Handoff time measurement

Jean Tourrilhes jt at bougret.hpl.hp.com
Mon Nov 4 16:16:49 EST 2002


On Sun, Nov 03, 2002 at 02:42:59PM +0100, Victor Aleo wrote:
> Jean Tourrilhes wrote:
> 
> >	That sounds like the proper number. Few hundred ms is totally
> >unrealistic, I don't know where you go this idea.
> >
> 
> Passive scanning is slow, I agree. That's why active scanning was
> introduced. The fact that active scanning might not be properly supported
> affects the quality of commercial products.

	Correct. Every manufacture has it's own priority.
	Note that even if you use active scanning, your worst case
scenario (traffic + interference) is still in the second range, you
gain only if the channels are relatively clear.

> I recommend to read this paper 
> to have an experimental understanding of the MAC layer handoff in 802.11: 
> http://www.it.kth.se/~aep/publications/measurements-kanter-maguire-escudero.pdf
> 
> The results shows that the measured handoff time was 157 ms. 

	They don't use an Intersil firmware, they use the Orinoco card.

> Anyway, note that handoff time can be reduced with passive scanning if a
> shorter beacon period is used. The drawback is a reduction of the channel
> capacity due to higher signalling traffic. It all depends on the scenario.
> If your users move fast, then it maybe worth to reduce the beacon period.

	Only if the card knows that the beacon period will always be
reduced. You always have to prepare for the worst case.

> Sure, this is true in general. But in my case I want to trigger handoff 
> only to distribute the load (thus, disassociation stations from the 
> APs), so I do not have that stability problem.

	Ok.

> I need it ;)
> 
> -- 
> Victor

	Jean



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