[asterisk-dev] Jitterbuffer and FAXes with E1
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 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.
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