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RE: OT? cRTP header compression

  • From: Mathew Lodge
  • Date: Thu Apr 11 15:55:16 2002 
At 03:19 PM 4/11/2002 -0400, Nathan Stratton wrote:

On Thu, 11 Apr 2002, Mathew Lodge wrote:
Note that turning on VAD does decrease the
> perceived voice quality a little, so whether it is worth it depends on
> where you want to make the trade-off between cost and voice quality.

I am not a big fan of VAD in my network, customers do not like the silence
they get on the other end.
Perhaps you are not using equipment that offers comfort noise generation when VAD is enabled. On a Cisco 2600/3600/5300, make sure you have comfort noise generation turned on, and the gain set to a level such that your users can hear it.

 As far as packet size, you want to use the
smallest you can get away with. I use 9 or 21 ms of G.726 on my network, I
know they are odd sizes, but we run our own version of CRTP directly over
AAL5 so we want to fill the cells correctly.
In a perfect world, you wouldn't bother with VAD, cRTP, longer sample sizes, expensive CODECs or any of the other technologies for optimizing bandwidth. But these are all valid technologies when bandwidth is expensive.

You can increase the sample size without affecting perceived voice quality, because perceived voice quality is a step function of latency. Human listeners don't notice voice latency until it passes a threshold, when suddenly it becomes very apparent and perceived quality (measured by MOS) plummets. Various standards bodies argue about where that threshold is, but my experience to date suggests it's around the 150ms mark -- your mileage may vary. Doubling the standard Cisco voice sample size (20ms) to 40ms only adds 20ms to end-to-end latency, halves the packet rate and doubles the ratio of payload to header.

Secondly, when link cost is high, it is often prohibitively expensive to buy circuits with higher data rates. And when this happens, serialization delay (the time it takes to get a packet on the wire) starts to become a major issue -- and that is directly impacted by the ratio of payload to header size.

Cheers,

Mathew



><>
Nathan Stratton                         CTO, Exario Networks, Inc.
nathan at robotics.net                  nathan at exario.net
http://www.robotics.net                 http://www.exario.net
| Mathew Lodge | [email protected] |
| Director, Product Management | Ph: +1 408 789 4068 |
| CPLANE, Inc. | http://www.cplane.com |