[asterisk-users] Fax from FXS to PRI

[Kevin P. Fleming]

On 09/20/2011 08:57 PM, Don Kelly wrote:

> This is a scary answer—you’re saying that what should be simple “TDM”
> FXS to PRI does not work?

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.

> Are you suggesting this is an Asterisk problem or a Digium hardware problem?

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.

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
Jabber: kfleming at digium.com | SIP: kpfleming at digium.com | Skype: kpfleming
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
Check us out at www.digium.com & www.asterisk.org