One of the most common problems that you will see in 802.11 wireless networks is what is called "Sticky Clients". This is when a client device has authenticated and associated with an Access point and then moved location further away from the AP and is getting a very poor signal level and data rate, and in this location, and there is an AP with stronger signal strength and it will not roam to this device.
The reason this happens is the decision to roaming between access points is decided by the client in what we call the Green Dimond decision algorithm.
Different client vendors and manufacturers use different algorithms to decided when to roam. Here are a few attributes they use to decide.
SSID
RSSI
SNR
Authentication Method
Channel Switch Announcements
De-authentication Frame
Dis-association Frame
Encryption Method
Error Ratio (Retries)
Heuristics
MCS/Data Rate
Minimum Basic Rate
Supported Data Rates
802.11k, 802.11r, 802.11v
Sadly these algorithms are kept proprietary and not in the public domain. So we can not predict exactly how each client will react.
Ways to improve roaming
As the decision is made by the client we can only adjust the BSS to help guide the client device into roaming. the following are a few ways in which we can make roaming more appealing to the client.
Protocols enablement - Enabling 802.11 protocols such as 802.11k and 802.11v. this will allow for the smoothest roaming but is subject to clients supporting these protocols and actually participating in an orderly fashion with the APs
Minimum data Rate - It is strongly recommended that you disable the 802.11b data rates on your wireless. This will prevent clients from connecting at very low speeds and decongesting the wireless. As users do not have to wait for the very slow device to finish with the channel. This will also reduce the BSS cell size and meaning devices have to be closer to the AP to transmit. Which in turn will force the device to roam when it moves further away from the AP and below the minimum data rate that the AP supports. I would recommend setting the minimum data rate to 12Mbps as a nice compromise. You can set it higher but then you have shrunk the cell size down quite a bit and this will lead to your clients roaming more regularly. which on some devices this could cause performance issues depending on how they handle the roaming and create coverage holes a well.
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