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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 57, NO. 8, AUGUST 2008
Experimental Study of Coexistence Issues Between
IEEE 802.11b and IEEE 802.15.4 Wireless Networks
Leopoldo Angrisani, Member, IEEE, Matteo Bertocco, Member, IEEE,
Daniele Fortin, Student Member, IEEE, and Alessandro Sona, Member, IEEE
Abstract—Coexistence issues between IEEE 802.11b wireless
communication networks and IEEE 802.15.4 wireless sensor networks, operating over the 2.4-GHz industrial, scientific, and medical band, are assessed. In particular, meaningful experiments that
are performed through a suitable testbed are presented. Such experiments involve both the physical layer, through measurements
of channel power and the SIR, and the network/transport layer,
by means of packet loss ratio estimations. Different configurations
of the testbed are considered; major characteristics, such as the
packet rate, the packet size, the SIR, and the network topology, are
varied. The purpose of this paper is to gain helpful information
and hints to efficiently face coexistence problems between such
networks and optimize their setup in some real-life conditions.
Details concerning the testbed, the measurement procedure, and
the performed experiments are provided.
Index Terms—IEEE 802.11b, IEEE 802.15.4, packet loss ratio
(PLR), signal-to-interference ratio (SIR), wireless local area network (WLAN), wireless sensor network (WSN).
I. I NTRODUCTION
W
IRELESS communication networks are receiving increasing interest in the scenario of digital communications. With respect to the wired counterpart, they avoid the need
for expensive cabled infrastructure, improve network scalability
and reconfigurability, and allow the efficient use of portable
and mobile terminals. A key issue of wireless networks is
their typical susceptibility to radio interference and the fact
that they are commonly deployed in areas that are already
crowded of radio disturbances, generated by nearby operating
electrical and electronic appliances and other wireless apparatuses. The presence of interference over the same frequency
band may even lead to disruptive effects in the transmission
of data packets among wireless stations (STs). Typical interference effects are loss of data packets, transmission delay, false
commands, false alarms, jitter, and loss of synchronization. A
critical case of interference involves IEEE 802.11b wireless
local area networks (WLANs) [1] and IEEE 802.15.4 wireless
sensor networks (WSNs) [2]. Two such networks exploit the
same frequency band, i.e., the 2.4-GHz industrial, scientific,
Manuscript received July 5, 2007; revised April 14, 2008.
L. Angrisani is with the Department of Computer Science and Control Systems, University of Naples Federico II, 80125 Naples, Italy (e-mail: angrisan@
unina.it).
M. Bertocco, D. Fortin, and A. Sona are with the Department of Information Engineering, University of Padova, 35131 Padova, Italy (e-mail: matteo.
bertocco@unipd.it; daniele.fortin@unipd.it; alessandro.sona@unipd.it).
Digital Object Identifier 10.1109/TIM.2008.925346
and medical (ISM) band, and are widely deployed in a number
of common applications in which they have to coexist in close
proximity [3]. In these cases, some interference effects should
always be expected, unless one would take into account specific
coexistence issues and adopt proper interference avoidance
solutions.
Coexistence issues between IEEE 802.15.4 WSNs and IEEE
802.11b WLANs have been studied in the past [3]–[8]. In
particular, in [3] and [4], the coexistence impact of an IEEE
802.15.4 network on IEEE 802.11b devices is analytically
investigated, and a predicting model is proposed. The model
is powerful but limited to the specific network configuration
analyzed and to the environmental conditions considered. In
[5]–[7], the interference effects of IEEE 802.15.4 networks
over IEEE 802.11b WLANs and vice versa are analyzed, both
analytically and through simulations. Simulation results are
given in terms of packet error rates, throughput, and transmission time at different distances between the two networks.
In [8] and [9], coexistence issues between wireless networks
(IEEE 802.15.4, IEEE 802.11b, and IEEE 802.15.1) are investigated through measurements. In particular, in [8], several
coexistence scenarios are analyzed, including the case of IEEE
802.15.4 and IEEE 802.11b coexisting networks. However, the
few reported results, although very interesting, do not provide
a comprehensive experimental overview of the phenomenon
under examination, which, to the best knowledge of the authors,
has yet to be reported in the literature. Helpful information
about how to arrange the testbed and to perform cross-layer
measurements is finally provided in [9] and [10], even if it does
not specifically refer to the case of an IEEE 802.11b WLAN
coexisting with an IEEE 802.15.4 WSN.
