2009 International Conference on Computational Science and Engineering
MAW: A Reliable Lightweight Multi-Hop Wireless
Sensor Network Routing Protocol
Kunjan Patel∗ , Lim Jong Chern† , C.J.Bleakley† and Wim Vanderbauwhede‡
∗ Institute
for System Level Integration, Scotland,UK
College Dublin, Dublin, Ireland
‡ University of Glasgow, Glasgow, UK
Email: see http://www.kunjanpatel.co.nr/contact, [email protected]
† University
Abstract—Wireless sensor networks consist of a number of
small wireless sensor nodes which take measurements and transmit them over wireless links. As wireless sensors are resource
constrained, the usage of energy and memory must be done
wisely to increase the lifetime of nodes. It is also necessary to
deliver data reliably to make any application more useful to the
end user. A reliable and lightweight routing protocol for wireless
sensor networks is presented in this paper. The protocol shows
more than 90% savings in number of transmissions compared
to the message flooding scheme when the same route is used to
transmit data messages. This saving increases exponentially as
the number of transmissions increases over a same route. The
protocol occupies only 16% of total available RAM and 12% of
total program memory in MICAz platform which makes it very
lightweight to implement in wireless sensor networks. Its self
healing capability to recover from livelock and deadlock enhances
its reliability as a routing protocol in wireless sensor networks.
Index Terms—AODV, wireless sensor network, routing protocol, TinyOS
in wireless sensor network which is a modified version of
the AODV routing protocol. We address some issues to make
this protocol suitable for wireless sensor networks as AODV
is designed for mobile nodes in an ad-hoc networks [5].
The major modifications in the proposed protocol compare to
AODV routing protocol and features which make this protocol
distinct from other protocols are follows:
•
•
I. I NTRODUCTION
With the increased demand of the functionality in the
embedded systems and networks the need of sensors is becoming more prominent. A sensor allows builtin intelligence to
systems. Though a single sensor seems to only provide trivial
efficacy when used in a group they are able to accomplish
difficult tasks such as event detection. High condition and
deployment demands of sensor network applications in various
fields have made the communication between these sensors
wireless. Networking protocols like TCP/IP are used for homogeneous purposes while the routing protocols for wireless
sensor networks are used for specific and collaborative purposes. One of the necessary requirements of a wireless sensor
network is to deliver data reliably due to the dynamic and lossy
nature of wireless transmissions. In wireless sensor networks
(WSNs), the energy dissipation in transmission of data directly
depends on the routing algorithm [1]. Hence, resources limited
wireless sensors require an optimum algorithm for routing.
Ad hoc On-demand Distance Vector (AODV) is a wellknown routing protocol for MANETs. In past few years
some implementations were presented on based on the AODV
routing protocol. Some of them were implemented for wireless
sensor networks (WSNs) [2] [3] [4]. However, some important
issues such as data broadcasting storm problems, message
traffic overhead, memory requirements were not fully addressed. We propose a lightweight protocol for unicast routing
978-0-7695-3823-5/09 $26.00 © 2009 IEEE
DOI 10.1109/CSE.2009.104
487
•
•
•
•
AODV routing protocol discovers a path between source
and destination node while the proposed protocol is able
to find the shortest possible path between two nodes
depending on the connectivity in the WSN.
As AODV was originally designed for mobile networks
its overall message packet length is comparatively higher
for WSN. Some of the fields in the AODV message type
like, lifetime, separate sequence number fields for source
and destination nodes can be avoided. The compact
message packet reduces the number of bits transmitted
per packet and hence reduces the power consumption.
The routing table length of the AODV routing protocol is
bigger for memory constrained wireless sensor nodes. It
stores information about each group and its group leader
in the network while the proposed protocol only stores
the information about the reachable neighbor nodes. This
makes the memory footprint smaller.
One of the most resources wasting problems in WSNs
are data broadcasting storm problems like, livelock and
deadlock. These problems are addressed to make the
protocol robust.
The connectivity in WSNs is dependant on a number
of ambient factors and hence the message delivery is
sometimes uncertain in WSNs. This protocol uses node
level acknowledgement scheme to increase the message
delivery reliability in WSNs. So, unlike some protocols
presented before, the proposed protocol is able to handle
point to point routing efficiency.
