Wireless laptop roaming through various access points

VDP · 2005-12-27T09:24:08+00:00

Goal: a roaming laptop should be custom-configured to "hop" fromone AP to another according to radio signal strength or networkconnectivity.Most client software seem to wait until the signal is *totally* lostand furthermore, they seem to wait for an extra timeout. Finally theyrescan for a stronger AP. However this behavior usually means that youcan have a 10-20 second downtime which is definitely annoying forTerminal Services or other "delicate" applications.What I would like to do sounds simple: if the laptop wifi softwaredetects that radio signal strength from the AP that it's connected todrops below an administrator-defined threshold then it shouldimmediately 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 nicsoftware 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 thiscorrectly and a 10-20 second downtime is produced whenever the clientswitches AP links (obviously, I am considering optimal radio coverage,i.e. the latter "downtime" occurs even when the laptop is close toan AP)b) I also downloaded and tried Intel's latest driver/utility. Ontheir web site they state that their software should take care of theroaming issue but in practice I don't see any difference with the waythe standard Windows client behaves (i.e. there is an unacceptabledowntime of 10-20 seconds, even when second AP is nearby, andconnections such as Terminal Service are annoyingly dropped)c) I finally downloaded Network Stumbler and launched it in thebackground. It takes control of the wifi nic and handles roamingperfectly. In other words, I can move around with a laptop from one APto another with a Terminal Service session open and never get thatannoying 10-20 second interruption. However, Netstumbler does not seemto work the...

Dec 27, 2005 21 Replies

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That's roughly the way the local hospital wireless LAN works. Anyone can connect, but without a VPN client and the necessary authentication and access, they go nowhere. The 192.168.1.xxx IP address issued by the Linux gateway DHCP server does nothing. Only after authenticating with the VPN server, does the VPN server issue a different

10.214.144.xxx IP address that works.

However, you might be trading one roaming problem for another. VPN connections do not do well when roaming from access point to access point. The one's I've tried are even more sensitive to packet loss. Lose a few bytes and they disconnect and renegotiate the entire key exchange cerimony. My guess is about 3-5 seconds delay after losing a few packets. You're going to lose something during the switch between access points. Fortunately, VPN authentication and header modification protection is based on IP addresses and not MAC addresses. Therefore, it should maintain the VPN connection when moving between access points.

There's also the problem of licensing VPN clients, although there are plenty of free ones available. Same with clients for PDA's.

In addition, with useable 192.168.1.xxx addresses assigned to clients, there's a real danger that these clients will turn your access points into their personal game network or something. I've had that happen to me. If you access points offer "client isolation" or something similar, use it.