In this paper, coexistence problems between an IEEE
802.11b WLAN and an IEEE 802.15.4 WSN operating within
a real-life environment are experimentally analyzed. In particular, an original comprehensive set of experiments based
on packet loss ratio (PLR) estimates is presented, involving
the cases of a WLAN under the interference of a WSN
and a WSN under the interference of a WLAN. A final
overview of the deduced coexistence conditions is also provided. Stemming from the past experience documented in [11],
the purpose is to deduce helpful information and hints for
designers and technicians, which are to be efficiently used
to optimize the network design and setup in common reallife coexistence conditions. A number of experiments are
conducted through a proper testbed, and a measurement approach that is similar to that proposed in [9] and [10] is
followed.
0018-9456/$25.00 © 2008 IEEE
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ANGRISANI et al.: COEXISTENCE ISSUES BETWEEN IEEE 802.11b AND IEEE 802.15.4 WIRELESS NETWORKS
Fig. 1.
WLAN frequency channels.
II. N OTES F ROM S TANDARDS
A. IEEE 802.11b WLAN
A WLAN is generally a set of computing devices and access
points (APs) communicating with each other through radio
waves and over common frequency channels [1], [12]. The
IEEE 802.11b standard defines a total of 14 frequency channels,
each of which characterized by a 22-MHz bandwidth (Fig. 1).
In the USA, channels 1–11 are allowed by the Federal Communications Commission, whereas in Europe, channels 1–13 can
be used. In Japan, one choice is allowed—channel 14.
Fig. 1 shows that only three channels at a time (e.g., 1, 6,
and 11) can be used without overlap. In the design of a WLAN,
the assigned channel and spatial position of each AP should be
fixed in such a way as to avoid frequency overlap over the whole
coverage area.
In a typical communication, an AP periodically transmits
a frame called beacon, which contains information such as a
network identifier (ID), the beacon and channel parameters,
and other traffic information. Each network ST receives the
beacon, and if it is aimed at accessing the network, it sends a
request for authentication. A series of tests is then performed to
ensure the identity of the ST. Once authenticated, an ST may
communicate to the AP or vice versa according to a carrier
sense multiple access with collision avoidance (CSMA/CA)
protocol. In particular, an ST (or the AP) wishing to transmit
senses the channel, and, if no activity is detected, it waits for an
additional randomly defined back-off period and then transmits
if the medium is still free. If the packet is received without
corruption, the AP issues an acknowledge (ACK) frame that
completes the process. If the ST does not detect the ACK
frame within a fixed timeout period, a collision is assumed, and
the data packet is retransmitted after another random back-off
period. Usually, to assess the channel status, the clear channel
assessment (CCA) technique in mode 1 is implemented [1].
In this mode, the channel is assumed as idle if the channel
power level is below a given user selectable threshold, which is
called the CCA threshold; otherwise, it is assumed to be busy.
A schematic representation of the described communication
protocol is shown in Fig. 2(a).
A packet fragmentation is performed before the transmission,
which allows breaking of large packets into smaller parts of
length d, which is useful in disturbed environments since large
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Fig. 2. Time diagrams of (a) WLAN and (b) IEEE 802.15.4 frames.
packets can be more easily corrupted. Two overhead frames are
then added to each data packet—a header h, which contains information such as the ST identifier number, the packet sequence
number, the sender and receiver addresses, and the total packet
length (d + h + c); and a control frame c, which allows errors
to be handled. In Fig. 2(a), the time interval covered by the
packet is denoted as τwlan , whereas the time interval between
two packets is denoted as Tpck . The two delays td1 and td2
between packets and the occurrence of ACK vary from one
packet to another, mostly depending on channel characteristics,
network topology, and back-off time. To indicate the percentage
of Tpck that is specifically dedicated to the transmission of the
whole packet (d + h + c), the duty cycle λwlan is commonly
introduced [9]. It is defined as follows: λwlan = τwlan /Tpck .
B. IEEE 802.15.4 WSN
IEEE 802.15.4 is a standard designed for a low-rate wireless
personal area network (PAN) characterized by limited coverage
area, reduced cost, and low energy consumption. It is one
of the most promising technologies for implementing WSNs
[2]. A variety of network topologies is known; an interesting
one is the star configuration, where a PAN coordinator (the
master) cyclically queries a set of slaves (sensor nodes), one
by one, and slaves reply by transmitting a short data packet
(usually tens of bytes) containing the information acquired by
the associated sensor. The transmission between the master and
the slaves exploits the 2.4-GHz ISM band at 250 kb/s and,
in particular, one of the 16 available channels. As shown in
Fig. 3, the channels have a 3-MHz bandwidth and are uniformly
distributed within the ISM band. In Fig. 3, three nonoverlapped
WLAN channels are also shown to highlight that WSNs and
WLANs can occupy the same frequencies and that the hazard of
in-channel interference is very high whenever a WSN operates
in the nearness of one or more active WLANs.
