We have a small office with a large number of wireless devices (a mixture of desktop machines, laptops, and wifi-enabled phones) all working from a single Apple Airport Extreme which extends our wired network. I've added another Airport Extreme for resiliency, since we've been seeing a decrease in performance and (as far as I understand) access points can only handle a small number of clients.
I set the new AP to extend the current network so that the clients weren't constantly switching between different wireless networks, however as soon as this AP was configured all the wireless devices started seeing network trouble, flicking on and off.
I'm assuming that this is because both APs are reasonably strong, and the client can't decide which to use.
What is the best route to follow to resolve this? What I'm aiming for is wireless resiliency; preferably having two APs share the network load, or if this isn't an option then having a primary AP with a "fail-over", should the primary go down for any reason.
I think the first step is to verify that your clients are indeed switching between the 2 APs. You can probably check this by looking at the BSSID for your connection before you drop and seeing what BSSID you get when you reconnect. If that is definitely the problem, then you need to decrease the size of each AP's cell by lowering the output power. If you can decrease the size of each cell, each client will get more bandwidth from the AP to which it's connected.
You should make sure that the APs are on different channels too. If the APs are on the same channel, you will see some co-channel interference degrading your performance. The only other problem I can think of here is you might have hidden node issues. Turning on RTS/CTS will fix that problem (you may need to lower your threshold if you can't manually turn on RTS/CTS).
If you have such a strong area of cross over for the access points, would you not be able to either just use the one or move them as far apart as your building allows?
alternatively, lower the power output of one of them depending on physical location in the building to keep the overlay to a minimum