asterisk users - Oct 2013 - dahdi fax catch-22

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.

Thanks,
--Greg

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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