[asterisk-dev] Jitterbuffer and FAXes with E1

[Martin Vít]

Zoa wrote:
> 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.
>   
Is the Slavs jitterbuffer patch for asterisk 1.2 the same as in 1.4? If 
yes, how is possible that in patched asterisk 1.2 both fixed and 
adaptive jitter works nice with fax?

> Zoa
>
> 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
>> ------------------------------------------------------------------------
>>
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>
>
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>   


-- 
Martin Vít
LAM plus s.r.o.
http://www.lam.cz/
Tel.: 605 267 610

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