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



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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