Mu Association Process - Motorola AP-51 Series Product Reference Manual

1.3.5 MU Association Process

An access point recognizes MUs as they begin the association process. An access point keeps a list
of the MUs it services. MUs associate with an access point based on the following conditions:
• Signal strength between the access point and MU
• Number of MUs currently associated with the access point
• MUs encryption and authentication capabilities
• MUs supported data rate
MUs perform pre-emptive roaming by intermittently scanning for access point's and associating with
the best available access point. Before roaming and associating, MUs perform full or partial scans to
collect statistics and determine the direct-sequence channel used by the access point.
Scanning is a periodic process where the MU sends out probe messages on all channels defined by
the country code. The statistics enable an MU to reassociate by synchronizing its channel to the
access point. The MU continues communicating with that access point until it needs to switch cells
or roam.
MUs perform partial scans at programmed intervals, when missing expected beacons or after
excessive transmission retries. In a partial scan, the MU scans 's classified as proximate on the
access point table. For each channel, the MU tests for Clear Channel Assessment (CCA). The MU
broadcasts a probe with the ESSID and broadcast BSS_ID when the channel is transmission-free. It
sends an ACK to a directed probe response from the and updates the table.
An MU can roam within a coverage area by switching access points. Roaming occurs when:
• Unassociated MU attempts to associate or reassociate with an available access point
• Supported rate changes or the MU finds a better transmit rate with another access point
• RSSI (received signal strength indicator) of a potential access point exceeds the current
access point
• Ratio of good-transmitted packets to attempted-transmitted packets falls below a threshold.
An MU selects the best available access point and adjusts itself to the access point direct-sequence
channel to begin association. Once associated, the access point begins forwarding frames addressed
to the target MU. Each frame contains fields for the current direct-sequence channel. The MU uses
these fields to resynchronize to the access point.
Introduction 1-27