Page 48 / 141
DTUS065 rev A.7 – June 27, 2014
V.7.5.1
Smoothing factor
Various parameters are meant to trigger events:
•
scan threshold
•
leave threshold
•
excessive signal detection threshold.
For the purpose of threshold crossing detection, all this parameters are
compared to the RSSI of the current AP.
The RSSI of the current AP is defined as an exponential moving average
computed over the most recent beacons received from the current AP. So,
the comparison is done, not against the current signal level, but against an
average. Note that only the beacons signal levels are used, since they are
transmitted at a stable bit rate and power level and they are received with
homogenous receiver sensitivity.
In order to favor more or less the recent beacons against the older ones in the
computed RSSI average, you can set the exponential factor of the moving
average. This factor is called the “RSSI smoothing factor”. It represents the
percentage attached to the most recent beacon in the computation.
The smoothing factor is a value between 0 and 1 in steps of 1/16
th
. For
example, a value of 3/16 means that the signal power levels of the previous
beacons are used like this:
•
for the most recent beacon,
3
16
=
18.75% of the signal value,
•
for the penultimate beacon,
3
16
×
13
16
=
15%,
•
for the antepenultimate beacon,
3
16
×
13
16
×
13
16
=
12%,
•
and so on.
Configuration
In the browser interface the factors are expressed as the percentage attached
to the last beacon. As an extreme case, using 100% (or 16/16
th
) means that
only the most recent beacon is used in the comparisons.
V.7.5.2
Off-channel configuration
You can shorten the duration of the off-channel probe request/response
sequences (the ‘B’ parameter in the “scan period” picture above). This
solves the situation where a great data flow is entering the AP which cannot
forward it to the client because it is scanning another channel, and the AP
has insufficient buffers. The ‘B’ delay is the sum of (1) a switching delay
(very quick), (2) a synchronization delay (ensuring that our probe will not
collide with another transmitter on the channel), (3) probe request
transmission (at the lowest rate available), (4) response waiting delay.
Also, the scanner can switch from channel to channel, without returning to
the current channel. This is limited to one beacon interval, though.
You can configure items (2) and (4) of the sum, and you can define the
overall off-channel duration.
