Faxing and PCI (was Re: [Asterisk-Users] Digium cards,

[Lee Howard]

Olivier Krief wrote:

> 1. Does using iaxmodem imply that, for example, incoming fax calls are 
> processed that way ?
> - PSTN calls are terminated on TDM board (Digium, Sangoma, ...)


Yes.  The PSTN call terminates there and (usually) takes a short IAX2 
hop over the loopback adapter to iaxmodem.

> - the media flow (G711 as it comes from PSTN) is forwarded without any 
> modification to the right instance of iaxmodem software process


Saying "without any modification" is risky.  Your Asterisk configuration 
may very well modify the audio independently.  The "without any 
modification" part depends upon your Asterisk configuration.

> - iaxmodem process are "managed" by hylafax application but are seen 
> and identified by Asterisk as IAX softphones (during above mentioned 
> registration)


Yes.  Also, iaxmodem doesn't *have* to register.  It's just one way of 
doing it.  If you have 30 iaxmodem processes all registering in the same 
fraction of a second then it can get nasty.

> - media is processed once, according hylafax config (that often means 
> forwarded to printer or email), after a single "move" from TDM board 
> to server processor through PCI bus


Yes, the media is processed by HylaFAX.

> 2. If it's correct, fax speeds and protocols (V29, G3, ...) are 
> defined by hylafax and iaxmodem.
> So, how do hylafax-iaxmodem speeds and protocols compare with those 
> from a standard fax machine ?
> I mean, if you had bought a new fax machine and had it connected to 
> the PSTN, would you get a different set of speeds and protocols ?


iaxmodem uses the spandsp library.  So currently its modulation protocol 
support is limited to V.27ter and V.29.  Partial (sending) V.17 support 
is available.  V.34 (Super G3) support is not.  So those are fax speeds 
2400 bps through 14400 bps for sending and 2400 bps through 9600 bps for 
receiving.  Most new fax machines that you would purchase off-the shelf 
would support either 2400-14400 or 2400-33600 depending on whether or 
not it supported Super G3.

> 3. How do iaxmodem and rxfax-txfax DSP functions compare ? Are they 
> using the same library ? Is there any standard allowing iaxmodem or 
> rxfax-txfax to benefit from hardware DSP inside TDM board ?


iaxmodem is a modem that uses spandsp and rxfax/txfax are fax 
applications that use spandsp.  Apples and oranges.

txfax/rxfax are flow-chart-equivalent to iaxmodem and HylaFAX together.  
There are a lot of functional and feature differences between 
txfax/rxfax and iaxmodem+HylaFAX.

iaxmodem does not access hardware directly, no.

> 4. How would that scale ? Up to 30 Calls ? Are incoming or outgoing 
> faxes more demanding ? Any hardware recommended ? 


You should be able to run a hundred or so iaxmodem instances with 
HylaFAX on most new systems without them making a dent in the system load.

Both incoming and outgoing faxes create more load in pre-processing (in 
the case of sending) or post-processing (in the case of receiving) than 
they do during the fax communication itself.  So the demand varies upon 
your configuration.  If you're going to be a lot of fax image processing 
(pre or post) then I would recommend offloading that to a different 
system somehow.

There are no hardware preferences for iaxmodem's sake.  However, you 
can't be experiencing so-called "frame slips", IRQ issues, or other 
issues that affect Asterisk audio.

Lee.