[Asterisk-Users] Which asterisk-friendly cards are fax-capable?

[Lee Howard]

Craig Guy wrote:

> I'm trying to figure out what an appropriate deployment model might 
> be. Whether to have iaxmodem installed on the hylafax server with a 
> switched ethernet connection for iax2 to the * server with the PRI, or 
> to have the iaxmodem on the PRI * server and channel the tty comms 
> across the network.


You can try running the IAXmodem IAX channel over your switched network, 
but my recommendation would be to always run IAXmodem on the Asterisk 
system (to prevent even minute audio corruption).  In my experience 
passing modulated fax audio over a small LAN has not been that big of a 
problem.  Everyone that plugs fax machines into SIP ATAs (and even 
IAXys, I've heard) are a testimony to that.  However, in those 
situations I think that either they have a very well-tuned network, a 
very low-traffic network, or the ECM capabilities and protocol error 
recovery features of their fax machines are managing to work around any 
audio corruption that may occur.  I wouldn't recommend passing modulated 
fax audio over a UDP/IP network for businesses where those faxes are 
critical.

As you may observe from the IAXmodem docs and patchset within the 
tarball, I have used IAXmodem in conjunction with termnetd+ttyd from the 
termpkg package.  In my testing and small production usage with that 
configuration I have not had any severe problems with the tty or with 
any degree of data corruption occurring.  However, I'm not yet convinced 
that the modem initialization, resetting, and other control handling 
that occurs on both ends of HylaFAX-faxmodem communications.  In other 
words, I'm not yet certain that I've tuned my termpkg usage perfectly 
for use on high-traffic deployments where one call may arrive moments 
after the last one ended.  If my concerns are confirmed and if there is 
no solution with termpkg to improve things, then I will have to create a 
"busy-out" AT command for IAXmodem that will tell the modem to return 
congestion until the "busy-out" setting is removed, and HylaFAX would 
busy the modem out during initialization and reinitialization cycles.  I 
do this already with other DID modems where "busying out" a "line" is 
possible.

> I suspect that the latter might work ok over a WAN so I could have a 
> central hylafax server with distributed * servers running iaxmodem at 
> the far end of wan links (up to 100ms latency).


I would suspect that you could run remote ttys over the internet and 
still use them for fax, yes... as long as IAXmodem is on or very close 
to the Asterisk server.

> The only issue is that I want to retain rxfax on the PRI * servers for 
> incoming faxes.
>
> Lee, if I install iaxmodem on a * server for outbound faxing from 
> hylafax, can I still use rxfax on the same server to receive faxes? 


If you're really so-possessed, yes.  ;-)

The only trick to watch out for is spandsp.  Both rxfax and IAXmodem use 
spandsp, so you'd want to make sure that the version of spandsp that 
you're using is happy with both rxfax and IAXmodem.

Lee.