03-28-2004, 06:19 AM
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Join Date: Dec 1969
Location: Waimauku, Auckland
| | Wifi Handover
So to start with Yes handover does work. I'll assume you are talking about using 11b. There is a number of things you need to get right
Make sure all the APs are sitting in the same IP subnet/VLAN. You don't want to have to re dhcp as you roam.
If you are using radius and 802.1x authentication to the AP, then make sure the APs support 802.11f (this provides handover between APs of the authenticated sessions. ) With out 802.11f you will need to re-authenticate to each AP. In larger installations (10 APs +) I'd install a system with a centralised security switch to solve this as the APs then only carry out the radio functions.
Hopefully your APs are correctly autoselecting channels. 11b only has 3 non overlapping channels (1, 6 11) and you MUST NOT have two adjacent APs using the same channel. This means you have to lay your APs out in a honey comb fashion and this gets even worse if you have to go 3D with a multifloor building. This is where 11a with 11 channels really comes into its own
1 / 1 /
6 | ----| 6 |----|
-----| 11 |---| 11 |
1 |-----| 1 |---|
/ 6 / 6
You may have to manually set the chanels on your AP. Performance will be CRAP is you have two APs sitting on the same (or close channels). Even if using channels 1, 6, and 11 you will still get some interfernce and this will result in some performance impact. Now some would say place your APs futher apart so as to avoid this, but then you would get very variable through out as you roam about. Also it makes handover harder.
For commercial installations I always do a site survey first and final quotes and numbers of APs are dependant on this. That said as a ruff rule to get started with a cell size of around 18m seams to provide a reasonable comprimise between number of APs and performance.
Handover occurs when there is a significant difference in signal strengt. A mobile unit (MU) will down speed rather than handover. Part of this is how the MU driver is coded. Some APs are now adding a feature that they will not accept an association request if the signal strength is below a preset threshold. This helps stop a problemseen where a MU will hang onto an AP for huge distances before roaming. This AP stickness results in reduced performance of all users on that AP as broadcasts etc to all users now have to be sent at this very low speed.
Bringing APs closer together means MUs see alternative APs of higher signal strength sooner and so are more likely roam. Generally we see it takes about 5 secs to hand over assuming there is no centralised rf control. It can be as high as 15 (observed values).