Wireless LAN Communication and Detection: A Project Review

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To figure out how to use the provided Wireless LAN card to retrieve signal information of surrounding wireless stations, the basic Wireless LAN operations need to be understood. Since the Wireless LAN technology is a topic too broad to be fully covered in few pages, only knowledge regarding station detection and communication are discussed in this paper. This paper critically reviews some projects in this area.

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(IJCSIS) International Journal of Computer Science and Information Security,
Vol. 6, No. 2, 2009

Wireless LAN Communication and Detection:
A Project Review
Debabala Swain, CIT M.Singh, KIST P.K.Nayak,KIST
S.P.Panigrahi, KEC S.M.Rout, KIST S.Swain, KIST

Bhubaneswar, Orissa, India, siba_panigrahy15@rediffmail.com
Abstract—To figure out how to use the provided Wireless LAN process, the 802.11 security implementation Wired Equivalent
card to retrieve signal information of surrounding wireless Privacy (WEP) allows the Wireless LAN networks to accept
stations, the basic Wireless LAN operations need to be or reject any association request from the mobile stations.
understood. Since the Wireless LAN technology is a topic too Once the association process is completed, the station is said
broad to be fully covered in few pages, only knowledge regarding
station detection and communication are discussed in this paper.
to have joined the network. Then, the station is free to transmit
This paper critically reviews some projects in this area. data within the network.
To support scanning, the IEEE 802.11 standard has specified
Keywords- WLAN, Location, Roaming that any 802.11 compliant devices require transmitting a
beacon frame periodically to declare its existence. The beacon
I. INTRODUCTION frame can be captured and informed by all other stations. The
beacon frame also carries the information about the belonging
It is important to realize that the Wireless LAN (IEEE station’s capabilities, such as supported data transfer rates.
802.11) is a standard that defines a common, shared operation
scheme for all the 802.11 compliant devices. However, the B. Station Communication
802.11 devices may be developed using different physical The IEEE 802.11 has defined two communication modes
designs that function with different signal modulation schemes to allow data transmissions between the stations: independent
and the devices using different designs may not be able to mode (ad-hoc) and infrastructure mode. In the independent
communicate. The IEEE 802.11 Architecture is shown to mode, the wireless stations communicate directly with each
better explain the differences. other within their coverage area. In the infrastructure mode,
IEEE 802.11 specifications define the Medium Access the stations communicate via wireless access points (AP).
Control (MAC) and Physical (PHY) Layer components for Every packet transmission from a mobile station reaches
wireless type of transmission in IEEE 802 architecture. The its desired destination by sending it through the APs. All the
MAC layer is the common operation method that is used by all sub standard (e.g. IEEE 802.11b) stations intercommunicate
the 802.11 or 802.11x (e.g. 802.11b) devices. In the PHY layer, using either ad-hoc or infrastructure modes.
designs are differentiated and devices using different PHY
There are many types of communication defined in the
layers may not be able to talk to each other. The IEEE 802.11
802.11 standard and each type may use different frame
has specified two underlying transmission techniques for radio,
Frequency Hopping Spread Spectrum (FHSS) and Direct formats. Some types require stations associating to a network
Sequence Spread Spectrum (DSSS). Both or either techniques beforehand but some do not. In the project, a communication
need to be supported by the 802.11 devices so basic type, called active probing is used to retrieve signal
communications can be understood. Since the FHSS and DSSS information.
are not interoperable with each other, a device that uses FHSS C. Wireless LAN Positioning
will not be able to interpret any information sent by a DSSS
device. In current markets, the DSSS has become the favored It has found that the most current solutions are based on the
implementation. The 802.11b and 802.11g added additional utilization of signal strength [2]. The signal strength values
modulation schemes to enhance the data transfer rates for from the reference stations (access points) are measured by the
actual data transmissions but the basic communications are still positioning device. And based on the use of signal information
done using DSSS [1]. (signal quality, signal strength, SNR and so on), there have
been two possible implementations – the empirical model and
II. USEFUL TERMS propagation model. The explanations of the two models with
corresponding related works are covered below.
A. Station Detection
III. USEFUL MODELS
The communication process in any Wireless LAN network can
be summarized into three steps – scanning, authentication and A. Empirical Model
association. A station first scans for existence of any wireless This empirical model is based on storing pre-recorded
network within the range. If there is one or more detected, the measurements in a database. A so-called “radio map” is
station then can choose which network to join, through the constructed before location positioning can begin. The radio
authentication and association processes. In the authentication map is the site map that contains markings of a series of

