A wireless sensor network is a wireless network consisting of tiny
devices which monitor physical or environmental conditions such as
temperature, pressure, motion or pollutants etc. at different
regions. The tiny device, known as sensor node, consists of a radio
transceiver, microcontroller, power supply, and the actual sensor.
Initially sensor network were used for military applications but now
they are widely used for civilian application area including
environment and habitat monitoring, healthcare application and so on.
Normally sensor nodes are spatially distributed throughout the region
which has to be monitored; they self-organize in to a network through
wireless communication, and collaborate with each other to accomplish
the common task. With the going time, sensor nodes are becoming
smaller, cheaper, and more powerful which enable us to deploy a
large-scale sensor network.
Basic features of sensor networks are self-organizing capabilities,
dynamic network topology, limited power, node failures & mobility
of nodes, short-range broadcast communication and multi-hop routing,
and large scale of deployment [12]. The strength of wireless sensor
network lies in their flexibility and scalability. The capability of
self-organize and wireless communication made them to be deployed in
an ad-hoc fashion in remote or hazardous location without the need of
any existing infrastructure. Through multi-hop communication a sensor
node can communicate a far away node in the network. This allows the
addition of sensor nodes in the network to expand the monitored area
and hence proves its scalability & flexibility property.
Presently there are different types of commercially available sensor
nodes. University of California at Berkeley has developed Mica mote
which is a special purpose sensor node. Other special purpose sensor
nodes available are Spec, Rene, Mica 2, Telos etc. Some high
bandwidth sensor nodes available are BTNode, Imote 1.0, Stargate,
Inryonc Cerfeube etc. [13].
1.1 Sensor Network Challenges
Wireless sensor network promise a wide variety of application and to
realize these application in real world, we need more efficient
protocols and algorithms. Designing a new protocol or algorithm
address some challenges which are need to be clearly understood.
These challenges are summarized below:
- Physical Resource Constraints: The most important constraint imposed on sensor network is the limited battery power of sensor nodes. The effective lifetime of a sensor node is directly determined by its power supply. Hence lifetime of a sensor network is also determined by the power supply. Hence the energy consumption is main design issue of a protocol. Limited computational power and memory size is another constraint that affects the amount of data that can be stored in individual sensor nodes. So the protocol should be simple and light-weighted. Communication delay in sensor network can be high due to limited communication channel shared by all nodes within each other’s transmission range.
- Ad-hoc Deployment: Many application or most of them requires the ad-hoc deployment of sensor nodes in the region. Sensor nodes are randomly deployed over the region without any infrastructure which requires the system to be able to cope up with random distribution and form connection between the nodes. As an example, for fire detection in a forest the nodes typically would be dropped in to the forest from a plane.
- Fault-Tolerance: In a hostile environment, a sensor node may fail due to physical damage or lack of energy (power). If some nodes fail, the protocols that are working upon must accommodate these changes in the network. As an example, for routing or aggregation protocol, they must find suitable paths or aggregation point in case of these kinds of failures.
- Scalability: In a region, depending upon the application, the number of sensor nodes deployed could be in order of hundreds, thousands or more. The protocols must scalable enough to respond and operate with such large number of sensor nodes.
- Quality of Service: Some sensor application are very time critical which means the data should be delivered within a certain period of time from the moment it is sensed, otherwise the data will be careless. So this could be a QOS parameter for some applications.
1.2
Wireless Sensor Networks vs. Traditional Wireless Networks
Though many existing protocol, techniques and concepts from
traditional wireless network, such as cellular network, mobile ad-hoc
network, wireless local area network and Bluetooth, are applicable
and still used in wireless sensor network, but there are also many
fundamental differences which lead to the need of new protocols &
techniques. Some of the most important characteristic differences are
summarized below:
- Number of nodes in wireless sensor network is much higher than any traditional wireless network. Possibly a sensor network has to scale number of nodes to thousands. Moreover a sensor network might need to extend the monitored area and has to increase number of nodes from time to time. This needs a highly scalable solution to ensure sensor network operations without any problem.
- Due to large number of sensor nodes, addresses are not assigned to the sensor nodes. Sensor networks are not address-centric; instead they are data-centric network. Operations in sensor networks are centered on data instead of individual sensor node. As a result sensor nodes require collaborative efforts.
- Wireless sensor networks are environment-driven. While data is generated by humans in traditional networks, the sensor network generate data when environment changes. As a result the traffic pattern changes dramatically from time to time.
- Another characteristic unique to wireless sensor network is the correlated data problem. Data collected by neighboring sensor nodes are often quite similar which makes possible to the development of routing and aggregation techniques that can reduce redundancy and improve energy efficiency. It also been observed that the environmental quantities changes very slow and some consecutive readings sense temporally correlated data. This advantageous feature can be exploited to develop an energy efficient data gathering and aggregation techniques.
1.3
Clustering in WSN
It is widely accepted that the energy consumed in one bit of data
transfer can be used to perform a large number of arithmetic
operations in the sensor processor [13]. Moreover in a densely
deployed sensor network the physical environment would produce very
similar data in close-by sensor nodes and transmitting such data is
more or less redundant. Therefore, all these facts encourage using
some kind of grouping of nodes such that data from sensor nodes of a
group can be combined or compressed together in an intelligent way
and transmit only compact data. This can not only reduce the global
data to be transmitted and localized most traffic to within each
individual group, but reduces the traffic and hence contention in a
wireless sensor network. This process of grouping of sensor nodes in
a densely deployed large-scale sensor network is known as clustering.
The intelligent way to combined and compress the data belonging to a
single cluster is known as data aggregation.
There are some issues involved with the process of clustering in a
wireless sensor network. First issue is, how many clusters should be
formed that could optimize some performance parameter. Second could
be how many nodes should be taken in to a single cluster. Third
important issue is the selection procedure of cluster-head in a
cluster. Another issue that has been focused in many research papers
is to introduce heterogeneity in the network. It means that user can
put some more powerful nodes, in terms of energy, in the network
which can act as a cluster-head and other simple node work as
cluster-member only. Considering the above issues, many protocols
have been proposed which deals with each individual issue.
