In a multi-AP WiFi environment — a mesh system, an office with multiple access points, or a home with a router and a range extender — the experience of moving between access points defines the quality of the whole system. A phone call that drops when you walk from the kitchen to the garden is not a signal problem; it is a roaming problem. Three 802.11 protocol amendments, operating together, are the mechanism that makes roaming fast enough to be invisible.