[asterisk-dev] Jitterbuffer and FAXes with E1

Zoa zoachien at securax.org
Thu Jun 28 10:12:06 CDT 2007

Because the jitter buffer will never be size zero if its a fixed jitter 
buffer and will fluctuate if its a variable length jitter buffer.
So whatever you choose, the timing will be altered.


Martin Vít wrote:
>>> 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
>> It is correct that a perceptual jitter buffer will play with the timing, 
>> and that will break things like faxing. 
> I need to investigate this, because without jitter there is no chance 
> to send fax on a little jittery network. (and there is no way to 
> enable or disable jitter or change jitter on the fly yet)
> On production system I have asterisk 1.2 patched with slavs 
> jitterbuffer implementation and digium 4port PRI. I've enabled SIP 
> adaptive jitterbuffer and with the same ATA reregistred to this 
> production system, i've send fax to my 1.4 with success.  (Jun 28 
> 16:16:04 VERBOSE[8475] logger.c:     -- ***[JB LOG]*** adaptive 
> jitterbuffer created on channel Zap/32-1)
> Complete scenario:
> FAX -> ATA SIP -> Asterisk 1.2 adaptive jitter -> Zap PRI ---> TELCO 
> SWITCH ---> Zap PRI asterisk 1.4 ---> iaxmodem.
> this work even though jitterbuffer is enabled on patched asterisk 1.2 
> which is almost the same as in 1.4. But there have to be some 
> differences which broken fax transmitions.
>> You need to disable clever 
>> jitter buffering algorithms when you need an unbroken stream. Typically 
>> VoIP equipment will do this automatically when they hear FAX tones.
> This is true for my testing ATA which is switching from adaptive to 
> fixed jitterbuffer.
> So the question is, why are frames modified when jitterbuffer is 
> active in 1.4 even though there is NO jitter (100mbit eth in lab 
> environment). I've disabled PLC in codecs.conf.
> Festr
> ------------------------------------------------------------------------
> _______________________________________________
> --Bandwidth and Colocation Provided by http://www.api-digital.com--
> asterisk-dev mailing list
> To UNSUBSCRIBE or update options visit:
>    http://lists.digium.com/mailman/listinfo/asterisk-dev

More information about the asterisk-dev mailing list