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Despite it's small size, this is the most difficult network design problem I've faced. There are three nodes in this network:

  • PC running Windows XP with an internal WiFi adapter.
  • Base station with both WiFi and a Wireless Modem (WiModem)
  • Mobile device with both WiFi and WiModem

The modem is a low-bandwidth but high-reliability connection. We'd like to use WiFi for high-bandwidth stuff like file transfers when the mobile is nearby, and the modem for control information. Here's the tricky part: we'd like the wifi traffic to go directly from the mobile to the PC, as rebroadcasting packets on the same WiFi channel takes up double the bandwidth.

We can do that with a manual configuration by giving the both the PC and the base station two IP addresses for their WiFi interfaces: one on a subnet shared with the mobile, and one on their own subnet. The routes on the PC are set up so that any traffic going to the mobile via WiModem goes through the secondary IP address so that return traffic from the mobile also goes through the WiModem. Here's what that looks like:

  • PC
    • WiFi 1: 192.168.2.10/24
    • WiFi 2: 192.168.3.10/24
    • Default route: 192.168.2.1
  • Base Station
    • WiFi 1: 192.168.2.1/24
    • WiFi 2: 192.168.3.1/24
    • WiModem: 192.168.4.1/24
  • Mobile
    • WiFi: 192.168.3.20/24
    • WiModem: 192.168.4.20/24

We'd like to move to having the base station automatically configure the mobile and PC, as the manual setup is problematic when you start having multiple mobiles and PCs. This means that the PC can only have 1 IP address and needs to be treated as being pretty simple. Is it possible to have a setup driven by DHCP on the base station that is efficient with bandwidth?

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2 Answers 2

If I understand correctly, the end result of this configuration would result in two network routes between the mobile and the desktop - with the possibility of routing loops if you aren't careful. I don't think DHCP would help here either. You could try RIP or other routing protocols, but I don't think that would simplify things too much.

What's more, WiFi is "point-to-point" - so I think you would have to get a second WiFi card for the desktop and configure it for a different channel and name. Then your mobile could choose which one you wanted - the router or the desktop.

If you move to multiple desktops and mobiles, then this gets exponentially worse - and more expensive.

I would choose the simpler possibility over the extra traffic: add WiFi routers and let the traffic flow. If the problem is just a transfer of files, I might use Bluetooth instead between the mobile and the desktop (assuming a one-to-one association).

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So you want to communicate using wifi on your LAN devices, then use the wimodem for anything else?

PC > Wifi > Base Station > Wimodem > Intrnet Right?

That's easy. Just use the base station as a router and make a default route on the base station out to your ISP's gateway. If local devices need to talk to each other, they'll use wifi, if they need to go out to the Internet, they'll use the wimodem.

Then configure your DHCP server to hand out IPs in the range you want with the base station as the default gateway.

I'm assuming that wimodem is connecting to the internet. You'll need to setup NAT as well probably.

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The WiModem does not connect to the internet. For the purpose of this question nothing is on the internet. WiFi and WiModem both connect the Mobile unit and the Base Station. The PC is also on the WiFi. The problem is that with the PC and the Mobile device both on WiFi, the mobile device always wants to send over WiFi. The WiFi channel is unreliable though. Not so simple, eh? –  IndigoFire Mar 31 '11 at 22:29

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