Goal: a roaming laptop should be custom-configured to "hop" from one AP to another according to radio signal strength or network connectivity.
Most client software seem to wait until the signal is *totally* lost and furthermore, they seem to wait for an extra timeout. Finally they rescan for a stronger AP. However this behavior usually means that you can have a 10-20 second downtime which is definitely annoying for Terminal Services or other "delicate" applications.
What I would like to do sounds simple: if the laptop wifi software detects that radio signal strength from the AP that it's connected to drops below an administrator-defined threshold then it should immediately drop its link to that AP and start scanning *immediately* for the strongest AP around.
Although this sounds easy it doesn't seem to me that most wifi nic software correctly deal with this aspect (although some do get close).
For instance, I've tried with two systems:1) IBM Thinkpad with Intel 2200BG wifi nic (Windows XP SP2) a) the native Windows wifi client doesn't seem to handle this correctly and a 10-20 second downtime is produced whenever the client switches AP links (obviously, I am considering optimal radio coverage, i.e. the latter "downtime" occurs even when the laptop is close to an AP) b) I also downloaded and tried Intel's latest driver/utility. On their web site they state that their software should take care of the roaming issue but in practice I don't see any difference with the way the standard Windows client behaves (i.e. there is an unacceptable downtime of 10-20 seconds, even when second AP is nearby, and connections such as Terminal Service are annoyingly dropped) c) I finally downloaded Network Stumbler and launched it in the background. It takes control of the wifi nic and handles roaming perfectly. In other words, I can move around with a laptop from one AP to another with a Terminal Service session open and never get that annoying 10-20 second interruption. However, Netstumbler does not seem to work the same way with every wifi nic (it failed with Broadcom and Ralink RT2500)
2) HP Compaq nx6125 with Broadcom wifi nic (Winodws XP SP2) a) the native Windows wifi client doesn't seem to handle this correctly and a 10-20 second downtime is produced whenever the client switches AP links b) Netstumbler doesn't work well and the network simply gets unaccessible c) I downloaded and tried HP Compaq's latest software for the Broadcom nic and seems to work a lot better than Intel's utility. Termnal Service managed to "survive" when switching from one AP to another. However, it's not as efficient as in the case of the IBM Thinkpad with Network Stumbler because in the latter example roaming was really fast (less than 5 seconds) but in this case, it lasted just under 10 seconds. I believe it may be because Broadcom's software doesn't try switching APs until the radio signal is really dead and after waiting a timeout. I can't seem to find an option (Windows registry? Config. Files?) to define a "radio loss threshold" that triggers a rescan of the nearest AP.
Can anyone please give me some tips/pointers on how to handle this roaming problem?
Do you know of any software (such as a sniffer , preferably free and nic-independant) that simply forces wifi rescan/reconnect (via native Windows wifi client for example) whenever radio signal strength goes below a user-defined threshold?
Or even a simpler solution comes to mind: I could write a program that simply pings a remote network server. If that fails then I could trigger the Windows operating system's WiFi client to rescan immediately for a stronger AP. How can this be done programmatically?
Is it possible to have two WiFi network cards on a single laptop, without interfering with one another (RF), and defining only certain APs on one nic and other APs on the other (so that neither nic connects to the same AP) ? Would it then be possible to bridge these two nics and have them act as one (single LAN IP)? I tested this with 2 wired Ethernet cards connected to the same switch and bridged both: it worked fine with the a single virtual IP for both nics. However, I never tried it with wireless nics and I'm not sure each nic can be assigned a list of "preferred APs".
I've also heard of the Virtual WiFi Projectbasically consists of using a single wireless nic to connect simultaneously to several APs (thus solving my roaming problem). However, the Virtual Wifi project is still experimental and lacks security options (APs must be "open"). Also, I don't mind buying two wireless nics if the bridge method above could be done.
Any suggestions would be greatly appreciated.