COEXISTENCE OF DIFFERENT WIRELESS SENSOR

NETWORKS

MAC Protocol Interference between X-MAC and Low Power Probing

Sven Zacharias, Thomas Newe, Sinead O’Keeffe and Elfed Lewis

Electronic and Computer Engineering, University of Limerick, Limerick, Ireland

Keywords: Wireless Sensor Network, WSN, Medium Access Control, MAC, Coexistence, Channel Interference,

Competition, Interoperability.

Abstract: Wireless Sensor Networks (WSNs) are an emerging technology that will be widely deployed in the near

future. Most WSNs operate on the 2.4 GHz band of the three free ISM frequency bands. The 2.4 GHz

frequency band is already used by different wireless systems. With an increasing number of WSNs, the

scenario of different WSNs operating on the same IEEE 802.15.4 frequency channel becomes more likely.

WSN Medium Access Control (MAC) Protocols used today were not designed with this problem in mind.

To date, the research focused on interference on the Physical Layer. This work analyses the jamming

potential and the robustness of MAC Protocols, namely X-MAC and Low Power Probing (LPP), at the level

of inter-network competition for medium access, when multiple WSNs are in range of each other operating

on the same channel. The following parameters have been investigated and their effect on interference is

shown: sampling time, channel check rate and payload.

1 INTRODUCTION

A Wireless Sensor Networks (WSNs) consists of

many, theoretically up to thousands of sensor nodes.

A single sensor node, called a mote, is a small and

inexpensive device that is built from the following

main parts: one or more sensors, a data processing

unit, a wireless communication interface, and an

energy source. Today’s most suitable wireless

transfer technologies for WSNs are based on the

Institute of Electrical and Electronics Engineers

(IEEE) 802.15.4 standard (IEEE, 2003), since it

provides a simple, low-power stack for the Physical

and Data Link Layers.

2 PHYSICAL LAYER

The IEEE 802.15.4 standard can physically operate

on the three free Industrial, Scientific and Medical

(ISM) frequency bands offering 27 channels: one at

868 MHz, ten in the 915 MHz band and 16 in the 2.4

GHz band. The only frequency band available

worldwide is at 2.4 GHz, which is the most used

ISM band utilised by many technologies and

therefore the band is crowded (Zhou et al., 2006).

Possible sources of interference in the 2.4 GHz

band can be the common microwave oven and

harmonics of monitors. However, as revealed by a

technical report of the Jennic Cooperation (Jennic,

2008) investigating the effects of different

Figure 1: Channels in the ISM 2.4 GHz base band used by IEEE 802.11b and IEEE 802.15.4. Bold channels are non-

overlapping channels that are normally used. Do not scale spectral mask or output power from this drawing.

197

Zacharias S., Newe T., O’Keeffe S. and Lewis E..

COEXISTENCE OF DIFFERENT WIRELESS SENSOR NETWORKS - MAC Protocol Interference between X-MAC and Low Power Probing.

DOI: 10.5220/0003829601970202

In Proceedings of the 1st International Conference on Sensor Networks (SENSORNETS-2012), pages 197-202

ISBN: 978-989-8565-01-3

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

interference sources, Wireless Local Area Networks

(WLANs) are the main source of interference. The

interference of WLANs based on the IEEE 802.11b

(IEEE, 2007) and the newer IEEE 802.11n (IEEE,

2009) standard on IEEE 802.15.4 have been studied

in detail (Yang et al., 2011); (Petrova et al., 2007);

(Bello and Toscano, 2009).

In order to minimise the risk of interference on

WSNs, a WSN channel outside the band of the used

WLAN channels is normally chosen, as shown for

IEEE 802.11b in Figure 1. But Petrova et al. (2007)

report that even outside of the used WLAN

channels, IEEE 802.11n interferes with WSNs.

Due to WLANs and other external factors, the

choice of non-interfered WSN channels is often

limited to four or less. The default, pre-set channel

in TinyOS and ContikiOS is 26. Hence it is likely

that many WSNs operate on channel 26.