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ISSN 1947-5500
(IJCSIS) International Journal of Computer Science and Information Security,
Vol. 6, No. 2, 2009
selected points. The locations (coordinates) of these points are thought out so it interprets receiving radio wave signals to
known and at each point the signal information, such as signal return the correct path loss values. The software needs to be
strength value from the APs are collected and store into a specifically written to understand the communication at the
database. When a device with unknown position requests it’s hardware layer so it obtains correct signal values for distance
positioning, the signal information from all the APs are calculations. As a result, system designs using the propagation
collected and sent to the database for comparison. An model are always much more complicated.
empirical model program created then compares and finds the In addition to the complication of the model, many existing
closest match in the entries of the known locations in the systems have shown that the accuracy of positioning will
database against the entry of a known point. The location of
decrease as the distance between the positioning device and
this matched known entry in the database is said to be the
the affiliated access point increases. It is mainly because that
position of the query point (the device). Sometimes, more than
closest matches are wanted and the final location is the radio signal will more likely be affected by more factors
determined by averaging the coordinates of these closest (e.g. change of humidity) as it travels in longer distances in the
known locations. air and the resulted path loss values of the signal when the
device receives will not be the same as predicted from the
As it can be immediately seen, there are two disadvantages radio propagation and path loss theories.
to the systems built using the empirical model. These systems To eliminate the aforementioned problems and increase the
will always require a considerable amount of manual efforts positioning accuracy, the system can be designed to give the
on radio map and database constructions, when they are to be primary access point (which has the least path loss value) a
used in a new environment. Another disadvantage the system higher weight in the triangulation algorithm. In addition, a
can lose some accuracy when the current environment
more complicated of the radio theories can be applied.
condition is different the condition when the radio map was
constructed. The radio wave properties in an indoor The idea of developing positioning tools with uses of the
environment vary greatly depending on number of objects 802.11 standard equipments is not entirely new. There have
(e.g. people). been various Wireless LAN positioning research and design
However, solutions are available to counter the projects based on the empirical and propagation models.
disadvantages. The map construction process can always be While the research projects concentrated on finding design
automated via some software written specifically to collect the ideas that would be derived from relevant theories, the design
data and write to the database. The using of such software can projects have actually gone into designing, manufacturing and
greatly speed up the construction processes. Several radio supported by test results.
maps of the various conditions, such as number of people in Here, any of projects that provided useful information towards
the environment, can be created and selected to use depending our design are discussed here.
on to the environment condition at the time of positioning.
IV. SOME PROJECTS
B. Propagation Model
On the other hand, the prorogation model tends to be more A. RADAR Project [3]
flexible. The model is based on the fact that as a radio wave The RADAR is an in-building radio frequency (RF) based
travels through an environment it loses signal and the amount user location and tracking system. The project was one of the
of signal strength that the radio wave is dependent on the very first research efforts into the Wireless LAN positioning
environment. The loss of the signal strength can then be technology and many later projects were started heavily
modeled by using known radio propagation and path loss replying on the methodologies it has represented. As the
theories. Using these theories, the distance from a wireless RADAR’s official document describes – “the system
device to an access point can then be calculated given the combines the empirical model measurements with the signal
received signal strength loss value. By having the distances to propagation modeling to determine user location and thereby
three or more access points, triangulation method can be enable location-aware services and applications”, which in fact
applied to determine the location of the device. The means that the two models were made into separate designs
propagation model is the ideal model to be used in the and tested and compared together.
The project was conducted and sponsored by Microsoft
Wireless LAN positioning or any other indoor positioning
Research to investigate the achievable degree of accuracy
systems. Because the location is computed depending solely when performs positioning using RF-based equipments. The
on the relative distances to surrounding access points, the Wireless LAN devices were used throughout the project
systems will be able to work in all environments as long as the testing. Instead of using Wireless LAN access points, base
positions of the access points in the area map are first known. stations were all Pentium-based PCs running Free BSD 3.0
Although the propagation model is the best solution to be equipped with wireless adapters. The mobile host waited-to-
used, there have been various difficulties in the actual be-tracked was a Pentium-based laptop computer running
implementations. In order to achieve accurate results, the Microsoft Windows 95.
signal path loss values that the positioning system received In the empirical model design, RADAR used the signal
from the access points have to be precise. In many cases, it information including signal strength and signal-to-noise ratio
means the systems have to be a complete design covering (SNR). The selected known points on the map radio and stored
hardware and software. The hardware needs to be carefully in the database did not only contain location coordinates (x
and y) but also directions (north, south, west and east). To

