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IAPP (Inter Access Point Protocol) is used for exchanging information between access points. In the process information regarding logged in WiFi devices are is transmitted. If a WiFi devices switches to another access point, this access point informs the old one, which removes the WiFi device from its station table. In doing so all login information is retained and the roaming takes place nearly seamlessly ("soft roaming"). Whereas when a "hard roaming" takes place, the WiFi device has to authenticate itself anew with the new access point.
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