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Recent researches and advancement in the wireless communication and electronics field have led to the development of low-cost, low power sensor nodes which are small in size and can communicate with each other even at short distances. These small size sensor nodes, which consist of sensing, communicating and data processing capabilities, give the idea of sensor networks. Wireless sensor networks represent great improvement over traditional networks.
A sensor network is composed of large number of sensor nodes which are distributed in the wireless environment. This feature allows a random distribution of the nodes in the disaster relief operations or inaccessible terrains. Another unique feature of sensor nodes is the cooperative effort of sensor nodes. It means that sensor nodes perform distributed operation and co-operate each other to perform a single task. Sensor nodes in the WSN are fitted with an onboard processor. Instead, of sending raw data to the nodes responsible for fusion, they use their processing capabilities to carry out simple computations locally and transmits only the required and partially processed data. 
Thus due to these features of wireless sensor networks they have a wide range of applications . These applications  include environmental control such as fire fighting or marine ground floor erosion, also installing sensors on bridges or buildings to understand earthquake vibration patterns, surveillance tasks of many kinds like intruder surveillance in premises, etc. Also classes of applications include car-to-car or in-car communication. The possibilities abound ââ‚¬" sensor networks could potentially become a disruptive technology when the basic size and cost problems are solved.
As sensor networks are infrastructure less networks similar is the case with mobile Ad-hoc Networks (MANETS). These both types of networks looks to be same in properties but there are a lot of differences between them. The Comparison between Sensor networks and MANETS can be explained as follows:-
1.2 Comparison of MANETS and Wireless Sensor Networks (WSN) 
MANETS (Mobile Ad-hoc Networks) and sensor networks are two classes of the wireless Ad-hoc networks with resource constraints. MANETS typically consist of devices that have high Capabilities, mobile and operate in coalitions.
Sensor networks are typically deployed in specific geographical regions for tracking, monitoring and sensing the surrounding environment. Both these wireless networks are characterized by their Ad-hoc nature that lack pre-deployed infrastructure for computing and communication. Both Share some characteristics like network topology is not fix, power is an expensive resource and nodes in the network are connected to each other by wireless communication links. WSNs differ in many fundamental ways from MANETS as mentioned below.
Sensor networks are mainly used to collect information while MANETS are designed for distributed computing rather than information gathering.
Sensor nodes in the sensor networks mainly follow broadcast communication technology but most MANETS are based on point-to-point communications.
The number of nodes in sensor networks can be higher in several orders than that in MANETS.
Sensor nodes may not have global identification (ID) because of large number of sensor nodes and large overhead while nodes in MANETS have global-ID.
Sensor nodes are much cheaper than nodes in a MANET and are usually deployed in thousands.
Sensor nodes are limited in computational capacities, memory and power whereas nodes in a MANET can be recharged somehow.
Usually, sensors are deployed once in their lifetime, while nodes in MANETS move really in an Ad-hoc manner.
Sensor nodes are much more limited in their computation and communication capabilities than their MANET counterparts due to their low cost.
1.3 Architecture of wireless sensor networks 
The architecture of sensor networks is influenced by factors such as scalability, fault tolerance and power consumption. The Two basic types of sensor network architecture are layered architecture and clustered architecture.
These two architectures can be explained as follow:
Figure1.1 Types of architecture in wireless sensor networks 
1.3.1 Layered Architecture :
In a layered architecture, there is a single powerful base station (BS) and there are layers of nodes around the base station. These layers have the same hop count to the base station (BS).
Layered architectures have been used with in-building wireless backbones and in military sensor-based infrastructure such as multi-hop infrastructure network (MINA). In the building scenario, the BS acts as an access point to a wired network and the small nodes form a wireless backbone to provide wireless communication medium. The users of the network have hand-held devices such as PDAs which communicate via the small nodes to the BS. Similarly in a military operation, the BS is data gathering and processing entity with a communication link to a larger network. A set of wireless sensor nodes is accessed by the hand held devices of the soldiers.
Figure1.2 Layered Architecture in wireless sensor networks 
The advantage of layered architecture is that each sensor in the network is involved only in short distance and low power transmission to the neighboring nodes.
1.3.2 Clustered Architecture :
In cluster based architecture, all the nodes of network are arranged in clusters and each cluster has a cluster head which is the controller of its particular cluster. These nodes have a function to exchange messages from one place to other. The sender first send the message to his cluster head and the cluster head further send the message to the base station. The clustered architecture is shown in the following figure:
Figure1.3: Clustered architecture of sensor networks 
Thus here in clustered architecture any message can reach BS in at most two hops. The cluster architecture has a feature of data fusion which ensures that the node first sends the data to the cluster head and only the needed information is further send to the base station . This is achieved through the network layer protocols such as low energy adaptive clustering hierarchy.
