I want to set up a bigger WLAN with devices which support OpenWRT. But therefore i need to know, if OpenWRT provides Handover. For example, I take two Linksys WRT54G and connect them by cabel. Then I gave them the same SSID and when a user logs in at one of them, he can switch to the other without lost of connection. Does OpenWRT support "Extended Service Set" or even IEEE 802.11f to do this?

Can nobody tell me something about that? Havn't nobody ever connected two APs by wire to enlarge his WLAN?

If two access points share the same ssid and encryption settings, the wireless driver on the client will automatically roam; it really has nothing to do with the AP.

Wow, that is very new to me! And what is about the channel? Does these two APs have to / are not allowed to share the same channel?

If the 2 AP's are in close proximity, you want them at least 5 channels apart to minimize interference.  I use channels 1, 6, 11 in rotation to cover large buildings.  It's been customary since the beginning of WiFi that you wire a building by using APs connected to a wired backbone LAN, with the same SSID and encryption settings.  There are several good books on WiFi technology you can read.

One such book is located here:
http://www.wndw.net

Association behaviour is a function of the CLIENT software not the AP by the way.  The AP just provides a bridge from wireless to wired.  The common client software behaviour is it associates with an AP and stays with it until it loses signal, then a reassociation attempt is triggered.  Some client sofware from Cisco is a bit smarter about it. The long-awaited 802.11r spec should enhance this behaviour considerably, but is not out yet.  Bottom line, don't expect to roam a large area with a WiFi phone and have uninterrupted VoIP calls with current cheap client software.

Thank you very much! This was what I was looking for all the time.

Now read the whole document and know much more.
But there is still a question, what is about handover in use with encryption? For example I use WPA? Or when VPN is used?

As far as I know, the association code is independent from encryption used.

Assumedly all AP are on some LAN.  So client should be able to reach it's target VPN server no matter which one it is associated to.

I also think so, but what is about WPA?

Using Kamikaze I can easily switch from one AP to another with WPA encoding, using WPA Supplicant (Webif automatizes Supplicant job call).
With WhiteRussian, I think that's the same if Nas is installed, I think I tried successfully once.
As vincentfox said, if you have all software you need, the association code is totally independent from encryption.