Trends like the Internet of Things and Machine

to Machine communication will lead to many

embedded wireless networks in the near future. So it

becomes more and more likely that there will be a

scenario of two WSNs operating on the same IEEE

802.15.4 channel and thus using the same Physical

Layer (maybe even the same radio chip), but

probably a different Data Link Layer.

3 MEDIUM ACCESS CONTROL

(MAC) SUB-LAYER

Since a network consists of many participants, but

the radio channel can only be used by a single

participant at any time, the task of the MAC Sub-

Layer is to avoid two or more nodes trying to

transmit at the same time (packet collisions).

In classical networks, all transfer modules are

always turned on due to the fact that energy is not

constrained, thus the communication medium can be

monitored at all times. This behaviour leads to the

simplest form of MAC Protocol, called Carrier

Sense Multiple Access (CSMA). The potential

sender listens in order to determine if the channel is

used by another device for transmitting. This check

is called Clear Channel Assessment (CCA). If the

channel is idle, the sender transmits its own

message. If the channel is used, the potential sender

performs a backoff algorithm, which means it waits

for a random time and retries. An extension of this

behaviour often used in WLANs is Carrier Sense

Multiple Access/Collision Avoidance (CSMA/CA)

with handshake. This handshake uses Request To

Send (RTS) and Clear To Send (CTS) messages

before each data transfer. It adds additional overhead

but takes into account that the receiving node can be

blocked by a device outside of the sender’s

communication range (Hidden Terminal Problem).

ContikiOS, which is the operating system used

for the following experiments, includes two network

stacks: uIP and Rime (Dunkels et al., 2007). uIP is a

small TCP/IP stack and Rime is a lightweight

communication stack designed for low power radios

used in WSNs. This work uses the Rime stack. WSN

MAC Protocols have to save energy, which is

realised by turning the radio unit off most of the

time. The channel check rate determines how often

the radio is switched on. In ContikiOS, the term

Radio Duty Cycling (RDC) Layer is used for the

lower part of the MAC Layer that manages the sleep

times of the radio. The MAC Layer Protocol in

ContikiOS can provide retransmissions (CSMA)

when the RDC Layer indicates a collision.

3.1 Radio Duty Cycling (RDC)

A huge number of RDC/MAC Protocols have been

published. Summaries and comparisons of the most

commonly used ones can be found in the literature

(Demirkol et al., 2006); (Kredo II and Mohapatra,

2007); (Roy and Sarma, 2010). A possible taxonomy

for MAC Protocols is:

 Unscheduled

o Push, Sender-initiated, e.g. Low Power

Listening (LPL), X-MAC

o Pull, Receiver-initiated, e.g. Low Power

Probing (LPP)

 Scheduled

o Time Division Multiple Access (TDMA)

Scheduled approaches have many advantages, but

they are mainly optimised for high traffic and

therefore adapt poorly to network changes and need

synchronisation (Cionca et al., 2008). Hence,

unscheduled protocols are used for dynamic WSNs.

In the following, the two unscheduled approaches

Push and Pull are studied in further detail by means

of X-MAC and LPP. Like most protocols, they have

been designed for stand-alone usage, so that both

coexistence with other protocols and competition for

the medium have not been considered yet.

3.1.1 X-MAC

X-MAC (Buettner et al., 2006) is a pushing protocol

based on the Low Power Listening (LPL) approach

(Moss et al., 2007), where the nodes turn off their

radios for most of the time. If a node is about to

send, it turns on its radio and sends short preambles,

called strobes, until it receives an acknowledgement;

SENSORNETS 2012 - International Conference on Sensor Networks

198

then the message is sent. Non-sending nodes wake

up for a short listening period after the sleep time in

order to monitor the channel for strobes. Due to this

behaviour, the idle listening time is reduced.

3.1.2 Low Power Probing

Low-Power Probing (LPP) (Musaloiu-E. et al.,

2008) can be roughly described as the inverse

approach to X-MAC. Instead of the sender initiating

the communication, the receiver is announcing its

ability to receive messages, basically pulling

messages. In LPP, all nodes are duty cycled and

wake up for just a short time. If a node is awake, it

sends a small packet, called probe, to signal that it is

awake and then it listens for a short time for replies.