The message retransmission scheme at the node level
in conjunction with the acknowledgment timeout scheme
reduces the number route discoveries and hence reduces
the message traffic overhead. On the failure of a particular
node next available node is automatically selected from
the routing table which further reduces the message traffic
overhead.
It also works independent of the topology which makes
it suitable for ad-hoc WSNs.
The proposed protocol is suitable for data query applications
used for environmental monitoring, habitant monitoring etc.
It also allows the user to query different types of data by
only changing the type of message in the message header
field which avoids unnecessary route discovery. For example,
an application monitoring temperature, if the user wants to
monitor humidity on the same route instead then he just needs
to change the message type.
•
II. R ELATED W ORK
Due to the popularity of wireless sensor networks extensive
research and development of different routing protocols have
been done. Flooding is one of the simplest approach for
routing in WSNs but it has a number of drawbacks [6]
and can not be used in applications where reliability is an
important issue. The authors of [7] proposed one of the earliest
data centric routing protocol for wireless sensor networks.
Three main deficiencies in a routing protocol for WSN,
implosion, overlap and resource-blindness, were handled by
negotiation and resource-adaption techniques. The proposed
approach showed 3.5 times energy saving than data flooding.
The authors of [8] presented an online power aware routing
method in wireless ad-hoc networks which does not require
the knowledge of message sequence. Two online algorithms,
Max-min zPmin algorithm and Zone based algorithm, were
presented. The Max-min zPmin algorithm performed more
than 80% better than optimal routing strategy [9]. However it
requires the knowledge of power level of each node and hence
the Zone based algorithm was presented which has nearly the
same performance as the Max-min zPmin algorithm [10]. A
distributed version of the Max-min zPmin algorithm which
does not depend on any centralization was presented in [10].
It showed significant improvement in the performance than a
distributed greedy-style algorithm. LEACH [11], TEEN [12]
and PEGASIS [13] are some of the clustering based protocols.
Clustering based algorithms forward data to the sink node
through the header node of each cluster. Although this kind of
routing protocols are energy efficient, they suffer from selective routing and message flooding to check the activeness of
nodes [14]. The TinyOS beaconing based approach presented
in [15] and [16] has issues with reliable routing, hotspot
and sometimes message flooding [14]. A routing algorithm
intended to increase the lifetime of the WSN is presented in
[1]. The presented algorithm showed network lifetime very
close to the optimal network lifetime and more than 3 times
longer than that of Minimum Total Energy (MTE) approach
presented in [17] and [18] and more than 2 times longer
than that of Max-Min residual Energy (MMRE) approach.
However, the presented algorithm is for static nodes and has
comparatively higher overhead for using it in WSNs.
The authors of [19] discussed different issues and design
strategies for AODV implementation. A detailed discussion
about AODV routing protocol for mobile nodes in an ad hoc
network was presented in [5]. An AODV like implementation
using TinyOS can be found in [4]. In this implementation,
after route discovery, bidirectional message routing requires
change in routing table alternatively which consumes more
power because of frequent access to the memory. In addition
to this, on failure of a node in the routing table it initiates the
route discovery which is the most power consuming part of
the AODV protocol. A detailed comparison of different AODV
based protocols is presented in [2]. The proposed protocol is
more reactive but memory consumption is 58.5% higher than
the AODV implementation of [3]. Although authors evaluated
the protocol for IEEE 802.15.4, an evaluation of message
traffic overhead is missing. A simplified AODV protocol,
AODVjr, was presented in [20]. The protocol discarded some
message types and fields from the message of the original
AODV specification [5] which are necessary to make the
protocol robust and suitable for WSNs. For example, it has
discarded the RERR message which is important when node
failure occurs. Consequently, this requires destination node to
send a message at particular interval to keep the route live
which leads to unnecessary overhead in the network. Most of
the research work has been concentrated on issues regarding
reliability or energy saving in mobile ad hoc network. But in
case of WSNs a point-to-point routing protocol having energy,
reliability and memory as design constraints is necessary. This
paper considers all these issues and presents a lightweight,
energy efficient and reliable routing protocol for WSNs.
