IMPROVEMENT OF WIRELESS NETWORK ISOLATION AND
SECURITY BY SHRUB BARRIERS
Iñigo Cuiñas, Paula Gómez, Manuel García Sánchez and Ana Vázquez Alejos
Universidade de Vigo, Dept. Teoría do Sinal e Comunicacións, rúa Maxwell, s/n, 36310 Vigo, Spain
Keywords: Isolation, security, indoor, wireless LAN.
Abstract: The increasing number of wireless LANs using the same spectrum allocation could induce multiple
interferences and it also could force the active LANs to continuously retransmit data to solve this problem,
overloading the spectrum bands as well as collapsing their own transmission capacity. This upcoming
problem can be mitigated by using different techniques, being site shielding one of them. If radio systems
could be safeguarded against radiation from transmitter out of the specific network, the frequency reuse is
improved and, as a consequence, the number of WLANs sharing the same area may increase maintaining
the required quality standards. The proposal of this paper is the use of bushes as a hurdle to attenuate signals
from other networks and, so that, to defend the own wireless system from outer interferences. A
measurement campaign has been performed in order to test this application of vegetal elements. This
campaign was focused on determining the attenuation induced by several specimens of seven different
vegetal species. Then, the relation between the induced attenuation and the interference from adjacent
networks has been computed in terms of separation between networks. The network protection against outer
unauthorised access could be also improved by means of the proposed technique.
1 INTRODUCTION
The proliferation of wireless local area networks
(WLANs) could be collapsed due to their own
success: an enormous number of corporate WLANs
are going to cohabit in office buildings sited in the
financial quarters of our cities, and a lot of
mismatches could appear as a result of the
interference among several of them. Although
wireless standards are prepared to solve connection
falls, mainly by retransmission of data, the
increasing number of systems using the same
spectrum allocation could force the active LANs to
continuously retransmit data, overloading the
spectrum bands as well as collapsing their own
transmission capacity.
The analysis of interferences on wireless
wideband communication systems is the topic of
different scientific works: considering both narrow
band (Giorgetti et al., 2005) and wideband
interferences (Yang, 2003). Several strategies have
been applied to reduce the interference between
adjacent networks. Among these proposals, the
control of the transmit power appears to be a
successful one (Qiao et al., 2007).
Another problem associated to the wireless
technology is network protection. The users do not
need to be physically connected to the network
nodes; so these users could access the system from
places that could be out of system manager’s
control. This fact allows external users to utilise the
network for private purposes or even for forbidden
actions.
These upcoming problems can be mitigated by
using different techniques, being site shielding one
of them (Van Dooren et al., 1992). If radio systems
could be safeguarded against radiation from
transmitters out of the specific area of coverage of
the network, the interference could be reduced and,
as a consequence, the number of WLANs sharing
the same area may increase maintaining the required
quality standards. The proposal of this work to
perform site-shielding is the use of trees or bushes as
a barrier to attenuate signals from other networks
and, so that, to defend the own wireless system from
outer interferences. Interior plants can be used to cut
the line of sight between adjacent radio equipment
of different networks. On the other hand, decorative
trees can be used in gardens around office blocks to
reduce the outdoor coverage of wireless networks
107
Cuiñas I., Gómez P., García Sánchez M. and Vázquez Alejos A. (2008).
IMPROVEMENT OF WIRELESS NETWORK ISOLATION AND SECURITY BY SHRUB BARRIERS.
In Proceedings of the International Conference on Wireless Information Networks and Systems, pages 107-112
DOI: 10.5220/0002022101070112
Copyright
c
SciTePress
around the own building. This outdoor coverage
reduction has another advantage, as it represents an
additional protection against hacker attacks or a limit
to external users, signifying an improvement in
network security (Cuiñas et al., 2006). This proposal
is softer and more ecological than conductive
shielding, and cheaper than frequency-selective
walls as introduced at (Sung et al., 2006).
A measurement campaign has been performed at
two different wireless frequency bands: 2.4 GHz and
5.8 GHz in order to validate this application of
vegetal hurdles. Measurements are focused on
determining the attenuation induced by lines of
small trees or bushes. Seven different species have
been considered, as well as five configurations: two
linear ones, a double line and two zig-zag
dispositions, one denser than the other.
