Wireless Sensor Network
Wireless Sensor Network (WSN) is a new category of networking technology that is progressively becoming attractive nowadays. A WSN is a wireless network containing of independent devices using sensors to monitor physical or environmental conditions. A WSN system includes a gateway that supplies wireless connectivity back to the wired world and distributed nodes as shown in Figure 1. The selected wireless protocol depends on your application requirements. Some of the available standards include 2.4 GHz radios based on either IEEE 802.15.4 or IEEE 802.11 (Wi-Fi) standards or proprietary radios, which are usually 900 MHz [1].
Figure 1
WSN Components, Gateway, and Distributed Nodes
The potential applications for WSNs include tracking, monitoring and controlling. WSNs are mainly used for habitat monitoring, object tracking, nuclear reactor control, fire detection, and traffic monitoring [2]. A common application of WSNs is area monitoring, in which the WSN is used over a region where some incident is to be controlled. For example, a large quantity of sensor nodes could be used over a battlefield to discover enemy invasions instead of utilizing landmines. When the event being controlled (heat, pressure, sound, light, electro-magnetic field, vibration, etc.) is detected by the sensors, this event has to be reported to one of the base stations, which in turn will take some appropriate actions for example they can send a message on the internet or to a satellite. Wireless sensor networks are widely used in the water/wastewater industries. Facilities not wired for power or data transmission can be monitored using industrial wireless I/O devices and sensor nodes powered by solar panels or battery packs [3]. Wireless sensor networks can make use of a wide variety of sensors to identify the presence of vehicles for vehicles detection. To control the temperature and humidity levels inside commercial greenhouses, wireless sensor networks can still be used. When the temperature and humidity falls below specific levels, the greenhouse manager can be made aware via e-mail or a message, or host systems can cause misting systems, open vents, turn on fans, or control a wide variety of system responses. The installation of some wireless sensor networks are easy thus they can be moved easily when the needs of the application change.
WSN System Architecture
Most common architecture for WSN follows the OSI Model. Basically in sensor network we need five layers: application layer, transport layer, network layer, data link layer and physical layer [4].
Figure 1.1
WSN System Architecture
WSN Network Topologies
WSN nodes are usually arranged in one of three types of network topologies:
‘ Star network
‘ Cluster tree network
‘ Mesh network
In a star topology, each node is attached directly to a gateway. In a cluster tree network, each node is connected to a node higher in the tree and then to the gateway, and data is routed from the lowest node on the tree to the gateway. Finally, the mesh networks feature nodes that can link to multiple nodes in the system and let data to pass through the most reliable path available. This mesh link is often referred to as a router.
Figure1. 3
Common WSN Network Topologies
A WSN node consists of several technical components. The different components are the radio, battery, microcontroller, analog circuit, and sensor interface. Important trade-offs have to be made when using WSN radio technology. In battery-powered systems, higher radio data rates and the constant use of radio consume more power. Nowadays, the WSN systems are based on ZigBee since it consumes less power. Wi-Fi is an interesting technology because battery life and power management technology are constantly progressing and because of the available IEEE 802.11 bandwidth.
Battery is the second technology consideration for WSN systems. Apart from battery life, the size and weight of batteries as well as international standards for shipping batteries and battery availability must also be considered. The low cost and wide availability of carbon zinc and alkaline batteries make them a common choice [5].
A WSN node periodically wakes up and sends data by powering on the radio and then switching it back off to save energy in order to prolong battery life. WSN radio technology must easily send a signal and let the system to go back to sleep with slightest power use. Therefore the processor involved must also be able to wake up, power up, and return to sleep mode easily. The trend of microprocessor for WSNs must be able to minimize power consumption while keeping or increasing the speed of the processor. When choosing a processor for WSNs, the power consumption and processing speed trade-off is considered to be very important. This makes the x86 architecture a crucial choice for battery-powered devices.
Figure 1.3 shows the different components of WSN node.
Figure1.4
WSN Sensor Node Components
TYPES OF SENSOR NETWORKS
Wireless sensor networks are used on land, underground, and underwater. A sensor network confronts different challenges and restrictions according to the environment in the sensor network deployed. There are five types of the wireless sensor network and these are [6]:
1. Terrestrial Wireless sensor network.
2. Underground Wireless sensor network.
3. Underwater Wireless sensor network.
4. Multi-media Wireless sensor network.
5. Mobile Wireless sensor network.
The advantages and disadvantages of wireless sensor networks
Advantages:
‘ Network setups can be completed without permanent infrastructure.
‘ They are perfect for the non-reachable areas such as across the sea, mountains, rural areas or deep forests.
‘ They are adaptable if there is ad hoc situation when additional workstation is necessary.
‘ It is cheap to implement wireless sensor networks.
Disadvantages:
‘ They are not safe because hackers can enter the access point and achieved all the information.
‘ Wired network is more rapid than wireless network.
‘ The configuration is more complicated compared to a wired network.
‘ They are easily affected by surroundings (walls, microwave, large distances due to signal attenuation, etc).