Tune that dial - Index of

here are the Others?
Wireless transfer of data can take place once a successful
connection has been established. Because the quality of
the connection can diminish – or even disappear completely
– when the user moves, the card scans at regular
intervals for other Access Points that may be within range.
Based on different algorithms that are implemented by
the manufacturer in the driver for the card, the card can
decide to establish a connection with another AP and
terminate the old connection. This is called a ‘handover’.
Which access point becomes the new connection
depends on the signal strengths of the various APs. The
signal strength is determined by the card based on data
packets that are sent by the AP specifically for this purpose.
The packets are called beacons and are transmitted
by the AP at regular intervals (about 10 times per
second). These beacons contain the unique MAC-address
of the AP, as well as the name of the wireless network, the
so-called SSID. Should the signal strength of the AP that
the card is currently connected to become too weak then
the card starts to search for better alternatives. This is only
valid of course if we have the use of a network with multiple
APs (refer to the example in Figure 1).
Relationship signal strength-position
The method described here implies that there is a relationship
between the position of the user (or more accurately:
of the WLAN card in the device that the user is carrying)
and the measured signal strength of the beacons that are
received from different APs. This relationship can also be
used to locate the user within the WLAN network. In principle
two different methods can be thought of to achieve
this:
1. Simulation: We try to calculate the expected signal
strength at each position in advance based on a model.
Afterwards we compare the measured values and try to
find out where the user is based on the results from the
model.
2. Calibration: Instead of calculating the expected signal
strength for each location, the signal strength is measured
at different, predetermined locations. This information is
stored and used as data for comparison for subsequent
measurements when the user is in an unknown location.
Various tests have shown that the first method is not only
significantly more complicated, but is also less accurate
than the second. The reason for this is that the signal
propagation in an indoor environment is so very complex
that even very extensive models cannot take all existing
factors into account.
Reading Signal strengths
The question now arises how we are going to obtain
these seemingly very important signal strengths. Windows,
after all, hides this information from us in the first
instance by translating the received signal strengths
into less accurate terms such as ‘very strong’ or ‘weak’.
The answer to this? NDIS! NDIS is a standard Win-
12/2006 - elektor electronics
dows-API for communicating with network cards. In this
way it is possible to obtain the necessary information
from the WLAN-driver in a pre-defined manner, irrespective
of the manufacturer. More information can be
found at [1] and [2].
Performing the calibration
Before our positioning system can be used, we must first
carry out the calibration. This consists of recording the
Figure 1.
This signal strength
information is normally
hidden from us by
Windows.
Figure 2.
Visualisation of the signal
strength of an AP, based on
a simulation. Red means
large signal strength,
blue means weak signal
strength.
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