Hybrid Optical and Wireless Sensor Networks

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Hybrid Optical and Wireless Sensor Networks

Lianshan Yan, Xiaoyin Li, Zhen Zhang, Jiangtao Liu and Wei Pan
Southwest Jiaotong University
Chengdu, Sichuan, China

1. Introduction
Wireless sensor network (WSN) has attracted considerable attentions during the last few
years due to characteristics such as feasibility of rapid deployment, self-organization
(different from ad hoc networks though) and fault tolerance, as well as rapid development
of wireless communications and integrated electronics [1]. Such networks are constructed by
randomly but densely scattered tiny sensor nodes (Fig. 1). As sensor nodes are prone to
failures and the network topology changes very frequently, different protocols have been
proposed to save the overall energy dissipation in WSNs [2-5]. Among them, Low-Energy-
Adaptive-Clustering-Hierarchy (LEACH), first proposed by researchers from Massachusetts
Institute of Technology [5], is considered to be one of the most effective protocols in terms of
energy efficiency [6-7]. Another protocol, called Power-Efficient Gathering in Sensor
Information Systems (PEGASIS), is a near optimal chain-based protocol [8].

WSN NodesWSN Nodes
WSN WSN
SINKSINK

Fig. 1. Illustration of a wireless sensor network (WSN) with randomly scattered nodes (sink
node: no energy restriction; WSN nodes: with energy restriction);

On the other hand, distributed fiber sensors (DFS) have been intensively studied or even
deployed for analyzing loss, external pressure and temperature or birefringence distribution
along the fiber link, ranging from hundreds of meters to tens of kilometers [9-13].
Mechanisms include Rayleigh, Brillouin or Raman scattering or polarization effects, through
either time or frequency-domain analysis. Compared with conventional sensors including
Hybrid Optical and Wireless Sensor Networks1
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wireless ones, optical fiber sensors have intrinsic advantages such as high sensitivity, the
immunity to electromagnetic interference (EMI), superior endurance in harsh environments
and much longer lifetime.
Apparently it would be highly desirable to have integrated sensor networks that can take
advantages of both WSNs and fiber sensor networks (FSNs). Such hybrid sensor networks
can find major applications including monitoring inaccessible terrains (military, high-
voltage electricity facilities, etc.), long-term observation of earthquake activity and large area
environmental control with tunnels, and so on. So far hybrid sensor networks have been
studied as well [14-16], while optical sensors in these networks are generally point-like (e.g.
fiber-Bragg-grating based), and such nodes can be regarded as normal WSN nodes after
optical to wireless signal conversion.
In this chapter, we first review typical WSN protocols, mainly about LEACH and PEGASIS,
then evaluate the performance of LEACH protocols for different topologies, especially the
rectangle one. We propose an improved algorithm based on LEACH and PEGASIS for the
WSN, finally and most importantly, we propose an O-LEACH protocol for the hybrid
sensor network that is composed of a DFS link and two separated WSNs. Most analyses
about performance are done in terms of lifetime of the sensor networks.

2. Overview of WSN protocols
Wireless sensor networks (WSNs) generally are composed of small or tiny nodes with
sensing, computation, and communication capabilities. Various routing, power
management, and data dissemination protocols have been specifically designed for WSNs
where energy awareness is an essential design issue. Among them, routing protocols might
differ depending on the applications and network architectures. In general, routing
protocols for the wireless sensor networks can be divided into flat-based, hierarchical-based,
and location-based in terms of the underlying network structures [17-19]. As some protocols
may be discussed intensively in other chapters of this book, here we give a brief review
about major protocols.
(1) Flat-based routing protocol
Sensor nodes in flat-based routing protocols have the same role and collaborate together to
perform the sensing task and multi-hop communication. Since the flat routing is based on
flooding, it has several demerits, such as large routing overhead and high energy
dissipation. Flat-based routing protocol is used in the early stage of WSNs, such as Flooding,
Gossiping, SPIN, and Rumor.
(2) Hierarchical-based routing protocol
Hierarchical-based routing protocol is the main trend for WSN’s routing protocols. In
hierarchical-based routing protocols, the network is divided into several logical groups
within a fixed area. The logical groups are called clusters. Sensor nodes collect the
information in a cluster and a head node aggregates the information. Each sensor node
delivers the sensing data to the head node in the cluster and the head node delivers the
aggregated data to the base station which is located outside of the sensor network. Contrary
to flat routing protocols, only a head node aggregates the collected information and sends it
to the base station. Due to these advantages, sensor nodes can remarkably save their own

