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So I'm trying to make a system in which I can walk through a building with my laptop and have no delay when switching access points. I have no control over the network itself.

I am currently planning on doing this by connecting my laptop to two wireless cards.

  • Card 1 will constantly monitor the network for the strongest AP

  • Card 2 is connected to an AP and is being used to establish an Internet connection for the computer.

When the access point that Card 2 is connected to becomes too weak, the cards will switch functionality, allowing Card 1 to become the Internet connection without having to scan all access points.

I'm mostly wondering how to pass off which card the OS is using for the internet connection at a given time.

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1 Answer 1

Ooh, this can (and probably will) lead to a lot of headaches. I think this is overall a bad idea. Here's why:

  1. If your network provisions you a new LAN address each time you connect, any active TCP connections will be terminated when the "handoff" occurs. The only way that your network could give you the same IP is if you set up a static IP, but then you would have to have two static IPs instead of one, and know which one to use at the right time by changing your routing table.

  2. If you have any programs listening on the LAN IP of one card, they will stop being able to receive connections (or communicate through existing sockets) once the old LAN IP dies. In the worst case, if the daemon isn't listening on 0.0.0.0, you'll have to restart the daemon to listen on the new IP.

This sounds like it's going to be very unreliable, no matter how you try to hack it. You see, the wifi spec has explicit support for physical layer (and link layer) handoff from multiple AP nodes in the same network. So if you have 5 wifi APs in the same building that are configured correctly, you should be able to walk from one to the next without getting any connection interruptions. It should "just work".

If it doesn't just work, then either the APs don't overlap in their broadcast range enough, or they aren't configured to be part of the same network, so each one tries to act like a router and tries to give you a new LAN IP.

More info: http://www.wireless-nets.com/resources/tutorials/how_roaming_works.html

A long shot, but some information about how you MIGHT approach this (but I think you'd need a lot of help from the IT department maintaining the wifi network): http://en.wikipedia.org/wiki/Multihoming

My suggestion? Use cellular broadband. Wifi sucks. Really, it does. See http://www.tomshardware.com/picturestory/571-wi-fi-beamforming-networking.html

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Thanks for the thorough response! Yeah, I can see how all of these are issues. However the people I'm working with say that there's still a multisecond delay when switching points, and that they've played with changing the threshold for finding a new AP. I'll play with these myself, but do you have any other suggestions for how to speed it up? –  user145182 Jul 10 '12 at 20:05
    
If you don't have control over the infrastructure, I'd say you're pretty much SOL. IT shops that lock their infrastructure up without letting customers (assuming you are a customer) voice complaints/concerns or making suggestions are the worst scum on the earth aside from criminals. It's one thing to have a badly designed network infrastructure; to ignore or disregard customer complaints or to react to them in a hostile manner is entirely another thing. The "blessed" way of dealing with this is implementing PROPER wifi roaming with significant AP overlap. –  allquixotic Jul 10 '12 at 20:07

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