802.11i is intended to help with the use of WLAN for speech connections (VoIP) in enterprise networks. Especially in connection with WLAN-based wireless telephony, quick roaming (switching from one access point to another without lengthy interruptions) is of special significance. In telephone conversations, interruptions of 100 milliseconds are irritating, but the full authentication process over 802.1x, including the subsequent key negotiation with the access point, can take significantly longer.
For this reason, the so-called PMK caching was introduced as a first measure. The PMK serves as the basis for key negotiation in an 802.1x authentication between client and access point. In VoIP environments it is possible that a user moves back and forth among a relatively small number of access points. Thus it may happen that a client switches back to an access point in which it was already registered earlier. In this case it wouldn't be sensible to repeat the entire 802.1x authentication again. For this reason, the access point can provide the PMK with a code, the so-called PMKID, which it transmits to the client. Upon a new registration, the client uses the PMKID to ask whether this PMK is still stored. If yes, the 802.1x phase can be skipped and the connection is quickly restored. This optimization is unnecessary if the PMK in a WLAN is calculated from a passphrase as this applies everywhere and is known.
A second measure allows for some acceleration even in the case of first-time registration, but it requires a little care on the part of the client. The client must already detect a degrading connection to the access point during operation and select a new access point while it is still in communication with the old access point. In this case it has the opportunity to perform the 802,1x negotiation with the new access point over the old one, which again reduces the "dead time" by the time required for the 802.1x negotiation.