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Hybrid Optical and Wireless Sensor Networks

energy. In general, a hierarchical routing technique is regarded as superior to flat routing
approaches. The classical Hierarchical-based routing protocols are LEACH, PEGASIS, H-
PEGASIS, TEEN, and APTEEN. We will discuss the LEACH and PEGASIS protocols in
more details later.
(3) Location-based routing protocol
Such protocol is based on the location information of senor nodes in WSNs. It assumes that
each node would know its own location and its neighbor sensor nodes’ location before
sensor nodes sensing and collecting the peripheral information. The distance between
neighbouring sensor nodes can be computed based on the incoming signal strength [17-18].

2.1 LEACH
Low Energy Adaptive Clustering Hierarchy (LEACH) was first introduced by Heinzelman,
et al. in [5, 20] with advantages such as energy efficiency, simplicity and load balancing
ability. LEACH is a cluster-based protocol, therefore the numbers of cluster heads and
cluster members generated by LEACH are important parameters for achieving better
performance.
In LEACH protocol, the sensor nodes in the network are divided into a number of clusters,
the nodes organize themselves into preferred local clusters, a sensor node is selected
randomly as the cluster head (CH) in each cluster and this role is rotated to evenly distribute
the energy load among nodes of the network. The CH nodes compress data arriving from
nodes that belong to the respective cluster, and send an aggregated packet to the BS in order
to further reduce the amount of information that must be transmitted to the BS, thus
reducing energy dissipation and enhancing system lifetime. After a given interval of time,
randomized rotation of the role of CH is conducted to maximize the uniformity of energy
dissipation of the network. Sensors elect themselves to be local cluster heads at any time
with a certain probability. Generally only ~ 5% of nodes need to act as CHs based on
simulation results. LEACH uses a TDMA/CDMA MAC to reduce intercluster and
intracluster collisions. As data collection is centralized and performed periodically, this
protocol is most appropriate when there is a need for constant monitoring by the sensor
network.
The operation of LEACH is broken up into rounds, where each round begins with a set-up
phase followed by a steady-state phase. In order to minimize overhead, the steady-state
phase takes longer time compared to the set-up phase. In the setup phase, the clusters are
organized and CHs are selected. In the steady state phase, the actual data transfer to the BS
takes place. During the setup phase, each node decides whether or not to become a cluster
head for the current round. A predetermined fraction of nodes, p, elect themselves as CHs.
A sensor node chooses a random number between 0 and 1. If this random number is less
than a threshold value T(n), , the node becomes a cluster head for the current round. The
threshold value is calculated based on Eq. (2-1):

p






1
p
1 *( mod
r
)
( )
0
ifnG
p
T n
otherwise

(2-1)
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Where p is the desired percentage of the cluster heads (e.g. p=0.05), r is the current round,
and G is the set of nodes that have not been cluster heads in the last 1/p rounds. Using this
threshold, each node may be a cluster head sometime within 1/p rounds. All elected CHs
broadcast an advertisement message to the rest of nodes in the network that they are the
new CHs. After receiving the advertisement, all non-CH nodes decide on the cluster to
which they want to belong based on the signal strength of the advertisement. The non-CH
nodes then inform the appropriate CHs to be a member of the cluster. After receiving all the
messages from the nodes that would like to be included in the cluster and based on the
number of nodes in the cluster, the CH node creates a TDMA schedule and assigns each
node a time slot when it can transmit information. This schedule is broadcast to all the
nodes in the cluster. During the steady state phase, the sensor nodes can begin sensing and
transmitting data to the CHs. The CH node must keep its receiver on to receive all the data
from the nodes in the cluster. Each cluster communicates using different CDMA codes to
reduce interference from nodes belonging to other clusters. After receiving all the data, the
CH aggregates the data before sending it to the BS. This ends a typical round.
Advantages of LEACH include: (i) by using adaptive clusters and rotating cluster heads,
LEACH allows the energy requirements of the system to be distributed among all the
sensors; (ii) LEACH is able to perform local computation in each cluster to reduce the
amount of data that must be transmitted to the base station. On the other hand, there are
still some drawbacks about LEACH: (i) LEACH assumes that each node could communicate
with the sink and each node has computational power to support different MAC protocols,
which limits its application to networks deployed in large regions. (ii) LEACH does not
determine how to distribute the CHs uniformly through the network. Therefore, there is the
possibility that the elected CHs will be concentrated in one part of the network. (iii) LEACH
assumes that all nodes begin with the same amount of energy capacity in each election
round, assuming that being a CH consumes approximately the same amount of energy for
each node. Hence LEACH is not appropriate for non-uniform energy nodes [17, 20].