A sending node turns its radio on and listens for the

probe of the communication partner.

Additionally, LPP simplifies routing. The data is

pulled hop by hop to the base station instead of

pushed for which the sender needs to know an

address of a node closer to the base station.

Figure 2 shows the principle of both approaches.

3.2 Competition between MAC

Protocols

The interference between different WSNs has been

studied only recently: Bello and Toscano (2009)

show the interference of WSNs operating on

adjacent channels. Bertocco et al. (2008) investigate

interference of two WSNs. They evaluate a ZigBee

WSN interfered by Bluetooth, WLAN and another

ZigBee WSN on the same channel and show how

CSMA/CA affects the Packet Error Rate (PER).

Boano et al. (2010) investigate the factors

influencing the robustness against interference of

WSNs and use a “Semi-Periodic Interferer” to

simulate a WSN on the same channel. Finally, they

present an enhanced, more robust version of X-

MAC. The security aspect of an intended jamming

of a WSN has also been studied in detail (Xu et al.,

2005).

4 EXPERIMENTAL SETUP

All experiments are conducted using TelosB sensor

nodes (MEMSIC, 2010) in a normal office

environment, thus the environment is full of WLAN

signals, laptop/desktop computers and other

electronic and metallic equipment. Channel 26 is

chosen, since this channel is less interfered with by

WLANs, as previously mentioned. To be sure that

missed packets are not due to external interference,

interfered and non-interfered trials are done in

alternating order. Also the received signal strength

indicator (RSSI) is logged for every packet to ensure

a good connection. Additionally, to rule out external

factors and gain better insight into the causes for the

results, all experiments have been simulated in

Cooja. The “Unit Disk Graph Medium” and a

random start up time of maximal 3 s (which is

comparable to the start of nodes by hand) have been

used. Due to the limited space and the fact that the

simulation corresponds to the experiments, only the

experimental results are presented here.

The software for the experiments is written in

ContikiOS version 2.4. X-MAC is used in two

versions, with and without CSMA. CSMA supports

a four packet big buffer and tries a single

retransmission. LPP provides a packet buffer size of

four packets. In the following setup, the observed

network will be called collector and the interfering

network will be called jammer.

Figure 2: (a) In X-MAC, the sender actively tries to establish the communication by sending strobes. (b) In LPP, the

receiver is pulling data with the help of probes. (c-f) The standard timing of X-MAC and LPP with a channel check rate of 4

Hz: (c) X-MAC listens for 6 ms at the beginning of each cycle and then sleeps for 244 ms. (d) If X-MAC wants to transmit,

it sends strobes until it receives an acknowledgement or reaches 50 reiterations. (e) LPP sends a probe every 250 ms waits

for incoming data and then sleeps. (f) A potential sender wakes up and listens for an incoming probe for a maximum of 1s.

COEXISTENCE OF DIFFERENT WIRELESS SENSOR NETWORKS - MAC Protocol Interference between X-MAC and

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199

The senders of the collector network send a

unicast message to the base station every five

seconds, 100 messages in total. The packet has a

payload of 4 bytes consisting of an unsigned integer

counter and a timestamp. Additionally, data from

lower layers are added, and thus the packet has a

total size of 36 bytes for X-MAC and 21 bytes for

LPP. This is considerably less than the 127 bytes

allowed for a single IEEE 802.15.4 data frame,

hence every message is sent in a single frame. The

message is only slightly longer than an X-MAC

strobe of 28 bytes or a LPP probe of 13 bytes. This

overhead is realistic for a simple temperature

measurement system for example. The default

values were used for the on- and off-times of the

radios, the number of retries and all other

parameters. Thus, both protocols have comparable

energy consumptions while being idle. The channel

is checked at a rate of 4 Hz, thus cycles of 250 ms

occur, the resulting standard timing is illustrated in

Figure 2 (c-f).

