HP ProCurve 420 Installation And Getting Started Manual page 31

Installing the Access Point 420
Sample Network Topologies
A wireless infrastructure can also support roaming for mobile workers. More
than one access point can be configured to create an Extended Service Set
(ESS). By placing the access points so that a continuous coverage area is
created, wireless users within this ESS can roam freely. All HP wireless
network cards, adapters, and access points within a specific ESS must be
configured with the same SSID.
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