III. OVERVIEW OF THE AODV A LGORITHM
AODV is a reactive protocol and a route from one node
to another node is only found when needed [19]. This is one
of the important advantages of AODV which makes it viable
for use it as a routing protocol in WSNs. When a source node
needs to communicate with a node in the network, it transmits
a Route Request (RREQ) message. Each intermediate node in
the network forwards the RREQ message until it reaches the
destination node. The destination node responds to the RREQ
message by transmitting the Route Reply (RREP) message.
As the RREP flows through the network, it determines the
route from source node to destination node. The activeness
of the route in the routing table is determined by a timer
associated with the route and the route will be removed if
it is not used after some time interval. The sequence number
is increased by each originating node and used to determine
whether the received message is the most recent one [5]. The
older routing table entries are replaced by the newer ones.
Active nodes in the networks are determined by broadcasting
a ”Hello” message periodically in the network. If a node fails
to reply a link break is detected and a Route Error (RERR)
message is transmitted which is used to invalidate the route as
it flows through the network. A node also generates a RERR
message if it gets message destined to a node for which a
route is unavailable.
IV. MAW P ROTOCOL
Figure 1 shows the brief flow chart of the proposed protocol,
Modified AODV for Wireless Sensor Network (MAW), which
488
is a modified version of AODV routing protocol. Although
AODV has been proven an efficient protocol for routing
purpose in an ad-hoc mobile network [5], it can not be directly
implemented in wireless sensor network because of the lossy
nature of wireless data transmission and reliability issues in
WSNs. There are two major issues with AODV in terms of
power consumption. One is route discovery phase and the
other is long header field of the message. We have also
addressed some other issues and proposed their solutions. The
modifications in AODV and addressed issues are as follows.
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The sequence number is used to determine the change in type
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same type of command messages in multicast routing. The
length of the payload can be predefined using DATA_LENGTH
depending on the application requirements. However, if an
application needs more than total 28 bytes then modifications
to TinyOS 2.X message types are necessary [22]. Here, 16
bytes are allocated for payload and 4 bytes are left reserved
for future extensions of the protocol.
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MAW PROTOCOL M ESSAGE F IELDS
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A. Types of Message in MAW protocol
There are five types of message defined in MAW protocol:
1) Route Request (REQ) Message: It is used to discover
a route to the final destination in WSN.
2) Route Reply (REP) Message: It is used to prepare
routing table from destination to source node.
3) Command (CMD) Message: On the reception of this
message the source node initiates route discovery in the
WSN for the destination node defined in the message.
User can define more command messages according to
the type of application. For each type of data query a
command message should be defined.
4) Data (DAT) Message: It is reply message to the Command type message. It returns the requested data from
destination node to the source node specified by the
Command message.
5) Route Failure (RTF) Message: A node broadcasts this
type of the message if the node fails to communicate
with all the nodes in the routing table.
B. Structure of the message in MAW protocol
In wireless sensors, the transmission consumes around 2000
times more power than processing a single instruction [21] and
so the number of fields in the message is kept minimum to
keep the overall length of the message as short as possible.
Figure 2 shows the structure of the message used in MAW
protocol. The data field of the TinyOS 2.x message is used
489
Length
(Bytes)
2
Hop Count
Type
1
1
Payload
16
Source Node Address
1
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1
Purpose
Differentiates between Command messages
Stores the value of hop count
Determines the type of the
message
Stores data depending on the
application
Store the node ID of the source
node
Stores the node ID of the destination node
C. Routing Table
One of the most constraint resources in the wireless sensor is
on-chip memory and hence it is important to use it wisely. The
routing table plays an important role in AODV routing protocol
and the whole routing procedure relies on the routing table.
However, spreading over a routing table with unlimited space
for routing information is not suitable for the wireless sensor
node. So, a memory constrained routing table was prepared
for the unicast routing. The routing table has two fields:
• Node ID: It stores IDs of nodes which are reachable.
• Hop Count: It stores the number of hop count required to
reach to the destination node using corresponding node.
D. Route Discovery Mechanism
The route discovery is initiated on the reception of the CMD
type message. The CMD type message can be inserted from
user end computer or source node can generate itself. The user
can define different types of command messages for different
kind of data queries. For example, user can define three kind
of CMD messages to query temperature, humidity and light
sensor readings. When a CMD message is received by the
source node, source node broadcasts a REQ message in the
WSN for the destination node defined in the CMD message.
