Handoff time measurement

Jean Tourrilhes jt at bougret.hpl.hp.com
Thu Oct 31 12:49:39 EST 2002


Victor Aleo wrote :
> 
> Hi,
> 
> Any one knows how much time it takes the handoff in a very simple WLAN 
> network with 2 APs (running the Prism2 driver as Master mode) and using 
> a station (as Managed mode) with also running Prism2?
> 
> I refer to the MAC (layer-2 handoff), without using MobileIP or anything 
> else. I have tried to measured it, but the results I have got are very 
> high: between 1 and 2 seconds!!! When it should be around a few hundred 
> milliseconds (200 at much, maybe). Of course, the handoff time depends 
> of the beacon period that in my case is the default, thus 100 ms.

	That sounds like the proper number. Few hundred ms is totally
unrealistic, I don't know where you go this idea.
	Just do the maths. You need to "probe" every channel to find a
new AP. If you use passive scanning, you need to wait at least one
beacon period on every single channel, so minimum 1.4s. If you use
active scanning, you can bring that down somewhat (two contention +
one max packet size per channel - depend on traffic), but I don't
expect the Intersil firmware to do that (at least, not without the
usual assortment of bugs).
	Not also that you don't want to trigger excessive handoffs, so
because of fading and interference (which may last in the order of
seconds), you will use a long term average to trigger handoff (like
SNR), which may delay the start of handoff in some situations. If you
try to trigger handoff faster (let say, every time you miss a beacon),
your system will become very unstable (so worthless).

> I would appreciate any suggestions,
> 
> Thanks
> 
> -- 
> Victor

	Good luck...

	Jean



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