OT? cRTP header compression

Thu Apr 11 19:50:59 UTC 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                 | mathew at cplane.com     |
| Director, Product Management | Ph: +1 408 789 4068   |
| CPLANE, Inc.                 | http://www.cplane.com | 