The Roaming Scan Feature; The Roaming Scan Mechanism - Alvarion BreezeACCESS SU-M-900 Installation Manual

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The Roaming Scan Feature

The Roaming Scan feature enables the mobile unit to perform faster
re-association with a new AU when the current AU is lost or the signal is
not good enough.

The Roaming Scan Mechanism

If the signal quality (RSSI) drops below a configurable threshold (Roaming
Decision RSSI Threshold), the SU will enter into roaming scan mode.
In roaming scan mode, the SU will scan other frequencies from time to
time. The frequencies that will be used in roaming scan are the same
frequencies that are used in regular scan. To minimize the effect on regular
traffic, the amount of time allocated for these scans (Roaming Scan dwell) is
relatively small; During this Roaming Scan dwell The SU will remain on
each frequency for 19 milliseconds, scanning each time a few frequencies.
The maximum number of frequencies that will be scanned in each Roaming
Scan Dwell depends on the regular Dwell Time: If the Dwell Time is 128 it
will be 5, if the Dwell Time is 64 it will be 2, and if the Dwell Time is 32 it
will be 1. The remaining time is used by the SU to perform normal traffic.
The interval between two consecutive Scan Dwells is set by the Beacon
Interval Between Scan Dwells parameter, ensuring that at least one beacon
can be received between two Scan Dwells. This ensures that the SU will not
loose too many beacons and will not de-synchronize from the AU before
finding a better one.
During each roaming scan, the SU will scan each frequency a configurable
number of times. The SU will finish the roaming scan after each frequency
will be scanned Number of Scanning Attempts In Roaming Scan times.
When the roaming scan is finished, any of the following actions may be
performed:
a. If a better AU (having a higher RSSI) was found the SU will associate
with this AU.
b. If the RSSI of the current AU increases above the Roaming Decision
RSSI Threshold, the SU will not continue the roaming scan.
c. Otherwise, the SU will start another roaming scan. The shortest time
between two roaming scans will be a single Roaming Scan Period (the
time period required for a full roaming scan). At each roaming scan that
failed (no AU found and the RSSI of the current AU is still below the
threshold), the SU will increase the time between roaming scans by a
time period equivalent to the Roaming Scan Period. However, there is an
BreezeACCESS SU-M-900 Installation Guide