The 802.11 standard defines how wireless devices behave when a station (re)associates. Several vendors have developed proprietary inter-AP protocols to facilitate handoff in homogeneous WLANs. To provide guidelines for handoff in multi-vendor environments, the IEEE developed a standard inter-AP specification (IAPP)-- see IEEE 802.11f.
More recently, the IEEE addressed security-related handoff delays in this year's 802.11i advanced security specification. 802.11i goes beyond basic roaming by letting stations use cached encryption keys and pre-authenticate with a new AP before losing contact with the previous AP. To learn more, see IEEE 802.11i and the WPA2 specification published by the Wi-Fi Alliance.
Finally, the new IEEE 802.11 task group R is working on fast roaming techniques needed to support latency-sensitive applications like VoIP more effectively. To learn more, see the 802.11 handoff-related presentations on Bernard Aboba's excellent 802.11 resource Web site.