The master dedicates a time interval, called the polling
window, of fixed time length Tpoll to each sensor node. As
schematically represented in Fig. 2(b), the master starts waiting
a back-off period td1 and then senses the air according to the
CSMA/CA protocol. If the channel is free, it transmits a frame
(query) q throughout the network, containing information such
as the destination node address, the sequence number of the
performed cycle, and the time stamps. After a pre-fixed short
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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 57, NO. 8, AUGUST 2008
Fig. 3. Frequency channels peculiar to (a) IEEE 802.15.4 and (b) IEEE
802.11.
Fig. 4. Adopted testbed.
delay td2 , the queried sensor node replies with an ACK and
waits for a second back-off period td3 . It then senses the air
and, if it is free, transmits the data packet d with a header frame
h and a tail field c. In the case of correct reception, the master
issues an ACK, and at the end of the polling window, it passes
to the subsequent sensor node. If the sensor does not receive the
ACK from the master, it waits for another back-off period and
then retries to transmit. At the expiration of the polling window,
if the ACK from the master has yet to be issued, the packet from
that queried sensor is considered lost.
III. T ESTBED
Experiments have been carried out by using the testbed
sketched in Fig. 4, which consists of an IEEE 802.15.4 WSN
and an IEEE 802.11b WLAN operating in close proximity.
The WLAN is simply realized through a standard AP and an
ST placed at a distance d = 13 m from it. Although limited to
only two terminals, the configuration can be considered general
and representative of situations with more than two WLAN
STs. In fact, according to the CSMA/CA mechanism, only
one WLAN ST may transmit at a time and, thus, interfere
with the WSN regardless of the number of WLAN nodes
that are involved. The AP is a D-link 524 device connected
to a PC via a wired link, whereas the ST is a notebook
Toshiba Satellite (Intel Centrino 1.60 GHz) with an Intel
Pro/Wireless integrated WLAN transceiver module. Traffic is
generated through the tool Distributed Internet Traffic Generator (D-ITG) [13], which allows the setting of the transport
layer protocol (Transmission Control Protocol, User Datagram
Protocol, Internet Control Message Protocol) and application
layer functionality—Domain Name System, Telnet, voice, or
custom. In the custom mode, the packet rate, the packet size,
and the interdeparture time statistical distribution can be varied
and set according to the desired configuration. Also, it allows
measurements of quality-of-service parameters, such as the
packet rate, the PLR, the packet average delay, and the packet
jitter, both at the receiver and transmitter sides.
The IEEE 802.15.4 network is a set of TmoteSky sensor
nodes (motes), available from Moteiv, randomly distributed
inside a circumference of radius r = 2 m and centered around
the ST. The motes operate according to the general-purpose
high-layer protocol based on a master–slave relationship. The
network includes one master M and N = 10 slaves, each
of which is equipped with the following basic elements:
1) one microprocessor; 2) one or more analog input sections,
which are to be interfaced with one or more sensing elements;
3) a buffer for acquired data; and 4) a radio communication
module. M is connected to a PC via a wired link for storing the
collected data and postprocessing. As described in Section II,
it executes periodical polling of each slave for receiving data
from monitoring sensors. All transmissions are handled by the
medium access control layer, which allows the radio channel
to be accessed and the data packets to be retransmitted in case
of losses or missing acknowledgments. Traffic is generated and
monitored through the tool WSN [14], which allows the setup
of the master and slaves and the display of the data collected by
the sensors.
Coexistence analysis has been performed in the following
two scenarios.
1) WLAN is a victim of WSN interference.
2) WSN is a victim of WLAN interference.
In both scenarios, the two networks have been forced to operate
on overlapped frequency channels, i.e., channel 11 for the
WLAN and channel 22 for the WSN.
Measurements have been conducted both at physical and
network/transport layers [9], [10]. In particular, at the physical
layer, the channel power and the SIR have been estimated
by using a National Instrument PXI 5660 module RF signal
analyzer and a receiving antenna Seibersdorf PCD 8250 biconical dipole. At the network/transport layer, the PLR, i.e., the
ratio between the number of lost packets and the number of
transmitted packets, has been estimated by using D-ITG [13]
and the WSN tool [14], running on the two PCs.
