[cisco-voip] 7920's

[josmon]

On Fri, Jul 01, 2005 at 08:14:28PM -0400, Jim McBurnett wrote:
> Has anyone had a need to put more than 7 7920's in a single area?
 
They might, but it probably doesn't work very well.  :-)

802.11b starts running out of packets per second (pps) about the time
the 6th or 7th call gets on any given AP.  802.11b simply isn't built
to transport lots of small packets quickly.  You *can* get more calls
through if you can convice the codecs to send more data in a given packet,
but with 20ms sampling, you don't get many calls through.

802.11g will move you from 6 - 7 calls to something in the high 20s.

> I happened into an site where there are about 40 roaming a bldg...
> More than 7 calls and it drops a call..

The 7th call probably makes *all* of the calls start to sound bad, and
by the time the 8th call hits, all of the calls are probably loosing
10% or more of their packet stream...

> How can you load share the AP's for this?

You'll have to design so that:
  - no more than 6 phones are ever on the same AP  (less if there's
    data on the network too!)
  - pack more than 20ms of voice into a given packet
  - use something other than 802.11b
  

Note:  It doesn't matter which codec you use, it's pps, not bandwidth
that is the limiting factor.


Some articles to look at (mind the wrap on the Cisco link):
  http://mmnetworks.stanford.edu/papers/hole_icc04.pdf
  http://www.nwfusion.com/reviews/2005/011005rev.html
  http://www.cisco.com/en/US/products/hw/phones/ps379/products_implementation_design_guide_chapter09186a00802a0a04.html