OT? cRTP header compression

Thu Apr 11 21:36:43 UTC 2002

On Thu, 11 Apr 2002, Mathew Lodge wrote:

> 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.

I still stand by my comment that customer notice and that is why I don't 
use it.

> 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.

Correct, I provide 2 voice lines and data over a 192k DSL circuit using 
only 1 AAL5 PVC because I don't own the DSLAMs. If you play that game you 
need to worry about header compression. I am not a big fan of G.729 or 
other low bitrate codes for business voice services. 

> 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) 

Well I am more of a PSQM guy myself. :-)

> 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.

Yes, I use 9 ms vs 21 ms most of the time because I want to keep end to 
end delay as low as possible. Sure I save 8 bytes if I use 21 ms, but I it 
requires me to up my jitter buffer. The other then you need to think about 
is packet loss. If I lose a 9ms same I may notice, if it is 21 ms I am 
MUCH more likely to notice.

> 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.

Yes, that is a big problem when you play with low speed links. I run voice 
and data over one PVC on a DSL circuit. If the user hits a web page and 
sucks a 1500 byte packet it will take over 93 ms to get over that link. I 
can't live with that amount of jitter so I fragment to a specified MTU 
that I can live with per link and then interleave the fragments between 
the voice samples after I compress the header.

><>
Nathan Stratton				CTO, Exario Networks, Inc.
nathan at robotics.net                  nathan at exario.net
http://www.robotics.net                 http://www.exario.net