WiFi handover

Does anyone around here know much about wireless handover between access points and making it seamless? Currently I have just two nodes, positioned relatively close to each other to test some software I'm building. It really relies on handover between access points for someone moving around. I understand that this is at least the /theory/ for WiFi, but can't quite get it going in practice.

I understand that CaféNet in Wellington have this working, as they've previously demoed a WiFi cellphone with someone walking along Courtenay Place (or was it Lambton Quay?) keeping a cellphone call going. My point is that I'm sure it's possible....

However, is it access point dependent? Are some better than others? Or is it more to do with the smarts that the client device has? I hoped that you'd just set the SSID the same and it'd all work ... alas disappointed!

Would appreciate any help from wizened souls!

We have tested WiFi phones and laptops handing over on cafenet, it works OK at low speeds, but is not totally seemless. However with surfing and even voice the slight delay while layers 2 and 3 sort themselves out is near to inperceptible. There are some proprietary features in some APs that can smooth handover. We have not set about to create seemless handover, but with out trying it appears to be not too bad. Generally the client tries to hold a link with an AP until a signal threshold is reached and then the client looks for the best signal again. I have not tried to tweak any parameters to optimise this process. Some client cards appear to perform better than others, but again we have not specifically tested that.

Very interesting - thanks for that. Perhaps we need to play around a big more with our access points, and maybe try a couple of different clients to investigate whether it's any different.

Definitely even more interesting that CaféNet wasn't built with that in mind specifically, because being able to do it has some very interesting uses with regards to mobility, and small devices such as PDAs which you can easily walk around with.

Maybe I'll have to go cruising around CaféNet today and test out the handover myself with different devices and see if it's similar to the performance I'm getting....

Still interested in any other feedback people have.... surely there's a bit of collective experience around here!

Hi guys,

Unsure if you guys are implementors of Cafe style systems. I was interested in what is a good implementation of billing for these type of setups. Is there a linux disto or all in one AP that will do billing?

Cheers,

Scott.

Alas I'm not... but I guess it depends on what basis you're wanting to bill on - time or data. AFAIK, using RADIUS is pretty common... and is included with most linux distros I think. Getting your AP to work with it is another kettle of fish....

Sorry I can't help more...

Well a hotspot / router / nat system i've found to be very good is m0n0.ch/wall . I bounce over one of these for my inet connection, ones in client mode and ones in AP mode.

I have 3 IP phones, one on my wireless, 2 on jetstart connections, when I make calls, there are no latency issues at all, sounds just like a GSM cellphone call. Then again these phones arn't wireless, they are RJ45 ethernet connected, but my connection goes over wireless, so yeah.

For bandwidth monitoring, i'm using freebsd, managed to get a paradise style ip accounting system going.

For those interested: http://www.albanywireless.co.nz/?page=banmon
user: icepick
pass: password

These login details due to change in the next week, so check it out while you can and let me know what you think Shot for the help linx.

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.

Authentication
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.

Channels
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.


AP location
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
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).

get yourself a little cisco with dual ethernets, and run cisco mobile IP on it..

designed to do exactly what you want, and offer a 100% seemless handover between network segments..

If your running a layer3 routed network, I think you will need more than one however..