Different configurations of WSN and WLAN parameters
have been considered. In particular, two values of Tpoll have
been taken into account—Tpoll = 30 ms, which is the minimum
allowed value, below which the number of lost packets in the
absence of interference abruptly worsens (about 100% [15],
[16]), and Tpoll = 100 ms.
In the two analyzed scenarios, a different meaning has been
given to the SIR. In scenario 1, SIR = Pwlan /Pwsn , where
Pwlan is the WLAN useful power, and Pwsn is the WSN
interference power, both measured over the WLAN channel
and at the space position occupied by the ST. In scenario 2,
SIR = Pwsn /Pwlan , where Pwsn and Pwlan are measured over
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ANGRISANI et al.: COEXISTENCE ISSUES BETWEEN IEEE 802.11b AND IEEE 802.15.4 WIRELESS NETWORKS
the WSN channel and by averaging the power readings
achieved at any sensor node position.
For the WSN and the WLAN, experiments have been executed in CCA mode 1, with a CCA threshold level Pth equal
to −76 dBm (the default value). Moreover, measurements have
been performed inside a nonshielded and nonanechoic room,
hence, with the potential occurrence of external interference
and collateral phenomena such as reflections from near metallic objects and structures. To also properly account for this
phenomenon, different positions of sensor nodes have been
considered.
IV. E XPERIMENTS IN THE C ASE OF THE WLAN V ICTIM
The WLAN behavior under the effect of WSN interference
has been investigated both in the case of the AP transmitting to
the ST and vice versa.
A. Transmission From the AP to the ST
The conducted experiments have aimed to assess the behavior of the WLAN under the effect of the WSN when
the ST (located in the proximity to the WSN) receives data
packets from the AP (located far way from the WSN). This
configuration has emulated the typical situation in which the
WLAN transmitter (here, the AP) senses an interference power
level (from the WSN) below its CSMA/CA threshold, i.e., the
WLAN channel is sensed free, thus enabling the transmission.
The first experiments have been conducted measuring the
PLR of the WLAN, i.e., PLRwlan , in the case of SIR =
−5.6 dB, for different values of the WLAN duty cycle λwlan
and the duration of the transmission window equal to 60 s.
PLRwlan has been estimated as the ratio PLRwlan = 1 −
Nrp /Ntp , where Nrp is the number of correctly received
packets, and Ntp is the number of packets delivered by the
WLAN transmitter (here, the AP) in ideal conditions (i.e., in
the absence of interference). The quantity Ntp − Nrp can, in
turn, be considered as the sum of two contributions, i.e., Ntp −
Nrp = Nerp + Nntp , where Nerp is the number of packets
erroneously gathered by the WLAN receiver (here, the ST) due
to signal collisions with WSN packets and/or channel errors,
whereas Nntp is the number of packets not delivered by the
WLAN transmitter (here, the AP) due to channel occupation.
According to the above assumption about the AP location with
respect to the WSN position, Nntp is expected to be null in the
analyzed configuration.
The obtained results are summarized in Fig. 5, in which three
diagrams are shown, where each referred to a specific size of
WLAN packets pswlan —600 B [Fig. 5(a)], 1112 B [Fig. 5(b)],
and 1563 B [Fig. 5(c)], with a payload of 512, 1024, and
1475 B, respectively. Different operative conditions of the
testbed have been analyzed—WSN off and WSN on with
Tpoll = 30 ms and Tpoll = 100 ms. The solid lines represent
the PLRwlan estimates, whereas the dashed lines give the percentage of packets that are not transmitted by the AP due to
channel occupation, i.e., Nntp /Ntp . Vertical bars also indicate
the standard deviation experimented under different positions
of the sensors.
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From the analysis of the results, the following conclusions
can be drawn.
• In Fig. 5(a) and (b), as expected, the percentage of nottransmitted packets Nntp /Ntp is always null, irrespective of
the chosen λwlan . This confirms that the AP always finds
the channel free, and it can transmit without interruption.
• In Fig. 5(c), Nntp /Ntp is not always null and increases
upon the increase of λwlan . This means that despite the fact
that the channel is always assessed free, the AP encounters
problems when managing very long packet sizes.
• PLRwlan assumes quite the same values for pswlan =
600 and 1112 B, whereas much higher losses are experienced with pswlan = 1563 B. For instance, with λwlan =
75%, WSN on, and Tpoll = 30 ms, the resulting PLRwlan
is 15%, 10%, and 100%, respectively.
