Re: [GnomeMeeting-list] packet loss

[Damien Sandras <dsandras seconix com>]

Le mer 12/03/2003 à 16:37, Marc Williams a écrit :
> I'm guessing that there no one "right" answer to these questions but I
> thought I'd ask anyway:

You are right, there is no "right" answer and no unique answer. I will
now answer about the "packet loss" problem, "bad voice quality" can be
due to "packet loss" but there are other possible causes.

There is a difference between "packets loss" and "late packets". When
you are talking, you are sending your voice compressed into packets to
the remote user, when the remote user receives the packets, it
bufferizes them (jitter buffer delay) and then uncompresses and plays
back the voice. Those packets are sent using RTP, via UDP, when using
UDP there is no warranty that packets will arrive to the remote user.

You are sending packets to the remote user, those packets are queued in
the jitter buffer (for example a jitter buffer could contain the packets
for 200ms of voice), when the jitter buffer is full (the 200ms are
elapsed) it plays the voice back. If one of the packets sent 300ms ago
arrives after 300ms when the jitter buffer is of 200ms, then it comes
too late to be played. You know that due to the nature of internet,
packets can arrive in no particular order. If the packet arrives too
late to be played, then it is simply dropped, that's what is called
"late packets" in GnomeMeeting. "Lost packets" are packets that were
sent by the remote user but never comes to you.

> 
> What's the primary cause (if there is just one) of lost and dropped
> packets?

The only cause is always a bad connection. 

Here are some possible causes for a bad connection:

- Some providers reduces downloads to a speed near of 0 when you are
uploading, with such providers your number of late/lost packets will
increase if the remote user is sending you a lot of data or if you are
uploading too much.

- Sometimes, the route between the remote user and you is going through
an overloaded router, that router can drop packets too. 

- If you are on a dialup connection, and if somebody on DSL is calling
you and sending video + audio, he can flood your dialup connection with
too much data.

- Some users will try to send video at 30 FPS, quality 100%, Large image
over a link that is not able to upload at high speed, packets can be
lost/dropped in that case. 

> 
> What can an individual do to lessen the effects of lost/dropped packets?
> 

They can identify the cause. If you are in a call with somebody who is
complaining that there is a big packets loss, try to call again without
video at all. If it works better, then it is a bandwidth problem, try to
reduce the quality and FPS for video. If it doesn't work better, then
the route between you and him is going through a broken or overloaded
router. 

If it is a bandwidth problem, you can be the cause of his packets loss
but he can also be the cause of it.

> What would be considered a "normal" amount of lost/dropped packets?

Packets loss < 10 for a 2 minutes connection. In general, if you or the
remote person sees the numbers increasing constantly, there is a
problem.

Am I clear ? :)

-- 
 _	Damien Sandras
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