The intermediate nodes keep forwarding the REQ message
until it reaches the destination node. Figure 3 shows the flow
chart of the Route Discovery mechanism. This flow chart
shows common Route Discovery mechanism for all nodes in
MAW protocol. When the REQ message is received at the
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The Flow Chart of Route Discovery Mechanism in MAW Protocol
destination node the destination node responds by transmitting
a REP message designated for the source node. This may lead
to numerous unnecessary message transmissions, depending
on the number of nodes and connectivity in the WSN, but the
acknowledgement scheme described in Section IV-H prevents
unnecessary data flooding.
E. Route Reply Message Forwarding Mechanism
When the REQ message is received by the destination node,
it replies with a REP message which is forwarded until it
reaches to the source node. Each intermediate node, on the
reception of the REP message, increments the hop count by
one and stores the hop count and node ID of the sender
node in the routing table. The flow chart of the Route Reply
mechanism is shown in Figure 4.
F. Command Message Forwarding Mechanism
On the successful reception of the REP message the source
node sends the Command type message to the nearest neighbor. The nearest neighbor is found using a neighbor sorting
mechanism which rearranges the routing table, prepared on
the reception of the REP messages, using the hop count information. The intermediate nodes follow the same procedure
until the message reaches the destination node. The CMD type
message can be a query for the temperature reading, humidity
reading etc. A programmer can define more than one type
of CMD messages depending on the type of application and
usability of nodes in the WSN.
490
H. Acknowledgement and Acknowledgement Timeout Scheme
The transmission range of a wireless sensor varies from 10m
to 50m, depending on the type of the transceiver available
on the wireless sensor [23]. As the range and link quality
depend on ambient factors, it is important to implement an
acknowledgement scheme in the routing protocol to ensure the
delivery of the message. In MAW protocol a node level acknowledgement scheme is implemented. In other words, each
node sends an acknowledgement message on the reception of
a message from the neighbor node.
The acknowledgement timeout scheme checks whether the
acknowledgement is received within a certain time period. If
an acknowledgement is not received within a predetermined
time period than the same message is sent again. This ensures
the delivery of the message to the next node. However, this
may lead to a number of transmissions if the designated node
does not respond or die. A maximum number of retransmissions is set to resolve this problem. If currently selected
neighbor node does not respond after a predefined number of
retransmissions then the transmitting node will select the next
nearest node from the routing table. This process is repeated
until the transmitting node tries out all nodes in the routing
table. If all the nodes in the routing table fail to respond then
the transmitting node broadcasts a RTF message.
I. Deadlock
The death of a node in the WSN being used in the routing
process may lead to unnecessary message transmissions by
other nodes. This kind of situation is called deadlock [24].
TABLE II
N ODE T OPOLOGY
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6,8,16
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50
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50
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neighbors
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14
15
16
17
18
19
20
21
22
23
24
25
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8,14,16,17
17
15,16,24,25
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18,20,22,23
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12,13,20
19,20
18,19,25
17,25
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52-53
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V. E XPERIMENTS S ETUP
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Fig. 5. The Flow Chart of Acknowledgement Timeout Scheme in MAW
protocol
This kind of broadcast storm problem can be handled by
constraining the number of retransmission [25] [24] and acknowledgement timeout scheme as shown in Figure 5. If a
required node in the the routing process dies, then after certain
number of retransmissions the node which wants to send the
message selects the next nearest neighbor from the routing
table. If all nodes in the routing table fail to respond then the
node broadcasts a Route Failure message.
J. Livelock
Livelock is less common than deadlock. But when it occurs
it leads to significant reduction in message throughput [24]
[26]. Livelock occurs in MAW protocol when two active
nodes in the routing process are the nearest neighbor of each
other. In this kind of situation each node transmits message
to each other and creates a message transmission loop which
never ends. This problem is solved by comparing the node
ID of recent sender to previous sender and message sequence
number. The problem can be also resolved by setting a global
timer at the source node which retransmits the message if a
response is not received within a certain time interval. But in
ad-hoc mobile network it would be difficult to determine such
491
The MAW protocol was written in nesC (network embedded
system C) language using TinyOS-2.x [27]. TinyOS is an
open source, component based and an event driven operating
system. It is written in nesC programming language [28] [29]
and designed for wireless sensors having limited resources.
