We gain diminishing returns by the short scan interval. The short
scan interval is used to hunt for a better AP if the RSSI of the
current AP drops. However, if we never roam, and the AP continues
to have low RSSI, at some point we should give up and return to
the slow background scan rate, otherwise we waste a lot of power.
Signed-off-by: Paul Stewart <pstew@google.com>
It is not guaranteed that we will get a CQM signal shortly after setting
up monitoring. In order to establish the correct initial background
scanning rate, poll directly for the signal strength.
Signed-off-by: Paul Stewart
The driver is likely to indicate an immediate signal event when the
threshold value is configured. Since we do this immediately after
association, there is not much point in requesting a new scan to be
started based on this event.
This allows bgscan modules to use more information to decide on when
to perform background scans. bgscan_simple can now change between
short and long background scan intervals based on signal strength
and in addition, it can trigger immediate scans when the signal
strength is detected to be dropping.
bgscan_simple takes following parameters now:
short interval:signal strength threshold:long interval
For example:
bgscan="simple:30:-45:300"
This fits better in wpa_supplicant/scan.c. Couple of remaining
scan_helpers.c functions are currently used in driver wrappers,
but they can likely be removed in the future.
This allows background scanning and roaming decisions to be contained in
a single place based on a defined set of notification events which will
hopefully make it easier to experiment with roaming improvements. In
addition, this allows multiple intra-ESS roaming policies to be used
(each network configuration block can configure its own bgscan module).
The beacon loss and signal strength notifications are implemented for
the bgscan API, but the actual events are not yet available from the
driver.
The included sample bgscan module ("simple") is an example of what can
be done with the new bgscan mechanism. It requests periodic background
scans when the device remains associated with an ESS and has couple of
notes on what a more advanced bgscan module could do to optimize
background scanning and roaming. The periodic scans will cause the scan
result handler to pick a better AP if one becomes available. This bgscan
module can be taken into use by adding bgscan="simple" (or
bgscan="simple:<bgscan interval in seconds>") into the network
configuration block.
