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

[Andrew Kohlsmith]

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