• In the absence of WSN interference, nonnegligible degradation of WLAN performance is visible for any considered
λwlan . In particular, PLRwlan is constant and nearly equal
to 5% up to a given threshold level of λwlan and is equal
to 75%, 75%, and 53% for Fig. 5(a)–(c), respectively,
whereas it increases even abruptly [see Fig. 5(c)] for
higher values.
• In the presence of WSN interference and Tpoll = 30 ms,
WLAN performance degrades further. This is particularly
true in the case of pswlan = 1563 B and λwlan ≤ 53%
for which PLRwlan changes from nearly 5% (without
interference) up to 70% (for λwlan = 53%).
• In the presence of WSN interference, the threshold values of λwlan , beyond which PLRwlan increases, reduce
from 75%, 75%, and 53% to 65%, 60%, and 30% for
Fig. 5(a)–(c), respectively.
• With a longer Tpoll of 100 ms, the effect of the WSN on
the WLAN is much weaker. In fact, with Tpoll = 100 ms,
the estimated PLRwlan values are very close to the results
achieved with WSN off.
• The dispersion of PLRwlan estimates around the corresponding mean values is almost always negligible. The
only exception is given in Fig. 5(c), with WLAN on and
Tpoll = 30 ms.
Other experiments have been carried out with different levels
of the SIR from −13 up to −3 dB, two values of λwlan =
56% and 75%, two values of Tpoll = 30 and 100 ms, and
pswsn = 1112 B. The SIR has been changed by acting only on
Pwlan . The obtained results are summarized in Fig. 6, and the
following considerations arise.
• In the transition range −13 ≤ SIR ≤ −7 dB, PLRwlan
abruptly decreases from 100% to very low levels, i.e.,
below 13%, upon the increase of the SIR.
• In the same range, the effect of Tpoll on PLRwlan is clearly
visible; stretching Tpoll from 30 to 100 ms, PLRwlan
considerably decreases. For instance, with SIR = −11 dB
and λwlan = 56%, PLRwlan is lowered from 34% to 10%
by increasing Tpoll from 30 to 100 ms.
• In the upper range, i.e., SIR > −7 dB, the effect of Tpoll
and λwlan on PLRwlan is negligible. In fact, the same values of PLRwlan , within 5%–12%, are achieved, regardless
of the chosen Tpoll and λwlan .
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Fig. 5. PLR of the WLAN versus λwlan for different packet sizes. (a) 600 B. (b) 1112 B. (c) 1563 B.
• For levels of the SIR below −13 dB, WLAN packets
are completely lost, i.e., PLRwlan = 100%, regardless of
the chosen value of λwlan and Tpoll . This is due to the
reduced level of Pwlan that, for the considered SIR values,
is comparable with or lower than the sensitivity level
of the ST, which is typically on the order of −84 dBm
(in the case of data rate = 11 Mb/s).
• The experimental standard deviation is almost always negligible, i.e., below 10%, with the exception of the highest
PLRwlan .
B. Transmission From the ST to the AP
The conducted experiments have aimed at assessing the
behavior of the WLAN under the effect of the WSN interference when the ST (located in the proximity to the WSN)
is transmitting data packets to the AP (located far way from
the WSN). This configuration has emulated the typical case in
which the WLAN transmitter (here, the ST) senses a high level
of the WSN signal beyond its CCA threshold Pth . In this case,
the presence of the WSN signal saturates the WLAN channel,
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ANGRISANI et al.: COEXISTENCE ISSUES BETWEEN IEEE 802.11b AND IEEE 802.15.4 WIRELESS NETWORKS
Fig. 6.
PLR of the WLAN versus the SIR for different λwlan .
Fig. 7.
PLRwlan versus λwlan for a packet of 1112 B.
and the ST, according to the CSMA/CA protocol, is forced to
defer the transmission. Consequently, data packets are delivered
from the ST to the AP with an amount of delay, and increased
values of PLRwlan can be experimented with.
The same procedure described in Section IV-A has been
applied in a number of experiments, in which the only case of
pswlan = 1112 B has been considered. The obtained results are
summarized in Fig. 7, and the following considerations from
their analysis are of concern.
• The percentage of not-transmitted packets Nntp /Ntp , represented by dashed lines in the figure, is not always null
and increases upon the increase of λwlan .
