Apple IOS devices maintain a connection with a WiFi access point (AP) a little differently from devices that run Windows, Linux and OS/X. Some of these differences are attempting to improve their battery life. Traditional design and deployment of WiFi AP solutions may not align well with this IOS behaviour. Here are some aspects of that behaviour when IOS devices connect to WiFi APs.
You can read more in these 3 articles:
Apple wireless roaming for Enterprise
Cisco Enterprise Best Practices for iOS devices and Mac computers on Cisco Wireless LAN
Aruba RF and Roaming Optimization for Aruba 802.11ac Networks
Many enterprise WiFi vendors offer a number of advanced connectivity features within the 802.11 standard. Apple has compiled a document that explains how IOS devices can use these features. Here is a brief overview of each of them from the AP vendors perspective:-
802.11k
This helps clients discover nearby APs, reducing the need for the client to perform off-channel scanning and potentially reducing battery drain on the device. WIth 802.11k enabled, the APs hold a table of neighbouring APs (called a neighbour report) and a WiFi client can request that list. The Neighbour Report helps the IOS device search quickly for nearby APs as the device will build an optimised list of channels for the target SSID.
802.11v
Not all IOS devices support this functionality, refer back to the link above in this section of the article.
802.11v allows the AP infrastructure to provide roaming recommendations to the WiFi client. This may result in the WiFi client understanding the best AP for roaming although it is not obliged to follow these recommendations.
802.11r
This is generally recommended where any SSID is using PSK and 802.1x security. 802.11r will reduce roaming times using a technique known as Fast Transition. This technique avoids full authentication handshaking as the client roams from one AP to another (within the same WiFi deployment).