We have already replaced the router, changed the channel, checked for over-riding WLANs, checked all the Windows...
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2000 Server settings, and (re)checked all client settings. What should I try next?
You can get a rough idea of RSSI by watching the client's network connection, or by running a shareware program like NetStumbler on the affected PC. See this WLAN Advisor column for discussion of RSSI and a list of freely-available measurement tools. Unfortunately, you're going to find that most tools don't support USB wireless adapters. If you have a laptop with a wireless PC card, try measuring RSSI from that laptop in the same location as a USB-connected PC. If RSSI looks strong from the laptop, that doesn't necessarily mean your USB adapter is faring as well, but this can help you diagnose weak coverage areas offered by your router. If your router's coverage simply isn't strong enough, you may try repositioning the router -- for example, move it to a central location, at desktop height, avoiding any obvious obstructions. You might also consider adding a second AP or an external antenna if the required coverage space is too large for one router.
If signal looks good, monitor a client that's very close to your router (10-20 feet) to capture/analyze traffic. To do this, you'll need a WLAN analyzer that works in RFMON mode -- you can use Ethereal on a non-Windows laptop, or a demo version of a commercial WLAN analyzer on a Windows laptop. Here again, you'll need a compatible wireless card, not a USB client. To learn more about WLAN analyzers, read this ISP-Planet article. A WLAN analyzer can tell you if the channel is over-loaded, if excessive errors are occurring, if someone is forcibly deauthenticating/disassociating your client, etc. Compare traffic before, during and after loss of connectivity to try to isolate differences and potential causes. If you are experiencing high error rates or bursts of noise, the culprit may be interference from a non-802.11 source, like a microwave oven, cordless phone or Bluetooth device.
Ultimately, you may find the culprit is a "wonky" USB adapter. Problems can be caused by loose USB connections, the orientation/placement of the USB adapter, transient obstructions between the adapter and the router, etc. I generally encounter more problems with wireless USB adapters than I do with wireless PC or PCI cards. You may find it expedient to simply replace one of your USB clients with another product/form-factor. For example, I've had success using a PCI-to-PC-card adapter and Atheros-based PC cards -- these also tend to be supported by Windows stumblers and WLAN analyzers, which will help you in debugging. If you find the replacement adapter solves your problem on one PC, move that PC around the office to confirm the solution before going to the expense of replacing all client devices.
Finally, since you've already re-checked client settings, I assume that you already eliminated basic configuration problems that might interfere with client connectivity. For example, a client with a mis-configured WPA-PSK will briefly connect to the WLAN and then become disconnected when authentication fails. A client running VPN software may obtain an IP address via DHCP before launching a VPN tunnel, but not be able to renew that IP address less than 10 minutes later, causing the tunnel to disconnect. If you verify coverage and absence of errors with another adapter and STILL experience intermittent loss of application, transport, or IP connectivity, start looking for higher-layer culprits.
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