[afnog] QoS/TE

[Brian Candler]

On Wed, Jan 18, 2006 at 01:25:34PM -0800, Jim Forster wrote:
> Depends on the link speeds and distances.   Think of this way: a 1000- 
> bit packet on a 1Gigabit link has a service time of 1 microsecond.    
> Speed of light time for 1000 km is about 3msec.   Link utilization of  
> 90% leads to an average queue depth of about 10 (engineering-level  
> queueing theory accuracy); 95% average utilization has an average  
> queue depth of 20.     So packets sit on queue for 0-20 microseconds  
> or so for the 90% case, then spend 3 msec on a 1000km link.   

Well, I have to admit that I thought the average queue depth would be larger
than that at those utilisations. In any case we're not so much interested in
the average queue depth, but the peak variation in queue depth over a period
of a few seconds, as this is what will affect the quality of audio delivery.
The 3ms overhead isn't important as it's constant.

I'm also not sure 1000 bits (only 125 bytes) is necessarily a representative
packet.

But of course you're right that the level of jitter induced may or may not
be important in your particular network or for your particular application.
All I know is this UK ISP thought it worthwhile to deploy, and gives them
some comfort that they can use VoIP to replace TDM links without too much
risk to quality.

Regards,

Brian.