The nodes were placed close (< 20 cm) to the

base station on a desk, in direct line of sight and thus

the distance or orientation of the nodes is not

affecting the transfer. The collector network builds a

single-hop star topology and the jammer network

consists of a sender and a base station. All nodes are

in communication range of each other.

The base stations in these experiments are duty

cycled as well, since they would just be a hop closer

to the real base station in a multi-hop network. They

do not send data messages. All nodes are started

manually, thus the start times vary slightly. Every

trial is conducted ten times in order to provide a

reliable data set.

4.1 Network Scalability

The scalability of a non-interfered collector network

is tested in order to provide a comparison between

high traffic and interference.

Table 1: Packet Error Rates (PERs) [%] of the scalability

experiment.

Scalability (sender)

3 5

X-MAC

mean

3.93 50.58

median

0.67 49.30

stdev

10.14 5.52

X-MAC/

CSMA

mean

3.47 15.44

median

0.17 17.00

stdev

5.36 5.90

LPP

mean

8.20 11.38

median

0.17 7.70

stdev

16.46 12.20

The resulting mean, median and standard

deviation of the PERs over ten repeats for this

experiment are shown in Table 1. Even with only

three sending nodes, some packets are lost due to

collisions. The PERs of all protocols increase due to

the traffic of two additional senders. The lost packets

almost always belong to a single node. If a deadlock

between nodes occurs due to bad timing then it stays

unresolved until the end of the experiment due to the

fixed duty cycling. This results in a constant, high

packet loss for a single node.

4.2 Interference Experiments

For all of the following interference experiments,

this work uses a collector network (consisting of a

base station and three sending nodes) and a jammer

network (consisting of a sender and a base station).

Experiment 1) Scalability versus Interference: This

experiment compares the effects of having two

additional senders in a network (Table 1) with two

interfering nodes using a different protocol. X-MAC

and X-MAC/CSMA are interfered by LPP and LPP

is interfered by X-MAC. The parameters used

(sampling time, channel check rate and payload) are

exactly the same for the collector and jammer

network. The effect of the interference, shown in

Table 2, is an increased PER that is lower than the

PER of the scaled up networks for X-MAC and LPP

and roughly the same for X-MAC/CSMA. The X-

MAC collector network is losing almost all packets

or packets at regular intervals from a sender. LPP is

almost unaffected.

Experiment 2) Decreased Sampling Time: To

increase the effect of interference the number of sent

packets on the jammer network is raised by setting

the sampling interval to 1 s. The packets contain the

same payload as the packets of the collector

network. Both networks use the same channel check

rate. As shown in Table 2, this only leads to an

increased PER for X-MAC.

Experiment 3) Decreased Channel Check Rate: To

investigate the effect of the channel check rate, the

sampling rate is set to 2 Hz for the jammer network.

This results in fewer data transfer contacts, but

increases the number of strobes/probes needed to

synchronise (the channel is checked twice in a

second and each second, a packet is sent). The PER

is not changing considerably (see Table 2), which is

due to the fact that the actual message is similar in

length compared to the strobes/probes.

Experiment 4) Increased Payload: The payload sent

by the jammer network is enlarged to see the effect

of increased packet length. A longer packet results in

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a longer continuous blocking of the channel. The

payload is increased to 42 bytes. The X-MAC and

X-MAC/CSMA collector is interfered by LPP with a

59 byte packet and LPP by X-MAC with a 74 byte

packet. The effect on the PER does not considerably

differ from Experiment 2, only X-MAC/CSMA

shows a higher PER, as shown in Table 2. This

shows that the payload and thereby the packet length

has a small effect due to the overhead of the strobes

and probes. X-MAC can send up to a maximum of

50 strobes to establish the data exchange. LPP

always sends a probe at each duty cycle. Compared

to these strobes/probes, the data packet length is

almost negligible.

Experiment 5) Pobes versus Data Packets: To show

that the overhead traffic generated by probes is more

relevant than the data packets, the sampling time is

set up to 25 s. Despite the sampling time change,

LPP still generates the same amount of probes. The

resulting PER, shown in Table 2, is still as high as in

Experiment 2 and Experiment 4 while the sent

application data decreased by the factor of five. This

shows that the overhead is more important than the

actual data sent.