1.4 Architecture of Sensor node
Wireless sensor networks are comprised of a number of spatially distributed sensor nodes which cooperate to monitor the physical qualities of a given environment.
Figure 1.4: architecture of a sensor node 
As shown in the figure 4, a sensor node consists of four basic units:
These four units can be explained as follow:
Sensing Unit :
A sensor is a device that measures some physical quantity and converts it into a signal which is processed by the microcontroller. A wide range of sensor types exist including seismic, thermal, acoustic, visual, infrared and magnetic. Sensors may be passive (active manipulation of the environment) or active (using active manipulation/probing of the environment to sense data, e.g. radar) and may be directional or Omni-directional. A wireless sensor node may include multiple sensors providing complimentary data. The sensing of a physical quantity results in the production of a continuous analogue signal, for this reason, a sensing unit is typically composed of a number of sensors and an analogue to digital convertor (ADC) which digitizes the signal.
The processing unit  is basically associated with a small storage unit and manages the collaboration of sensor nodes with each other to carry out a specific task. This is the main unit to process the data and contains a memory and a processor to complete the task.
A transceiver unit allows the transmission and reception of data to other devices connecting a wireless sensor node to a network.
Wireless sensor nodes typically communicate using an RF (radio frequency) transceiver and a wireless personal area network technology such as Bluetooth or the 802.15.4 compliant protocols ZigBee and MiWi.
Wireless sensor nodes are supported by a power unit which is typically some form of storage (that is, a battery) but may be supported by power scavenging components (for example, solar cells). Energy from power scavenging techniques may only be stored in rechargeable (secondary) batteries and this can be a useful combination in wireless sensor node environments where maintenance operations like battery changing are impractical. To conserve energy a power unit may additionally support power conservation techniques such as dynamic voltage scaling.
1.5 Topology used in wireless sensor networks 
As the sensor networks consists of large number (several hundred to thousands nodes) of nodes which are deployed in a small area. So, the dense deployment of these sensor nodes requires careful topology maintenance . The types of topologies that can be used in sensor networks can be explained as follows:
Star Network (Single Point-to-Multipoint) :
In a star network topology, a single base-station communicates with other remote nodes in the network. This single base station can send or receive messages from other nodes. The remote nodes are not allowed to communicate with each other directly. So, they send or receive messages through the base station. The advantage of star network is that it is simple and reduces the power consumption by remote nodes. Star network also allow low latency time communication between the base station and the remote node. The disadvantage of this type of network is that the base station must be within the radio transmission range of all the remote nodes and is not as much robust as the other networks are because here a single node manages the whole network .
Figure 1.5: star Topology 
Mesh Network :
In the mesh network each node can communicate with every other node which is in radio transmission range of each other. It allow multi-hop communication i.e. if a node wants to send a data packet to any other node which is out of its radio transmission range  then, it can use an intermediate node to forward this packet to the destination node. This network topology has an advantage of scalability and redundancy. If the node fails to work, a remote node can still communicate with any other sensor available in its range, which can further forward the packets to the destination node. The range of the whole network is not limited by the range in between individual nodes. This range of network can be simply extended by adding more nodes in the system. The disadvantage of mesh network is that the power consumption of the nodes that have the capability of multi-hop communications are generally higher than for the nodes that donââ‚¬â„¢t have this capability of multi hop communication. Thus, the battery life becomes limited. Also, when the number of hop count for communication to a destination node increases, then the time to deliver the message also increases especially if low power operation of the nodes is a requirement .
Figure 1.6: Mesh topology 
Hybrid Star ââ‚¬" Mesh Network 
A hybrid of the star and mesh network gives a versatile and robust communications network, while maintaining the ability to make the power consumption by the nodes of network at a minimum point. In this network topology, sensor nodes with lowest power are not able to forward the messages. Hence, maintenance of minimum power consumption is must. However, other nodes in the network have multi-hop capability that allows them to forward messages from the low power nodes to other nodes in the network. Basically, the nodes with multi-hop capability have higher power and they can be plugged into the electrical mains line if possible. This is the topology implemented by the upcoming mesh networking standard called as Zig-Bee .
Figure 1.7 Hybrid Topology