CHAPTER FIVE - CONECTIVITY
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Note!
The following roaming settings are not applicable if your WLAN
communication is based on Cisco wireless LAN infrastructure with
lightweight access points and with a controller unit (Cisco Information
Element), not to mention if your WLAN uses only one access point (AP).
However, in the case of one AP it is worth considering the values of an
‘Aggressive Scan’ and ‘BG Channel Set’. (As far as Cisco wireless LAN
infrastructure goes, there is a global setting ‘CCX features’ that affects
whether or not the roaming settings below are overridden.)
•
Roam Trigger:
When there is an association between a PL3000 and an
access point, the PL3000 monitors received signal strength (RSS) from the
access point (AP). When RSS drops below the value of the Roam Trigger the
PL3000 starts to scan other APs having the same SSID as the current AP has
(referred to as roaming scan). Higher values of the Roam Trigger will tend to
result in more frequent roaming scans than lower values (be aware that the
values are minus numbers, that is, (-) 70dBm is higher than (-)85dBm).
•
Roam Delta:
During the roaming scan the PL3000 monitors received signal
strength from all the APs that respond to its probe requests. The value of the
Roam Delta is a threshold of starting an actual handoff. If RSS from some AP
is at least the value of the Roam Delta stronger than the RSS of the current
AP, the PL3000 renews its WLAN association through the AP from which it
received at least Roam-Delta stronger signal.
•
Roam Period:
RSS monitoring does not resume immediately after the roaming
process has ended with or without re-association (it is possible the PL3000
holds its WLAN connectivity without renewing it through another AP), but
PL3000 starts to monitor RSS again after the ‘Roam Period’ has elapsed. The
point of the ‘Roam Period’ together with the Roam Delta is to prevent the ping-
pong effect. You can set the Roam Period in five-second increments in the
range of 5 to 60 seconds.
•
BG Channel Set:
PL3000 searches for the APs within range by sending probe
requests. To be successful in searching the PL3000 probes the set of the
channels being available in the 2.4GHz frequency band. The number of the
available channels varies between 11 and 13, depending upon regulatory
domain. Because the most of the available channels are framed to overlap with
each other, only the non-overlapping channels are typically used. Using the
‘BG Channel Set’ you can limit the scanning to the non-overlapping channels
to decrease scan time, which improves e.g. roaming. Note that the ‘Custom’
value is not in use, meaning that, it is not possible to create and to use the
channel subset of your own.
•
Aggressive Scan:
To sustain WLAN connectivity, there is a special
aggressive scan mode on the PL3000 for re-association. When the Aggressive
Scan is set to ‘On’ (a factory default), the aggressive scanning mode (aka
aggressive roaming) takes control of roaming as follows. If the PL3000 does
not receive the number of beacon frames it expects within the certain period
from the AP (referred to as ‘current AP’) that it has associated with, the
aggressive roaming takes place. (Be aware that the AP defines the interval of
its beacon frames.) The aggressive roaming does not obey the roam
parameters such as the Roam Delta, but it finishes with a successful re-
association with the AP (referred to as ‘better AP’) from which the PL3000
received stronger signal compared to the current one. The aggressive roaming
may also end if the beacon frames are captured again at the expected ‘rate’
from the current AP. If the association with the current AP is lost before finding