The section 2 contains the description of
measurement equipment and set-up, followed by the
procedure used to get the data, and the vegetal
species used during the experiment.
The section 3 is intended to the results obtained
in the measurement campaign, taking into account
the median values, as well as its variability and
confidence. This section is finished by the
evaluation of the improvement of the interference, in
terms of the reduction in the shortest distance
between adjacent networks to maintain the quality of
service.
Finally, section 4 contains the conclusions
extracted from these results.
2 MEASUREMENTS
Narrow band measurements have been used to
characterise the effect of the vegetal barriers in the
radio channel. The measurement setup is based on
commercial equipment used as transmitter and
receiver, in co-ordination with an automated linear
positioner. Five barrier configurations were
considered, involving elements of seven different
species.
2.1 Measurement Setup
The measurement set-up consists of separate
transmitter and receiver segments, and an automated
linear positioner, as depicted in figure 1. The
distance between transmitter and receiver, is 6
meters, and the vegetal barrier is placed just in the
middle, at 3 meters from transmitter and receiver.
The transmitter segment is built around a signal
generator Rohde-Schwarz SMR-40, which provides
pure tones in the frequencies of interest. This
generator feeds a log-periodic antenna
Electrometrics EM 6952, gaining 4.7 dB at 2.4 GHz,
and 4.8 dB at 5.8 GHz. The antenna is placed in a
fixed location, at 1.25 meters height.
The receiver segment is based on a spectrum
analyser Rohde-Schwarz FSP40, which receives the
signal captured by another log-period antenna,
identical to the transmitting one. The antenna, placed
at 1.25 meters height, is installed on a positioning
system, which consists of a 2.5 meter linear table
with a millimetre screw along it. This positioning
system is controlled from a PC, by means of an
indexer. The same software manages the operation
of the spectrum analyser, constructing an automated
measurement system.
Tran smit ter
3 m 3
m
2.5 m
Receiver
Vegetal barrier
Figure 1: Basic geometry of measurement system.
2.2 Measurement Procedure
Measurements were performed in two steps: a free
space measurement used as a reference, and a
blocked-by-trees measurement. The first of them
consisted of a free space measurement. Placing the
transmitter antenna at its position, a complete
movement along the receiver locations was done,
getting 8001 samples of received power at each
measurement point. Then, the vegetal barriers were
installed, and this measurement procedure was
repeated. This method is applied in both horizontal
and vertical co-polar situations.
Five barrier configurations were considered. The
figure 2 depicts the linear disposition (configuration
number 1), which consists of nine trees, separated 40
cm among adjacent elements. The configuration 2
contains just the central five elements in the barrier.
The configuration 3 consists of two parallel lines
of trees, each of them containing five elements. The
separation between tree lines is again 40 cm.
WINSYS 2008 - International Conference on Wireless Information Networks and Systems
108
Tran smitt er
Receiver
Figure 2: Linear disposition (configuration 1).
The configuration 4, the zig-zag, is composed of
nine trees installed in two parallel lines, separated 40
cm each, as depicted in figure 3. Finally, the
configuration 5 is like the fourth one, but doubling
the distance between adjacent trees within the same
line.
Tran smitt er
Receiver
Figure 3: Zig-zag disposition (configuration 4).
2.3 Vegetal Species
Both indoor and outdoor vegetal species have been
considered in this study, as indoor could be
interesting to reduce the interference, whereas
outdoor could be used to protect the network against
external attacks.
The indoor species analyzed in the measurement
campaign are: Heptapleurum arboricola gold
capella (commonly known as schefflera), Dypsis
lutescens (areca), and Ficus elastica (ficus). All of
them are common indoor species used frequently as
decorative elements at buildings and indoor yards.
The outdoor species have been chosen either for
been typically used to make private fences in a short
time, or because of massive foliage that favors
electromagnetic shielding. In order to analyze both
characteristics, the species considered have been:
Callistemon laevis (commonly known as callistemon
or bottlebrush), Camellia japonica (camellia),
Juniperus communis hibernica (Irish juniper) and
Thuja atrovirens (white cedar).
