Re: [hylafax-users] T1 PRI Question, Can this be done and has anyone ever done it

[Lee Howard <faxguy@xxxxxxxxxxxxxxxx>]

John Warren wrote:

> I'd be running 22 channels doing outbound fax at night with one taking
> inbound. During working hours, when the voice traffic is higher, all fax
> traffic would be limited to 6 channels.  I've read that jitter can be a
> problem with IAXModem but I don't know if that's true or not. Also, I don't
> know if anyone has run that many fax channels with IAXModem.
>
> The system configuration is a Dell 2650 Dual Xeon 2.8 GHz. Currently running
> 2GB of ram but that's going up if I have any reason to think I'll run into a
> swapping issue.

As IAXmodem generally is run on the Asterisk server itself, and as the 
network path over the loopback adapter is assumed to be jitter-free the 
jitter isn't so much an issue with IAXmodem as it is with Asterisk 
itself.  If using a Zap device for PSTN interfacing it's extremely 
important that your zttest results are never (ever) worse than 99.98%.  
If they are worse than that it indicates that the zaptel driver is not 
able to get adequate attention from the OS, and thus audio samples 
_will_ be missed, and because PLC (packet loss concealment) is not 
performed on Zap channels, those missing audio samples will be 
translated into silence somewhere along the way.  20ms of silence in the 
middle of Phase C does horrible things to synchronicity between the two 
fax endpoints.

Usually these kinds of problems are resolved by moving the Zap card to a 
different slot, a lower IRQ setting, and where it does not share an IRQ 
with another device.  I've only heard of rather rare circumstances where 
the user needed to change the motherboard, although I have heard that it 
was necessary for some.

Upcoming development for IAXmodem will include a fax-specific 
jitterbuffer in the libiax2 code which should hopefully minimize the 
damage that missing audio can cause.  As I've never written a 
jitterbuffer before it may take some time... I'm not sure.

I've personally run a maximum of 10 iaxmodem instances on a single 
server, and the system (nothing particularly spectactular) handled it 
and the HylaFAX processes without even a noticeable sweat at all.  I've 
done calculations, and it should be possible to run hundreds of iaxmodem 
instances without a problem ... the first bottleneck you'll run into 
will be disk I/O from HylaFAX reading-from or writing-to the disk... 
but, with current HylaFAX code, I've only ever been able to hit that 
disk I/O problem when I deliberately try to with 24 modems faxing back 
to another 24 modems.

I don't have any hands-on experience with more than 10 iaxmodem 
instances, but I strongly suspect that there are several iaxmodem users 
that are using it to full-T1-PRI capacity at least.  And I certainly 
wouldn't expect that such usage would be a problem for iaxmodem in the 
least.

Anyway, all of that is probably off-topic... better-suited for the 
iaxmodem-users@xxxxxxxxxxxxxxxxxxxxx list.

Thanks,

Lee.


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