Symbol Technologies Spectrum24 AP-4131 - Manual (Page 30)

Introduction
20 AP-4131 Access Point Product Reference Guide
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 AP. The MU continues
communicating with that AP until it needs to switch cells or roam.
MUs perform full scans at start-up. In a full scan, an MU uses a sequential
set of channels as the scan range. For each channel in range, the MU tests
for CCA (Clear Channel Assessment). When a transmission-free channel
becomes available, the MU broadcasts a probe with the Net_ID (ESS) and
the broadcast BSS_ID. An AP-directed probe response generates an
MU ACK (Mobile Unit Acknowledgment) and the addition of the AP to the AP
table with a proximity classification. An unsuccessful AP packet transmission
generates another MU probe on the same channel. If the MU fails to receive
a response within the time limit, it repeats the probe on the next channel in
the sequence. This process continues through all channels in the range.
MUs perform partial scans at programmed intervals, when missing expected
beacons or after excessive transmission retries. In a partial scan, the MU
scans APs classified as proximate on the AP table. For each channel,
the MU tests for CCA. The MU broadcasts a probe with the Net_ID (ESS)
and broadcast BSS_ID when the channel is transmission-free. It sends an
ACK to a directed probe response from the AP and updates the AP table.
An unsuccessful AP packet transmission causes the MU to broadcast another
probe on the same channel. The MU classifies an AP as out-of-range in the
AP table if it fails to receive a probe response within the time limits. This
process continues through all APs classified as proximate on the AP table.