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Troubleshooting the WLAN

Brien M Posey Tech Republic

Published: 19 Nov 2002 12:02 GMT

Verify the SSID
A while back, I took my laptop to a friend's house to work. Because my friend had a wireless network in place, I decided to connect to his network for the duration of my visit. Upon returning home, I didn't use my laptop for a couple of weeks. The next time that I went to use my laptop, it wouldn't connect to my network. The problem was that I had forgotten to reset the Service Set Identifier (SSID) back to my own network identifier. Remember, if the SSID doesn't specify the correct network, you won't be able to ping the access point. Instead, your laptop will ignore the access point's existence and search for an access point with the specified SSID.

Verify the WEP key
Next, check out the wired equivalent privacy (WEP) encryption configuration. If WEP is configured incorrectly, you will not be able to ping the access point from a wireless client. Different brands of NICs and access points require you to specify the WEP encryption key differently. For example, one brand requires you to enter the encryption key in hex format, while another brand requires the key to be entered in decimal format. Likewise, some brands support 40-bit and 64-bit encryption, while other brands support only 128-bit encryption. For WEP to function, all settings must match exactly between the client and the access point. I have run into several situations in which clients that seemed to be configured perfectly simply could not communicate with an access point that was using WEP. During these situations, I usually had to reset the access point to the factory defaults and reenter the WEP configuration information. Only then did WEP begin to function.

Tricky WEP configuration issues
By far the most common configuration-related problems involve the use of the WEP protocol, so WEP deserves some more discussion. Troubleshooting a WEP problem can be especially tricky, because a WEP mismatch has symptoms that are similar to a more serious failure. For example, if WEP is configured incorrectly, a wireless client won't be able to get an IP address from a DHCP server (even if the access point has a built-in DHCP server). If the wireless client is configured to use static IP addresses, the wireless client won't even be able to ping the access point's IP address, thus giving the illusion that no connection exists.

The trick to figuring out whether a problem is related to a WEP configuration error rather than a hardware malfunction is to be aware of the diagnostic capabilities built in to the NIC driver and the operating system. For example, one of my laptops is running Windows XP and has a Linksys wireless NIC.

Notice in Figure A that if I move my mouse pointer over the top of the wireless icon in the taskbar, I see a summary of my connection information. In the Figure, the connection strength is Excellent. As long as the channel and SSID are configured correctly, you can connect to the access point, even with a WEP configuration error. Had there been a physical connection problem, the connection strength would be None, not Excellent. Linksys cards will show you the connection strength whether WEP is configured correctly or not. So you can validate that a connection exists, even if you can't ping the access point.

Figure A

The signal strength is a big clue as to the nature of your problem.

If you right-click on the wireless networking icon in the taskbar and select the View Available Wireless Networks command from the resulting menu, you'll see the Connect To Wireless Network dialog box. This dialog box displays the SSID of any wireless network on your present channel to which you are not currently connected. If the name of your wireless network shows up on this list, but you can't seem to connect, rest assured that your connection is good and that you have a configuration problem.

Note
An interesting side note is that the Connect To Wireless Network dialog box also includes a field where you can enter a WEP key when you try to connect to a wireless network. There have been times when I absolutely could not connect to a particular wireless network unless I went through this dialog box and manually entered the WEP key. After doing so, the network became available to me.