The table 1 summarises the sizes of the
specimens, as well as of their leaves.
Table 1: Dimensions of the shrubs in cm.
shrub leaf
Specie
height diameter length width
areca
150 70 25 1
schefflera
160 60 10 4.5
ficus
170 55 7 3
callistemon
150 80 7 4
camellia
165 90 8 6
Irish juniper
205 55 2 0.5
thuja
165 45 0.5 0.2
3 RESULTS
3.1 Attenuation
At any location, 8001 samples of received power
have been measured. Median values at any point
have been considered, in order to eliminate spurious
or outlier measured values. Then, attenuation due to
vegetal barriers has been computed by comparing
received power in free space condition to received
power in obstructed line of sight situation.
Figures 4 and 5 show the median attenuations
induced by barriers conformed by Irish junipers,
with horizontal polarisation, at 2.4 GHz and 5.8 GHz
respectively. Both figures contains the results
measured with linear and zig-zag dispositions, this
means, configurations number 1 and 4, respectively.
Figure 4: Median attenuation (dB) due to Irish junipers at
2.4 GHz, with horizontal polarisation.
IMPROVEMENT OF WIRELESS NETWORK ISOLATION AND SECURITY BY SHRUB BARRIERS
109
Figure 5: Median attenuation (dB) due to Irish junipers at
5.8 GHz, with horizontal polarisation.
The table 2 contains the median attenuations at
2.4 GHz for horizontal polarization, and the table 3
for vertical polarization. Median is the statistics used
as it is less sensitive to measured outliers than mean.
Configurations 3 and 4 provide higher attenuation
than the others.
Table 2: Median attenuation (dB) at 2.4 GHz, with
horizontal polarization.
Hurdle configuration
Specie
C1 C2 C3 C4 C5
areca
0.1 0.2 0.4 2.9 0.1
schefflera
0.4 0.8 1.6 1.9 0.7
ficus
2.2 2.8 4.3 4.7 2.7
callistemon
2.1 2.5 3.5 3.3 1.5
camellia
3.1 3.2 3.9 5.9 2.9
Irish juniper
5.2 5.2 8.9 6.2 4.5
thuja
1.6 1.5 1.8 2.1 1.4
Table 3: Median attenuation (dB) at 2.4 GHz, with vertical
polarization.
Hurdle configuration
Specie
C1 C2 C3 C4 C5
areca
3.2 3.5 2.5 2.1 3.2
schefflera
0.1 0.9 1.5 1.0 0.4
ficus
2.1 4.1 5.1 5.9 2.0
callistemon
3.3 3.0 6.9 5.4 3.0
camellia
5.4 5.5 6.9 8.2 5.2
Irish juniper
9.6 9.8 10.7 10.1 8.0
thuja
3.5 3.8 5.6 6.2 3.5
The table 4 contains the median attenuations at
5.8 GHz for horizontal polarization, and the table 5
for vertical polarization. As observed for 2.4 GHz,
configurations 3 and 4 provide stronger attenuations
than the others. This is probably due to they are
denser and more compact than configurations 1, 2
and 5. Shrubs at these configurations configure
lighter barriers.
Table 4: Median attenuation (dB) at 5.8 GHz, with
horizontal polarization.
Hurdle configuration
Specie
C1 C2 C3 C4 C5
areca
0.1 0.1 0.9 4.1 0.1
schefflera
0.1 0.2 5.6 6.7 1.0
ficus
6.2 5.4 9.3 11.3 5.3
callistemon
1.1 4.0 6.5 7.7 2.4
camellia
10.1 12.4 12.1 13.2 10.7
Irish juniper
6.8 5.9 13.2 10.6 8.1
thuja
3.9 5.1 6.3 6.8 4.7
Table 5: Median attenuation (dB) at 5.8 GHz, with vertical
polarization.
Hurdle configuration
Specie
C1 C2 C3 C4 C5
areca
2.0 2.4 5.1 3.8 2.6
schefflera
2.5 2.9 6.1 6.4 2.4
ficus
7.1 8.4 10.7 9.9 7.1
callistemon
10.5 13.0 14.5 14.6 11.1
camellia
10.4 11.3 14.2 13.5 10.5
Irish juniper
15.7 13.7 21.2 19.8 15.4
thuja
5.2 7.2 12.0 8.8 4.4
3.2 Variability
The measured attenuations present certain variability
around its first order statistics. This variability is the
result of several effects: the measurement system
noise itself, the movement of the leaves that is
translated into changes in the barrier configuration,
the differences in humidity of the plants during the
measurement period, and so on.