• The three dashed lines completely overlap the corresponding solid lines that are obtained with the same value of
Tpoll and with WSN on/off, i.e., PLRwlan = Nntp /Ntp for
any considered λwlan .
• PLRwlan increases significantly for higher λwlan values.
For instance, with λwlan = 87%, PLRwlan is nearly 27%
and 31% for Tpoll = 100 and 30 ms, respectively.
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• With respect to Fig. 5(b), quite the same values of PLRwlan
have been obtained in the presence of interference.
C. Comments and Hints
Relevant comments and hints deriving from the obtained
results are given in the following.
1) Both with and without WSN interference, the WLAN
PLR strictly depends on λwlan and increases for λwlan
values beyond a given threshold. It is advisable to set
λwlan below such a threshold, which represents a key
parameter in the design stage, to be accurately known
and, if necessary, measured, as shown above.
2) Both with and without WSN interference, the WLAN
PLR also depends on the packet size. In particular,
whereas very similar results have been achieved with
pswlan = 600 and 1112 B, much higher values have been
experienced with pswlan = 1563 B and for λwlan > 55%.
The use of a packet size of 1112 B is suggested; it
allows the improvement of PLRwlan in the presence of
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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 57, NO. 8, AUGUST 2008
Fig. 8. PLRwsn versus λwlan for different WLAN packet sizes—600, 1112, and 1563 B—and Tpoll = 30 ms.
3)
4)
5)
6)
7)
interference with respect to pswlan = 1563 B and higher
throughput with respect to pswlan = 600 B. This result
agrees with the values of the packet length obtained by
simulations in [6] for the same case of an IEEE 802.11b
WLAN under the interference of an IEEE 802.15.4 WSN.
The WLAN PLR also strictly depends on the SIR. In particular, PLRwlan may abruptly worsen for SIR values that
are below a given threshold, whereas for upper values,
it assumes low levels and becomes quite independent of
WSN characteristics such as polling time. The SIR should
be always greater than this threshold, which can, in turn,
be efficiently measured as described.
For the deployed testbed, the effects of WSN interference,
in terms the WLAN PLR, are the same, regardless of the
communication direction between the AP and the ST.
A different interference phenomenon arises depending on
the case of the AP transmitting to the ST or vice versa.
In particular, in the case of the AP transmitting to the
ST, i.e., when the WLAN source is located far way from
the WSN, interference comes out as collisions between
WSN and WLAN data packets and/or channel errors.
Conversely, in the case of the ST transmitting to the AP,
i.e., when the WLAN source is closely located to the
WSN, interference comes out as channel occupation due
to the presence of WSN emissions in the WLAN channel
and the use, in the WLAN transmitter, of the CSMA/CA
mechanism.
The increase in WSN polling time from 30 to 100 ms allows the mitigation of the interference effects of the WSN
on the WLAN, particularly for λwlan and SIR values
corresponding to when the estimated PLRwlan is greater.
In all the analyzed cases, very short standard deviations
have been experienced. External interference or collateral
phenomena, such as reflections, have not introduced significant perturbations on conducted measurements.
V. E XPERIMENTS IN THE C ASE OF THE WSN V ICTIM
A second set of experiments has been carried out with the
WSN victim and the WLAN interferer. Only the configuration
of the AP transmitting to the ST has been considered. It allows a
better investigation into the behavior of the WSN under WLAN
interference, avoiding the effects of the WSN on the farawaylocated AP. In fact, because of the low power level received
by the AP from the WSN, i.e., below its CCA threshold, the
AP always finds the channel free, and consequently, it can continuously transmit, interfering the WSN without interruption.
This is not true in the other case, i.e., the ST transmitting to
the AP, because of the near operation of WSN nodes, which
periodically require accessing the same channel, hence with the
hazard of disturbing the ST itself.
A. Results
Measurements have been conducted with three different
WLAN packet sizes—pswlan = 600, 1112, and 1563 B—at the
same level of sensor node output power Pmote = 0 dBm
and Tpoll = 30 ms, which guarantee, in the absence of radio interference, a null WSN PLR (denoted here as PLRwsn )
[15]. The first experiments have been carried out with SIR =
−3 dB and Pwlan greater than the CCA threshold of WSN
motes Pth .
