iOS supports improved client roaming in enterprise WLAN networks. With 802.11 workgroup standards "K" and "R", wireless clients can move more seamlessly from access point to access point within the same network.

IEEE 802.11k

With 802.11k, an iOS device can quickly discover nearby access points that are available for roaming. When the signal strength of the current access point decreases and your device has to switch to a new access point, it has already detected the best access point to switch to.

IEEE 802.11r

When your iOS device moves from one access point to another on the same network, 802.11r optimises the authentication process with the Fast Basic Service Set Transition (FT) feature. FT allows iOS devices to connect to access points faster. Depending on the manufacturer of the WLAN hardware, FT can work with the authentication methods Preshared Key (PSK) and 802.1X. The fast detection of the target access point by 802.11k and the faster connection method of FT together improve the performance of the application and should make the Wi-Fi experience on iOS more pleasant. Apple describes which iOS devices support 802.11k and 802.11r with iOS in the following article Wi-Fi Roaming 802.11k and 801.11r (Link).