[Asterisk-Dev] CVS HEAD (20040807) jitter buffer questions

[Steve Kann]

The LAG measurement is pretty meaningless in the present implementation, 
if the clocks skew between both sides.    Unless both ends of the 
connection are using ntp (for a while, and have stabilized), you can't 
trust it.  [even then, I don't remember if ntp is accurate enough for this].



Andrew Kohlsmith wrote:

>I've been keeping an eye on the jitter buffer ever since upgrading and using 
>Steve's patch to fix the 65s dead air problem but I've been noticing 
>something...
>
>... Asterisk frequently gets "lag" and "jitter" mixed up.
>
>This directly affects the jitter buffer, as the jitter buffer only grows when 
>jitter grows.
>
>To better explain what I mean I'll show you the output from 'iax2 show 
>channels':
>
>*CLI> iax2 show channels
>Peer             Username    ID (Lo/Rem)  Seq (Tx/Rx)  Lag      Jitter  JitBuf  
>Format
>192.168.2.2      benphone    00009/16384  00047/00050  00006ms  0001ms  0051ms  
>GSM
>1 active IAX channel(s)
>
>This is good.  
>
>*CLI> iax2 show channels
>Peer             Username    ID (Lo/Rem)  Seq (Tx/Rx)  Lag      Jitter  JitBuf  
>Format
>192.168.2.2      benphone    00003/16385  00008/00005  00000ms  0000ms  0050ms  
>GSM
>192.168.2.2      benphone    00009/16384  00072/00074  00002ms  0000ms  0050ms  
>GSM
>66.225.202.72    benshaw     00013/00207  00005/00008  00000ms  0015ms  0080ms  
>GSM
>
>
>This is not.  How can I possibly have 0ms lag?  This was a decent call, some 
>have like 182ms jitter which is even beyond normal lag measurements I have 
>seen (normally 26-35ms, sometimes as high as 50 to 80ms).  
>
>The measurements are consistent too -- I can run the command a 20 times in a 
>minute and get 0ms lag on the channel(s) every single time with the jitter 
>staying consistent.  It's not a "one-off" thing.
>
>Sometimes it's one channel where the others look "normal", sometimes it's a 
>half dozen channels showing 0ms lag and high jitter with the few remaining 
>channels showing normal, sometimes they're all normal.  
>
>*CLI> iax2 show channels
>Peer             Username    ID (Lo/Rem)  Seq (Tx/Rx)  Lag      Jitter  JitBuf  
>Format
>192.168.2.2      benphone    00003/16385  00017/00015  00000ms  0000ms  0050ms  
>GSM
>192.168.2.2      benphone    00009/16384  00088/00090  -00001ms  0000ms  
>0050ms  GSM
>66.225.202.72    benshaw     00013/00207  00015/00017  00000ms  0015ms  0080ms  
>GSM
>
>Similar here -- I see the meter itself is fixed (not 6553434665234265ms lag 
><g>) but how is negative lag possible?  I believe I have also seen negative 
>*jitter* -- and this causes the jitter buffer to grow without necessity.
>
>Relevant iax2.conf settings:
>
>pingtime=5
>lagrqtime=5
>
>jitterbuffer=yes
>dropcount=2
>maxjitterbuffer=500
>maxexcessbuffer=100
>minexcessbuffer=50
>jittershrinkrate=1
>
>All peers have trunk=off (I was getting "bunched and gappy" audio the odd 
>time... definitely not every time but especially noticeable right after a 
>clean shutdown and restart of Asterisk)...  
>
>By "bunched and gappy" I mean the received audio sounded like this:
>
>"....Hellohow....areyouto.....dayIhaven't.....heardfromy.....ouina.....while"
>
>I understand that a few months ago this was a consistent bug when enabling 
>trunking and jitter buffer but I've been running trunking and jitter buffer 
>for a while now without issue (well just the once in a while issue I just 
>mentioned here).
>
>I'll be able to provide iax2 debug logs and packet dumps if anyone's 
>interested.
>
>Regards,
>Andrew
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