In Fig. 8, the obtained results in terms of the WSN PLR,
i.e., PLRwsn , upon varying the WLAN duty cycle λwlan are
summarized. The diagram clearly shows that, in the presence
of WLAN interference, PLRwsn abruptly degrades from 0%
(with WLAN off) up to values that are always greater than
50% for λwlan beyond 40%. Moreover, PLRwsn increases quite
linearly and quickly toward a saturation level comprised within
75%–85%. In these cases, the probability for a WSN to find the
channel free, hence to begin transmission, is very low, and it
worsens for shorter WLAN packet sizes. This phenomenon is
clearly visible for values of λw up to 35% and is simply due to
the fact that, for a given λw , a shorter time interval between two
consecutive WLAN packets (Tpck − τw ) is available for WSN
transmission.
To avoid such critical degradation of WSN performance in
the presence of WLAN interference, an efficient solution has
consisted of increasing Tpoll , for instance, from 30 to 100 ms.
Some tests have been performed with the suggested solution,
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ANGRISANI et al.: COEXISTENCE ISSUES BETWEEN IEEE 802.11b AND IEEE 802.15.4 WIRELESS NETWORKS
Fig. 9.
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PLRwsn versus λwlan for two values of Tpoll —30 and 100 ms.
Fig. 10. PLR of the WSN versus the SIR for different polling window values—30 and 100 ms.
and the obtained results are summarized in Fig. 9; only one
packet size, i.e., 1112 B, and two values of Tpoll , i.e., 30 and
100 ms, have been taken into account.
Fig. 9 highlights that in the case of Tpoll = 100 ms, reduced values of PLRwsn (below 10%) have been achieved with
λwlan ≤ 35%.
Further results are given in Fig. 10, which are obtained with
psw = 1110 B, with λwlan = 60%, and upon varying the value
of the SIR. The diagram shows that PLRwsn proportionally
decreases upon the increase of the SIR, which is always below
a given threshold. Such a threshold value is nearly 16 and 8 dB
for Tpoll = 30 and 100 ms, respectively. Beyond the threshold,
PLRwsn assumes quite constant values, i.e., nearly 10% and 0%
for Tpoll = 30 and 100 ms, respectively.
2) Interference effects of a WLAN on a WSN can be
mitigated increasing Tpoll and reducing λwlan below a
suitable threshold. Therefore, a WSN may coexist with
a near-operating WLAN but to the detriment of the
throughput of both the WLAN (lower λwlan ) and the
WSN (higher Tpoll ).
3) The maximum λwlan and the minimum Tpoll to guarantee
the desired levels of PLRwlan are essential parameters to
be taken into account in a design stage. They can be determined through measurements such as those described
above.
4) PLRwsn strictly depends on the SIR, which should not
be lower than a given threshold. Such a threshold varies
upon varying Tpoll and can be experimentally determined,
as shown above.
B. Comments and Hints
Relevant comments and hints deriving from the obtained
results are given in the following.
1) A WLAN may considerably degrade the performance of a
nearby operating WSN, increasing the values of PLRwsn ,
even up to 80%.
VI. C OEXISTENCE I SSUES
The obtained results have underlined the importance of
the two parameters pswlan and λwlan in the optimization of
the performance of both IEEE 802.15.4 and IEEE 802.11
networks when operating in the same environment and
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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 57, NO. 8, AUGUST 2008
Fig. 11. (Dashed lines) PLRwsn and (solid lines) PLRwlan for a different setup of a WLAN.
sharing the same frequencies. Their choice can be simplified
and optimized by using comprehensive diagrams such as that
in Fig. 11, which is obtained by gathering the results of
Figs. 5 and 8.
The diagram shows two sets of curves—the dashed ones give
the values of pswlan and λwlan that guarantee a given level
of PLRwsn (indicated through vertically oriented numbers),
whereas the solid ones represent the values of pswlan and λwlan
that guarantee a given level of PLRwlan (indicated through
horizontal-oriented numbers). The two sets of curves refer to
Tpoll = 30 ms.
The diagram fixes the couple of pswlan and λwlan values
allowing the increase in the WLAN throughput (which can be
pursed increasing pswlan and/or λwlan ) and the minimization of
PLRwsn and PLRwlan . For instance, with a maximum PLRwsn
of 24.4% and a maximum PLRwlan of 9.0%, the diagram
shows that the two parameters pswlan and λwlan must be in
the region on the left of the curve with PLRwsn = 24.4%,
which means that a very low λwlan must be chosen, and
consequently, low WLAN throughput is obtained. Instead, in
the case of a maximum PLRwsn of 79.0% and a maximum
PLRwlan of 9.0%, a much greater region of pswlan and λwlan
values is available. In this case, it can be noted that to increase the WLAN throughput, a solution is to reduce the
WLAN packet size, which allows greater values of λwlan
to be set.
