Roaming to the Wrong AP
May 17, 2007
Poor signal or performance issues? Your wireless card may be connecting to the 'wrong' access point.
When you connect to a wireless network, you are connecting to a single access point (AP) -- though the radio card (A.K.A. client card, network interface card, or NIC)
If your wireless network is outfitted with multiple APs, and users experience poor signal strength and performance despite proper coverage in the area, the users radio card may not be connecting to the closest or most ideal access point for that specific location.
Here are a few symptoms of a client connecting to the wrong AP:
- Low signal strength
- Poor performance
- Intermittent connectivity
Since these symptoms are similar to other network issues, you should try to verify that the radio client is connecting to the wrong AP, and not just sitting too far away from the right one.
One way to verify this problem is to find where the nearest APs are located in relation to the location of the problem client, using a facility map of the AP locations. Check to see which AP the client is connected to when problems occur. This can be done by checking the association lists on the APs, or by using a Wi-Fi analyzer. Then check if theres a closer or more ideal AP for the client, keeping in mind the environment of the facility.
For example, say you checked your AP association lists and saw that the client is connected to AP 2, which is located on the next floor down. However, the facility map indicates that AP 1 is slightly closer to the client and doesnt have any obstructions in the way. Therefore, you can conclude that the radio is likely connecting to the wrong AP.
Here are a few things to try in order to temporarily fix the problem:
- Restart the radio card. Simply disabling and re-enabling the radio card may work. During initial scanning, the client may choose the correct AP.
- For mobile clients, move around. Moving the client around may make the radio card realize theres a better AP in range.
- Use MAC address filtering. You could block the client from connecting to incorrect AP(s) by only allowing clients with specific MAC addresses.
- Change AP transmit power. You could slightly decrease the transmit power on incorrect AP(s) or slightly increase the transmit power on correct AP(s).
Fixing the Underlying Problem
Although you may be able to find a temporary fix for this problem, you may want to do some in-depth analysis of the network and RF conditions. For example, check to see if occasional RF interference causes the problem, and analyze AP placement. You should also keep in mind that the underlying cause of the problem may be the client cards scanning methods and techniques; therefore, you could try replacing the card, or see if theres a driver update that may fix the issue. You may also want to contact the manufacturer to report the issues youve found.
Eric Geier is the founder and president of Sky-Nets, Ltd., which operates a Wi-Fi hotspot network serving the general aviation community. He has also been a computing and wireless networking author and consultant for several years. Erics latest book is Wi-Fi Hotspots: Setting up Public Wireless Internet Access, published by Cisco Press.