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

Fri Dec 16 16:15:16 UTC 2005

Thus spake "Mikael Abrahamsson" <swmike at swm.pp.se>
> 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).

There's two problems, actually.  The first is serialization delay, and 
afflicts any link under about 3Mb/s regardless of utilization.  Access 
speeds are finally climbing past this, but for links where they haven't you 
need something like MLPPP for fragmentation and interleaving.

The second is queueing delay, and that tends to only matter when average 
utilization passes 58% (someone with a stat background explained why, but my 
math isn't good enough to explain it).  LLQ and WRED solve this well enough 
for end systems to cope with the result.

> 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.

Adaptive jitter buffers are old technology; Skype is hardly the first 
company to use them.  Most phones and softphones have them; it's the 
gateways at the other end that are usually stuck with static ones.

S

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