The Qos PipeDream [Was: RE: Two Tiered Internet]

Fri Dec 16 08:13:10 UTC 2005

On Fri, 16 Dec 2005, Christopher L. Morrow wrote:

> ah-ha! and here I thought they wanted buzzword compliance :) From what 
> sales/customers say it seems like they have a perception that 'qos will 
> let me use MORE of my too-small pipe' (or not spend as fast on more 
> pipe) more than anything else.

When you're running voip over a T1/E1, you really want to LLQ the VOIP 
packets because VOIP doesn't like delay (not so much a problem) nor jitter 
(big problem), nor packetloss (not so much a problem if it's less than a 
0.1 percent or so).

So combining voip and data traffic on a link that sometimes (more often 
now when windows machine have a decent TCP window) go full, even just in a 
fraction of a second, means you either go QoS or do what Skype does, crank 
up the jitter buffer when there is high-jitter, which means latency for 
the call goes up.

So prioritizing packets in the access and core is good, for access because 
it's usually low-bandwidth and going to higher bw to remove congestion 
might mean factor 10 higher bw and a serious cost, in the core it's good 
to handle multiple faults, if the things that never should happen, you're 
not dropping your customers VOIP packets when the pipe is full that 0.1% 
of the time.

But, if you take the above model and start to always run your pipes full 
and use core packet prioritization as an everyday thing to support your 
lack of core bw, then you're in much bigger doo-doo.

-- 
Mikael Abrahamsson    email: swmike at swm.pp.se