[asterisk-dev] Jitterbuffer and FAXes with E1

Martin Vít vit at lam.cz
Thu Jun 28 07:59:12 CDT 2007

Steve Underwood wrote:
> Martin Vít wrote:
>> Hello,
>> I'm testing 1.4.5 (no modifications) with E1 card and i've found problem.
>> I've this scenaro
>> FAX -> SIP - ATA -> asterisk -> E1 port A --loopback--> E1 port B --> 
>> iaxmodem.
>> When jbenabled is off FAX is delivred OK. But when jbenable = on FAX are 
>> failing in almost every line. I've compared WAV extracted from network 
>> with WAV recorded (app monitor) before it gets to iaxmodem and there is 
>> small differences. Network jitter is about 2ms (lab environment).
>> Where could jitterbuffer modifies frames? I've tried to disable plc in 
>> plc.c but without success. Any idea?
> This is correct behaviour. Most VoIP kit has the ability to disable 
> jitter buffering when a FAX is in progress.

I still dont understand, what is correct. I'll be more specific:

FAX frames are delivering from SIP ATA to asterisk box, routed through 
E1 and returned back to the same E1 and then to iaxmodem (it is silly 
but just for test).

If jbenable = yes in zapata.conf and in sip.conf, every rows in fax are  

    Jun 28 14:48:10.25: [11153]: RECV/CQ: Bad 1D pixel count, row 763, 
got 2355, expected 1728

If jbenable = no in zapata.conf and sip.conf, everything are OK.

I've checked debug file from jitterbuffer and it seems that all frames 
are delivered ok

            JB_GET {now=136}: Delivered frame with ts=49147553 and len=20
            JB_GET {now=156}: Delivered frame with ts=49147573 and len=20
            JB_GET {now=176}: Delivered frame with ts=49147593 and len=20

I've tried adaptive and fixed jbiml, always the same

So i've tried simulate fixed latency on my router (hold data for 200ms 
from ATA to asterisk) and disable jb at all. FAX are OK. If jbenable = 
yes - problem.

There is something wrong with jbenable or am I missing something?

Maybe there is somewhere activated PLC or some another filter? whenever 
jbenable is turned on?

Codec is alaw

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