[asterisk-users] dahdi fax catch-22

[Richard Mudgett]

On Wed, Oct 30, 2013 at 8:40 AM, Greg Woods <greg at gregandeva.net> wrote:

> I've got a Digium Wildcard TDM410P with one POTS line and three
> extensions. One of the extensions is connected to a fax modem. This kind
> of works, but there's a gotcha. If I set faxdetect=incoming in
> chan_dahdi.conf, then incoming faxes do get routed to the modem and this
> all works, but outbound faxes fail, with a message like this:
>
> [Oct 29 21:57:09] WARNING[16732][C-00000000] app_dial.c: Unable to
> create channel of type 'DAHDI' (cause 17 - User busy)
>
> If I set faxdetect=no, then outbound faxes work, but of course incoming
> faxes do not get routed to the modem. I'm guessing that what is
> happening is that the "outbound" call coming in from the modem's channel
> is an "incoming" call on that channel, asterisk detects the fax and
> tries to route it to the same channel, resulting in busy.
>
> Is there any way around this catch-22? Is there some way to set
> faxdetect=incoming only on the one POTS line channel?
>
> The only alternative I can think of would be a kludgy Radio Shack-style
> solution like a manual switcher, that switches the modem between the
> connection to the dahdi channel (receive) and a direct connection to the
> POTS line (send). I'd really like not to have to resort to that.


One thing you must remember about the [channels] section in chan_dahdi.conf
is that *all* configuration in the section is cumulative.   For example:
[channels]
echocancel = yes
faxdetect=incoming
signaling=fxs_ks
channel=1
; Channel 1 is created at this point with echocancel enabled, faxdetect is
incoming,
; and fxs_ks signaling.

faxdetect=no
channel=2
; Channel 2 is created at this point with echocancel enabled, faxdetect is
no,
; and fxs_ks signaling.  The echocancel and signaling settings were set
before
; channel 1 was created and those settings are still in effect when channel
2
; is created.

Richard
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