Opportunistic PMK Caching improves
fast roaming in order to create a transparent environment for users of latency-sensitive
wireless applications.
Figure 6.4: Opportunistic PMK Caching.
Figure 6.5: Performance of Opportunistic PMK Caching.
In a wireless switch environment, the switch has access to all PMKs from all
connected access points. Depending upon the switch policy configuration, it is possible for
a PMK from any one of the access points to be used for all connected access points.
Therefore, a client may perform PMK caching with any other PMK that is available,
bypassing the 4-way CCMP authentication handshake. This can greatly speed up the
Understanding and Achieving Next-Generation Wireless Security
143
roaming process between access points, and in some cases will lead to virtually no
disruption in service - critical to wireless VoIP and other devices.
Motorola??™s own testing has shown that the access point handoff can occur in less than
40 milliseconds - far below the 150 milliseconds time considered the threshold of VoIP
latency as perceptible to humans. Motorola achieved this key breakthrough with its switchbased
architecture, making possible seamless interoperation with several third-party clients
and transparent wireless roaming for demanding user applications.
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