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Dropped connections when clients switch APs

Lisa Phifer takes the mystery out of dropped connections in a medical office building using 802.11g where clients are unable to roam.

We are implementing wireless 802.11g in a large medical office setting with many closed-in exam rooms (more than 50 per floor). I have installed APs using a "rule of thumb" of 1 AP per 3000 sq ft. Our clients are using Windows XP, and would like to be able to "roam" seamlessly between APs. We notice disconnection and reconnection when the clients switch APs, with complaints from users. Any suggestions?

802.11 stations automatically try to associate with the "best" AP with a given SSID. "Best" can be based on many factors, including signal strength, error rate, and link speed. When a station associated with one AP decides that another AP might be better, it disassociates from the old AP and associates to the new AP. This roaming process of course takes time.

If your APs are in the same subnet with no security enabled, roaming may be barely noticeable. If your APs require WPA or WPA2-Personal, PreShared Key authentication will add many milliseconds of latency. If your APs require WPA or WPA2-Enterprise, the full-blown 802.1X authentication can add seconds of latency, which users perceive as broken connections. If your APs are in different subnets, every TCP and UDP session must be restarted: the user's worst case scenario.

Read the rest of this expert response in this tip.

This was last published in November 2006

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