Wifi hotspot through cell modem

I promised that I could set up a wifi hotspot for a meeting where there is no internet access. Pure hubris.

I ordered and received a cellular broadband modem in less than a week. It only talks to Windows, so I couldn't just mimic the configuration that I have with my home wifi configuration via Linux. I started by trying to figure out the right configuration for the hand-me-down Apple Airport that I would be using. That turns out to be a bad idea, since WinXP internet connection sharing demands that you have a pipe configured by your ISP at one end and the magic IP address of 192.168.0.1 at the other. There goes my preferred 10.0.0.0 network setup.

So here is the configuration that I found to work:

WinXP

cell modem: CHAP as per ISP-defined configuration

advanced network properties:

Allow other network users to connect through this device

Establish dial-up connection on demand

Shared with network: wired LAN

wired ethernet: 192.168.0.1 as per connection sharing rules. This is set automatically and cannot be changed.

Airport

ethernet uplink: DHCP

dhcp server off (it is provided by WinXP connection sharing)

When the cell modem is disconnected, the Airport picks its own IP address since it has no DHCP server to listen to.

When the cell modem is connected, the Airport gets an IP in the 192.168.0.0 range.

I thought there might be an issue about the boot order of the devices, but it seems to work regardless. The cell modem dials quickly on demand, so in the amount of time that you are used to waiting to establish a wireless connection at a wifi hotspot, it is actually dialing the cell modem. It seems to take about 10 seconds to dial.

It disconnects after about 5 minutes of inactivity.

Bandwidth is about 1Mb/sec through the modem.

This is so cool. I can set up a laptop and an airport anywhere there is power, including in our car, and have an instant hotspot. Although, driving across cell borders is bad for your youtube video or mp3 stream...