Mu Association Process - Symbol AP-51xx Product Reference Manual

1-22 AP-51xx Access Point Product Reference Guide
Intercepting and decoding a direct-sequence transmission requires a predefined algorithm to
associate the spreading code used by the transmitting access point to the receiving MU. This
algorithm is established by IEEE 802.11b specifications. The bit redundancy within the chipping
sequence enables the receiving MU to recreate the original data pattern, even if bits in the chipping
sequence are corrupted by interference.
The ratio of chips per bit is called the spreading ratio. A high spreading ratio increases the resistance
of the signal to interference. A low spreading ratio increases the bandwidth available to the user. The
access point uses different modulation schemes to encode more bits per chip at higher data rates.
The access point is capable of a maximum 54Mbps data transmission rate (802.11a radio), but the
coverage area is less than that of access point operating at lower data rates since coverage area
decreases as bandwidth increases.

1.3.5 MU Association Process

An access point recognizes MUs as they begin the association process with the access point. 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 pointand 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 access point 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 access point'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 access point and updates the table.
An MU can roam within a coverage area by switching access points. Roaming occurs when: