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

 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

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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