2.2 PEGASIS
In [8], an enhancement over the LEACH protocol called Power-Efficient Gathering in Sensor
Information Systems (PEGASIS) was proposed. The basic idea of the protocol is that nodes
only receive from and transmit to the closest neighbours, and they take turns being the
leader for communicating with the BS. This reduces the power required to transmit data per
round as the power draining is spread uniformly over all nodes. Hence, PEGASIS has two
main objectives: (i) to increase the lifetime of each node by using collaborative techniques; (ii)
to allow only local coordination between nodes that are close together so that the bandwidth
consumed in communication is reduced.
PEGASIS adopts a homogenous topology. In this topology, the BS lies far from sensors with
the fixed position. The data is collected and compressed before sent to the next node. Hence
the messages maintain ideally a fixed size when they are transmitted between sensors. To
locate the closest neighbour node in PEGASIS, each node uses the signal strength to
measure the distance to all neighbouring nodes and then adjusts the signal strength so that
only one node can be heard. The chain in PEGASIS consists of those nodes that are closest to
each other and form a path to the BS. The following describes the protocol briefly:

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1)
The chain starts with the furthest node from the BS to make sure that nodes father
from the BS have close neighbours. Based on the greedy algorithm, the neighbour
node joins into the chain with its distance increases gradually. When a node dies, the
chain is reconstructed in the same manner to bypass the dead node.
To gather data in each round, a token is generated by the BS to set the aggregating
direction after the token sent from the BS to an end node. Each node receives data
from one neighbour, fuses with its own data, and transmits to the other neighbour on
the chain.
Only one node transmits data to the BS in certain rounds, the leader is the node
whose number is (i mod N) where N represents the number of the nodes in round i.
2)
3)
PEGASIS is better than LEACH in terms of energy saving due to following facts: (i) During
the data localization, the distances that most of the nodes transmit information are much
shorter compared to that in LEACH. (ii) The amount of data for the leader to receive is much
less than LEACH. (iii) only one node transmits to the BS in each round.
Though PEGASIS has obvious advantages, it has some shortcomings. Firstly, though most
sensors are joined on a chain to form a basically homogenous structure, a sensor with too
much branches may perform many times of data receiving in a certain round thereby
resulting in unbalanced energy problem. Secondly, all the nodes must keep active before the
token arriving. This means there will be a large percentage of active nodes with nothing to
do from the beginning, meaning a waste of energy and time. Thirdly, once a sensor on the
chain was captured the whole net may be under the control by the attackers. The weak
security could be a great threat [17, 20].
LEACH that is a cluster-based protocol and PEGASIS that is a chain-based protocol are the
most classical Hierarchical-based routing protocols. They both have attracted intensive
attention, and lots of routing protocols are based on these two. Next we will investigate
some issues in details.

3. WSN Topologies
3.1 Shapes of different topologies
According to the shape of WSN monitoring area, application requirements and monitoring
of different targets, different topologies should be chosen for deploying the WSN: circular
topology is preferred for applications such as harbour, stadium etc. [21]; square topology is
suitable for irrigation in agriculture, nature reserve area etc.; rectangular topology can be
chosen for highway, railway, mine and other areas [22].
Here we study the life time of WSN in round, square, rectangular shapes of topology, and
the three topologies are shown in Figs. 3.1 (a-c). In Fig. 3.1(a), the circular area is 10,000m2
(same as square, rectangular areas) with the radius R= 56.419m and the base station is
located at the center of the circle, i.e. (0, 0). In Fig. 3.1(b), the size of the square area is
100x100m2 with the base station located on (0, 50) or (50, 175). In Fig.3.1(c), the size of the
rectangular area is 50*200m with the base station located on (0, 25) or (100, 150).
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