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(IJCSIS) International Journal of Computer Science and Information Security,
Vol. 6, No. 2, 2009
determine the unknown point’s location, “k-nearest neighbor most probable position of the user. The server was used to
search” algorithm (later explained) was used to compute k have the calculations being performed somewhere else than on
number of the known locations closest to the unknown point the client, in order to save processing power on for example
by matching the database entries. The k value from 1 to 10 handheld devices or slower computers. The map setup tool
was experimented. created the maps that would be used on location display
In the propagation model, RADAR used Wall Attenuation machines (e.g. handheld). The map setup tool assigned
Factor (WAF) to calculate the distances using the path loss. coordinates and had the ability to place user specified objects
The WAF equation was described as following: such as stairs and toilets on the maps.
Unfortunately, the project’s design was not fully completed
at the end. It was unclear if any graphical interface for
displaying the positions has been created (either on server or
clients). However, the project did produce some testing results
n indicates the rate at which the path loss increases with and in particular, the propagation model implementation has
distance, P(d 0 ) is the signal power at some reference shown positioning errors between 7.3 and 2.8 metres.
distance d 0 and d is the transmitter-receiver separation C. Amulet Project [5]
distance. C is the maximum number of obstructions (walls) up The Amulet ((Approximate Mobile User Location
to which the attenuatiactor makes a difference, nW is the Tracking System) project is practical design, making uses of
number of obstructions (walls) between the transmitter and similar empirical model proposal also from the RADAR
receiver, and WAF is the wall attenuation factor. In general, project. Amulet is a kind of modular software. This software
n , the path loss exponent and WAF depend on the building was developed solely by Mr. Blake M. Harris in the University
layout and construction material and are derived empirically. Of Rochester, USA and the application operates on recent
Based on the measurements in its testing environment, a WAF versions of Linux desktop operating systems.
of 3.1dBm and C of 4 was chosen for the WAF equation. A The Project is a practical presentation of Wireless LAN
positioning technology using the empirical model. The Amulet
value of n as 1.523 and d 0 of 58.48dBm were selected by system breaks the design into three modules with
finding that the values for all three access point are similar, intercommunicating with each other – Access Point Statistics
despite their different physical locations and surroundings. Recorder (APSR), Nearest Neighbor Association Module
In the final results published, RADAR has shown an (NNAM) and Map GUI. The APSR module is a shell script
estimation error of 2.94 metres when using the empirical model that continuously logs signal information from all the
and 4.3 metres when using the propagation model. requested access points. The module collected signal
information from the access points by using Linux Wireless
B. Advanced Wavelan Position Project [4] Tools iwspy interface and allowed maximum 8 entries at each
The Advanced Wavelan Position Project [4] was a student operation (Linux Wireless Tools is discussed in later
team project in Lulea University of Technology and intended chapters). The NNAM implements the k-nearest neighbor
to re-experiment both empirical and propagation models algorithm, same as what was used in the RADAR project. The
proposed in the RADAR project. In the implantation using the NNAM reads in the most current access point information
empirical model, signal strength was the only parameter taken from the APSR, obtains the coordinates of the k nearest
during the radio map construction and used later in the neighbour in signal strength space and output those
positioning. In the propagation model, a simpler radio coordinates to the application that needs access to them. The
propagation method was used: Map GUI module is simply an application to visualize the
points included in the radio map and the location of the
positioning device. The Map GUI was written in Java Swing
of the Java Foundation Class (JFC) [6].
P (d ) is the signal strength value measured at distance, “Signal quality” was the signal information collected
d , P(d 0 ) is the value measured at some reference distance
during the radio map and database constructions. Using the
signal quality values instead of signal strength has shown
d 0 . d is unknown distance to be calculated. results in no much difference to the positioning accuracies.
The system design consisted of a client, a database, a server A 3 to 5 metres resolution was achieved and was slightly
and a map setup tool. The client was the positioning device that less accurate than the RADAR system due to its less
was responsible for collecting the signal strengths it currently detailed empirical measurements. The RADAR system used
can hear, sending them to the server for processing and finally more advanced methods used in averaging signal values, as
presenting the calculated location on the selected map. The well as gathered user orientation data when collecting radio
database was used to store all pre-measured values if the map points.
empirical model was chosen. If the propagation model was
used, the database will be omitted.
The server provided functionalities to receive the current
signal strengths from the client and calculate and return the

339 http://sites.google.com/site/ijcsis/
ISSN 1947-5500
(IJCSIS) International Journal of Computer Science and Information Security,
Vol. 6, No. 2, 2009
TABLE I. IMPIRICAL & PROPAGATION MODEL COMPARISION calculating the device coordinates and stores the calibration
data.
Being a commercialized product, it has claimed to achieve
Characteristics Empirical Model Propagation Model
up to 1 metre average in the positioning accuracy. The client
and server modes enable the ability to locate positions of
Design multiple devices and display all the points on any of the device.
Low High
Complexity
Positioning
Good Fair F. Other Projects
Accuracy
Reusability Low High There were two other projects related the Wireless LAN
positioning technology. Having not been researched in depth
Set-up Cost High Low
during the project development, they are only listed here for
readers who are interested in further studying of the topic.
• CoSCo Group – University of Helsinki [9]
The group later founded and developed the aforementioned
Ekahau Positioning Engine.
• SpotOn Project – University of Washington [10]
The SpotOn is an application combining both hardware
and software designs and it focuses on finding distance
between two radio transceivers (Ad-hoc location).
G. Comparisions
Figure 1. IEEE 802.11 Architecture
Having looked through various projects involved in the
Wireless LAN positioning development, the advantages and
D. Halibut Project [7] disadvantages of the empirical and propagation models can be
Halibut is a research project uses the propagation model summarized as in the table 1.
exclusively for the positioning and considers various
parameters affecting the radio wave propagation, including V. CONCLUSION
standard free space loss, signal attenuation, diffraction, multi- The designs using propagation model have shown to be
path fading and a random variable to model log-normal more cost effective than the empirical model in the sense that it
shadowing. An equation similar to the WAF theory used in the does not require detail measurements to generate a signal
RADAR project: strength map. To design a positioning system balanced between
the cost and performance, both models should be used.

PI (d ) is the path loss measured at distance d, PI (d 0 ) is the REFERENCES
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analysis. The Positioning Engine is a server application for

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