Experiment 6) LPP jammed by X-MAC/CSMA: In all

the experiments described so far, LPP is jammed by

X-MAC without CSMA. In this experiment, LPP is

jammed by X-MAC/CSMA. The retransmissions

caused by CSMA generate additional traffic on this

jammer network. As shown in Table 2, this

additional traffic slightly increases the PER.

4.3 Discussion

The PER is not equally distributed among the

repetitions of the experiments. In the results of the

single trials three different cases can be identified:

full loss of all 100 packets of a single sender,

packets loss occurring in regular intervals, or no

packet loss. These patterns match with patterns

shown in Cooja simulations, so it can be asserted

that the packet errors are not caused by external

interference. Since theses packet losses are due to

collisions between the jammer and collector

network, timing is a vital key factor.

For X-MAC and X-MAC/CSMA, the

interference has roughly the same effect as the

increased traffic of more communication partners

using the same MAC Protocol.

LPP seems not to be affected by the jammers.

This result matches with Boano et al. (2010), since

the latter shows that a “Packet Queue with Fast

Drain” helps to avoid interference. LPP is using a

similar concept by default, since it has an included

buffer, which is transmitted in a row when the

handshake succeeds. But LPP seems to fail in

scaling to more nodes and jams other WSNs with its

permanent probing. Since the major traffic is

generated by the overhead in the network, the actual

data sent has a small effect on the PERs.

5 CONCLUSIONS

To date, robustness against interference with other

technologies has been widely investigated, but the

interference between WSNs operating on the same

channel has not been focused on to date. In WSN

MAC Protocol design, the assumption of an isolated

WSN is still dominant. This work gives a first

attempt to direct attention to the problem of inter-

WSN interference.

The parameters: sampling time, channel check rate

and payload of the jamming network have been

experimentally investigated. For the setup being

used, their effects on the PER are surprisingly small,

since most of the traffic is overhead. The results

show that the packet errors caused by a second

network using a different protocol are roughly the

same size as the packet errors caused by traffic of

additional nodes on the same network.

In this work, only the parameters of the

Table 2: Packet Error Rates (PERs) [%] of the interference experiments.

Jammer network (sampling interval, channel check rate)

5 s, 4 Hz 1 s, 4 Hz 1 s, 2 Hz

1 s, 4 Hz,

payload

25 s, 4 Hz,

probes

1 s, 4 Hz

CSMA

Experiment number 1 2 3 4 5 6

X-MAC

mean

38.77 53.10 40.23 50.67 50.13 -

median

36.83 47.33 45.67 44.00 53.67 -

stdev

20.55 26.47 14.30 26.09 16.03

X-MAC/

CSMA

mean

18.10 19.17 17.13 28.80 21.37 -

median

15.17 15.00 10.83 20.83 22.33 -

stdev

15.26 16.99 15.36 23.74 7.70

LPP

mean

0.17 0.23 6.63 4.50 - 5.87

median

0.17 0.17 0.00 0.33 - 0.33

stdev

0.17 0.26 19.46 12.83 - 16.39

COEXISTENCE OF DIFFERENT WIRELESS SENSOR NETWORKS - MAC Protocol Interference between X-MAC and

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201

interfering network have been changed. Valuable

insight into the problem could be provided in future

studies by investigating the parameters of the

collector network and their effect on its vulnerability

to interference. A larger setup including multi-

hopping would exacerbate the interference problem.

The conduction of the experiments in a shielded

environment would exclude external interference

and thereby deliver less noisy measurements.

A modification of the MAC Protocol duty

cycling supporting variable timings could avoid the

deadlock. The authors believe that the problem of

inter-WSN interference will gain more research

interest in the near future.

ACKNOWLEDGEMENTS

The authors wish to thank the following for their

financial support: The Embark Initiative and Intel,

who fund this research through the Irish Research

Council for Science, Engineering and Technology

(IRCSET) postgraduate Research Scholarship

Scheme. Thanks to the anonymous reviewers for

their valuable comments and suggestions.

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