If the central statistic were the mean, the natural
measure of variability would be the standard
deviation. But as we used the median as the central
statistic, this variability could be evaluated as a
function of the inter quartile range (IQR) of each
measured series. The IQR corresponds to the
distance between the first and the third quartile,
indicating how much concentrating the
measurements are around the median (which is the
second quartile). The maximum IQR measured with
indoor species at the barrier resulted to be 12 dB,
whereas with outdoor species that was 6 dB.
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3.3 Application to Interference
Mitigation
The measured attenuation values could be used to
compare the influence of the hurdles in the control
of interference. The standard IEEE 802.16a defines
different minimum signal to noise ratios (SNR) to
maintain its various modulation schemes: QPSK, 16-
QAM and 64-QAM (IEEE Standard, 2003).
Table 6: Minimum SNR to maintain each modulation
scheme, IEEE 802.16a.
modulation coding rate SNR (dB) at receiver
1/2 9.4
QPSK
3/4 11.2
1/2 16.4
16-QAM
3/4 18.2
2/3 22.7
64-QAM
3/4 24.4
A WLAN element could receive signals from
other elements at its own network, and from other at
different networks. Considering the co-channel
interference as a kind of noise, and applying the
Friis equation with the limited SNR values assuming
all the network transmitters are emitting the same
power, we can compute the improvement provided
by the vegetal barrier in terms of interference
reduction: comparing the minimum distance to
assure the interference would not degrade the
performance of the network with and without the
hurdle. Thus, a reduction in this distance indicates
how much the hurdle improves the network security:
it allows installing two networks closer than without
the hurdle, and maintaining their performances.
By using the mean of the measured attenuations,
the minimum security distance, using a QPSK
scheme, is reduced from 2.95 m without barrier to
0.8 m when the hurdle is installed. If a 16-QAM
scheme is implemented, the distance reduces from
6.6 m to 1.79 m. And if a 64-QAM modulation is in
use, the distance decreases from 13.65 m to 3.69 m.
3.4 Application to Protection Against
External Attacks
The presence of the vegetal barrier provides an
additional attenuation to the propagation channel:
this means that the physical distance to be connected
to the network is shorter than when the fence is
absent. When installing such fences in the gardens
around office buildings, the coverage of the WLANs
in the surroundings is reduced, and this means that
the outdoor area from which an attack could be
shouting is compacted. Thus, this outdoor coverage
could be modelled to extend no far away the limits
of the parcel around the company, not allowing the
network access from public places as the street.
Uncontrolled accesses could be reduced compared to
the open coverage situation with the proposed
method.
4 CONCLUSIONS
The use of hurdles constructed by trees is proposed
as a method to reduce the interference among
WLANs in high-traffic areas.
Measurements of radiofrequency attenuation due
to rows of trees, done at two WLANs frequency
bands, appear to support this proposal. Results show
a reduction up to 10.7 or 21.2 dB at 2.4 and 5.8
GHz, respectively. Vegetal species susceptible to
conform the barriers have to fit more conditions that
just electromagnetic absorption: they must have
everlasting leaves, be tall enough to cut the line of
sight between transmitter and receiver, and present
densely foliated specimens.
The improvement in terms of reduction of
distance between networks to assure the interference
will not degrade their performances has been also
evaluated. The minimum security distance, using a
QPSK scheme, appears to be reduced from 2.95 m
without barrier to 0.8 m when the hurdle is installed.
If a 16-QAM scheme is implemented, the distance
reduces from 6.6 m to 1.79 m. And if a 64-QAM
modulation is in use, the distance decreases from
13.65 m to 3.69 m.
This reduction in coverage is an advantage to
control external attacks, too.
ACKNOWLEDGEMENTS
This work has been supported by Xunta de Galicia,
Project Ref. PGIDIT05TAM32201PR.
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