Wireless laptop roaming through various access points
- From: "VDP" <rentorbuy@xxxxxxxxx>
- Date: 27 Dec 2005 04:24:08 -0800
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 Project
(http://research.microsoft.com/netres/projects/virtualwifi/software.htm)
which basically 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.
Regards,
Vieri
.
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