TinyOS simulator (TOSSIM) was used to simulate the WSN
made of total 26 nodes [30]. TOSSIM for TinyOS-2.x can
also simulate noise and interference present in radio links
in the WSN [31]. Table II shows the list of nodes, their
neighbors and noise present in radio links between nodes. All
the simulations were done for the micaz platform [32]. The
maximum number of neighbors per node was set to 7 and the
number of maximum acknowledgement trials was set to 3. The
protocol was simulated for unicast routing only and for a WSN
by varying number of nodes. The WSN was simulated for 4,
7, 10, 16, 19 and 25 nodes. Different Python scripts were
created to simulate the WSN for different number of nodes
and to inject messages in the WSN. As TOSSIM for TinyOS2.x generates massive log files after simulations, shell scripts
were created to analyse the log files and get the statistics of
the simulation results.
VI. R ESULTS
Figure 6 shows the number of transmissions, retransmissions, acknowledgement messages, command messages and
data messages required to discover a single route and to query
a single data message on the discovered route in the WSN
as shown in Table II. While the number of acknowledgement
messages does not vary so much, the number of transmissions
and retransmission messages is linearly proportional to the
number of nodes in the WSN. It can be seen from the results
that major overhead is in finding a route from the source
node to the destination node but once a route is discover then
the number of command and data messages are minimal. As
command and data messages travel on almost same route in
most of the cases, the number of command and data messages
is the same. The variation in some cases is due to the noise
present in radio links which leads to retransmission of the
message or change in the route.
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Memory footprint on MICAz wireless sensor platform
Fig. 6. Number of transmissions, retransmissions and acknowledgments
required for one route discovery and one data query
The chart presented in Figure 7 shows a comparison of total
all types of message transmissions required using MAW protocol and without MAW protocol for 10 data query messages
and 20 data query messages. It is clear from the results that
after route discovery phase when the same route is used to send
messages, it saves overall energy by more than 90% compared
to message flooding scheme. These figures also include the
number of transmissions required to discover the route to the
destination node.
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introducing different acknowledgement and timeout schemes.
Its memory constrained routing tables and low memory requirements make it lightweight. The protocol acquires only
12% of the total program memory and 16% of the total
RAM in MICAz platform. The protocol shows more than 90%
savings in overall number of transmissions compared to the
message flooding scheme. These savings increase exponentially as the communication rate increases on the same route
in an ad-hoc wireless sensor network. After a route from the
source node to the destination node is discovered, the source
and destination node can communicate with minimal message
traffic overhead. Results show that as the number of packets
sent over a single route increases the energy efficiency of
MAW when compared with the message flooding mechanism.
The protocol is also able to detect and recover from the
broadcast storm problems [24], livelock and deadlock. This
enhances its reliability in WSNs. This kind of characteristics
make it useful for data query applications in WSNs.
VIII. F UTURE W ORK
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Fig. 7. Comparison of number transmissions required with and without
MAW protocol
Figure 8 shows the memory footprint for the MAW protocol
on MICAz wireless sensor platform. The proposed protocol
requires around 12% of the total program memory and around
16% of the total RAM available on the MICAz wireless
sensor platform which lower than the memory consumption
mentioned in [2]. This shows very low memory requirements
of this protocol which makes it lightweight for implementation
in WSNs.
VII. C ONCLUSION
This paper proposes a reliable lightweight routing protocol
for unicast routing in WSNs. The protocol is made reliable by
492
As a real implementation is complex and difficult to debug
[33], only simulations were done for the proposed protocol.
An implementation on a WSN network is necessary to check
the reliability in real life and to identify modifications which
might be necessary. After the implementation, it would be
also useful to get actual energy consumption readings of
nodes present in the WSN to get an idea of the lifetime of
the WSN. An implementation of the protocol using Contiki
operating system [34] may allow more extensive tests of the
proposed protocol. Integration of the protocol with other layers
of Open Systems Interconnection (OSI) [35] like Medium
Access Control (MAC) would be also useful to save more
power in WSNs.
ACKNOWLEDGMENT
The authors would like to thank Dr. Robert Forbes for his
valuable and thoughtful suggestions. The authors would like
to thank iSLI and University College Dublin for providing
funding for publishing this paper.
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