In general, the graph confirms that the predominant effect
of interference is from the WLAN to the WSN. In fact, with
pswlan ≤ 1112 B, all the PLRwsn curves are on the left with
respect to the corresponding ones (having the same PLR level)
of PLRwlan . Therefore, in this case, for any chosen λwlan ,
PLRwlan PLRwsn .
VII. C ONCLUSION
The experimental analysis has confirmed that the WLAN
and the IEEE 802.15.4 WSNs may coexist when operating
in close proximity but to the detriment of some performance
parameters such as the PLR and the throughput. In the case
of the WLAN victim, the WLAN duty cycle must always be
set lower than a given threshold value, which depends on the
WLAN packet size, decreases in the case of WSN interference,
and can be determined as described above. Moreover, in the
presence of WSN interference, the WLAN PLR does not significantly depend on the number and the position of WSN nodes
inside a fixed circumference that is centered around the WLAN
receiver.
In the opposite case, i.e., the WSN victim, much more visible
loss of WSN packets arising when the WLAN transmits has
been observed. In particular, at the beginning of the WLAN operation, the PLR abruptly increases from 0% to values beyond
70%. In this case, to enhance the WSN reliability, a solution
may be either to increase the time duration of the polling
window in such a way as to have more possibilities to correctly
retransmit or to reduce the WLAN duty cycle. Unfortunately,
this solution causes lower throughput of the WSN, if a longer
polling window is set, or of the WLAN, if the duty cycle is
reduced.
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Nov. 2007.
Leopoldo Angrisani (M’08) was born in Nocera
Superiore, SA, Italy, on April 16, 1969. He received
the M.S. degree (cum laude) in electronic engineering from the University of Salerno, Salerno, Italy, in
1993 and the Ph.D. degree in electrical engineering
from the University of Napoli Federico II, Napoli,
Italy, in 1997.
Since 2002, he has been an Associate Professor
with the Dipartimento di Informatica e Sistemistica,
Università degli Studi di Napoli Federico II. His
research activity is focused on typical topics of
electrical and electronic measurements and, in particular, on the definition
and the implementation of new digital signal processing methods for the
performance assessment of telecommunication systems and apparatuses; the
definition and the implementation of new methods for communication network
test and measurement; the definition and the implementation of new digital
signal processing methods for the detection, measurement, and classification
of nonstationary signals; design, realization, and metrological characterization
of automatic measurement instruments based on digital signal processors; and
the definition and the implementation of alternative methods for uncertainty
evaluation in indirect measurements.
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Matteo Bertocco (M’92) was born in Padova, Italy,
in 1962. He received the Laurea degree and the
Ph.D. degree from the University of Padova, Padova,
both in electronics engineering in 1987 and 1991,
respectively.
In 1994, he joined the Department of Electronics
and Informatics, University of Padova, as a Researcher, where he became an Associate Professor
with the Electronic Instrumentation and Measurement in 1998 and, since 2005, has been a Full
Professor. He has taken part in a number of national
and international research projects, including the European Union fourth and
fifth framework programs. He is the author of over 140 scientific papers
and textbooks for academic courses. His research interests include digital
signal processing, estimation, virtual systems, automated instrumentation, and
electromagnetic compatibility.
Daniele Fortin (S’07) was born in Padova, Italy,
in 1977. He received the Laurea degree in telecommunications engineering and the Ph.D. degree in
information engineering, with a course in bioelectromagnetism and electromagnetic compatibility, from
the University of Padova in 2004 and 2008, respectively.
He is currently with the University of Padova. His
research interests include wireless communications
(DVB, WLANs, and WSNs), which are focused
on measurement methodologies for the performance
assessment of digital communication systems.
Alessandro Sona (M’05) received the Laurea degree (summa cum laude) in electronics engineering
from the University of Padova, Padova, Italy, in
1999 and the Ph.D. degree in electronic instrumentation from the University of Brescia, Brescia, Italy,
in 2002.
Since then, he has been with the Department of Information Engineering, University of Padova, where
he is currently an Assistant Professor of instrumentation and measurement. His main interests are in the
field of measurement uncertainty, electromagnetic
compatibility, and wireless communication systems and networks. In particular,
his research is focused on new measurement techniques and tools for the
analysis of electromagnetic compatibility phenomena and on performance
evaluation of digital communication systems in the presence of interference.
He is currently responsible for several internal projects on electromagnetic
compatibility, digital communications, and sensor networks.
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