve it.

    For many people, with modern CPUs, current versions of DAHDI and
    Asterisk, and appropriate configuration (using the faxbuffers option in
    chan_dahdi.conf, for example), such a system can be setup to work very,
    very close to 100% of the time.