30 Wireless Sensor Networks

Suchit Purohit

epgp books

Learning Objectives:

  • Introduction to sensor and wireless sensor networks
  • Characteristics of wireless sensor networks
  • Comparison with wireless ad-hoc networks
  • Components of wireless sensor network
  • Architecture of sensor node
  • Protocol Stack of Wireless Sensor Network
  • Topologies of Wireless Sensor Network
  • Applications of Wireless Sensor Network

Introduction

 

“A person has six senses, sensors sense more than that.”

 

Sensing is a skill used to assemble information about a physical object or process, including the occurrence of events (i.e., changes in state such as a drop in temperature or pressure). Sensor is an object performing such a task of sensing. For example, the human body is covered with sensors that can capture optical information from the surroundings (eyes), acoustic information such as sounds (ears), and aroma related information such as smells (nose). These are examples of remote sensors, that is, they do not need to touch the object to be monitored to gather information. Briefly defining, a sensor is a device that converts parameters or events in the physical world into signals that can be measured and analyzed. Recent advances in micro- electro-mechanical systems(MEMS) technology and use of low power embedded computing devices made it feasible to develop sensors.

 

A sensor network is a group of tiny, generally battery-powered devices and wireless infrastructure that monitor and record conditions in any number of environments.

 

A wireless sensor network (WSN) is a wireless network that is designed using spatially distributed self-governing devices that uses sensors for monitoring physical or environmental conditions. Generally parameters such as temperature, pressure, humidity, direction of wind and speed, illumination, intensity of vibration, intensity of sound, power-line voltage, chemical concentrations, pollutant levels and vital body functions are monitored. Data from these parameters are passed through the network to a base station where the data can be observed and analyzed. A sink node or gateway acts like an interface between users and the network. Wireless Sensor Network (WSN) is a self-configured and infrastructure-less wireless network and consists of gateway and sensor nodes.

 

Fig. 1. WSN Components, Gateway, and Distributed Nodes

 

Wireless sensor networks are all around because of ease of implementation (no long cable runs), ability to operate in difficult environments, easy in troubleshooting and repairing, and high level of performance.

 

Characteristics of Wireless Sensor Networks:

  • Power efficiency: Nodes are remotely located and don’t have direct power access. So, they have limited power capacity. Batteries are used as a power source.
  • Fault tolerance:  Node is prone to failure. WSN is resilient, that is, it should have the ability to maintain network functionalities in case of node failures.
  • Mobility of nodes: Some nodes require mobility to increase communication efficiency or according to the type of applications.
  • Heterogeneity of  nodes:  Nodes  in  WSN  are  of  different  types  but  need  to  work cooperatively in a single network.
  • Scalability: In WSN nodes are densely deployed. So, WSN should be scalable to manage large number of nodes.
  • Responsiveness: WSN should be capable to adapt quickly to the changing topology through which sensor nodes are connected.
  • Communication failures: In WSN, it should be quickly informed to the base station or gateway node, when any node fails to exchange data with other nodes.

Comparison with wireless ad-hoc networks

 

Wireless sensor networks are a type of ad-hoc networks with few differences. Table 1: Comparison between wireless sensor network and wireless adhoc network.

 

Wireless sensor network Wireless adhoc network
1. Consists of sensors sensing Different parameters in specific situations or events. No sensing ability.
2. Small in size, ranging from the size of a grain of sand up to the size of a shoe box. Larger in size. E.g. Laptops, PDAs etc.
3. Limited power capacity Larger power capacity
4. Sensor nodes are inexpensive More expensive compared to sensor nodes.
5. Highly     dense    and    highly     redundant network. Low density and less redundant network.
6. Nodes remain unattended Nodes don’t remain unattended. Battery can be replaced.
7. Transmission range is small (approx. 3 to 30 meters) Transmission range is large (approx. 10 to 500 meters)
8. Memory and processing power are limited Memory size is big and have high Processing power.
9. Data    moves     from     many     nodes     to one(gateway node). Data moves from one node to many nodes (broadcasting).

Components of Wireless Sensor Network:

 

The component of a wireless sensor network creates wireless connectivity inside the network that connects an application platform at one end of the network with one or more sensor or actuator devices in any portion of the network.

 

Gateway: A gateway interfaces the application platform and the wireless nodes on the wireless sensor network. Information which is received from the wireless nodes is aggregated/manipulated by the gateway and then forwarded to the application. Here, the application may run on a local computer or a networked computer. In the opposite direction, the gateway relays or forwards the information to the wireless sensor network, whenever a command is issued by the application program to a wireless node. Protocol conversion can be performed by all gateway so that wireless network can work with other industry or non-standard network protocols.

 

Sensor/Actuator: This device is used for interaction with the physical system that is to be monitored and/or controlled. An example is a sensor monitoring the temperature in a room and controlling the air-conditioned equipment.

 

Sensor Node: A sensor node, also known as a mote, is a node in a sensor network that has the capability of performing some processing, collecting sensory information and communicating with other connected nodes in the network.

 

Architecture of sensornode:

 

A sensor node normally consists of mainly four basic components: sensing unit, processing unit, communication unit and power unit. Other optional components are application dependent which is location finding system, mobilize and power generator.

Fig. 2. Architecture of Sensor node

 

Sensing unit is generally made up of two sub units: sensor and analog to digital converter (ADC). Sensors are used to capture data from environment. The analogue signals generated by the sensors are converted to digital signals by the ADCs, and then supplied into the processing unit. The processing unit is usually associated with a small storage unit or memory and micro controller/ micro processors. Memory used here is on-chip micro controller memory or flash memory. The micro controller performs assigned sensing tasks, processes data and controls the functionality of other components in the sensor node. A transceiver forms communication unit that connects the node to the network. One of the most important components of a sensor node is the power unit or battery. Power units can be supported by a power scavenging unit such as solar cells.

 

Protocol Stack of Wireless Sensor Network

 

WSN follows the OSI model as the most common architecture. In Sensor network five layers are considered, which are application layer, transport layer, network layer, data link layer and physical layer. Three cross layer planes are also there which are power management plane, mobility management plan and task management plane. These planes monitor power, movement and task distribution between sensor nodes and helps them coordinate the sensing task and reduction in overall power consumption.

Fig. 3. Protocol stack for WSNs

 

  • Application Layer: This is the top most layer indulged in different types of application. Software can be built depending on sensing task. It provides the user with interfaces for interest dissemination and is useful for lower layer operations. It also provides interfaces for issuing queries and response of queries.
  • Transport layer: This layer helps when system is planned to be accessed by external network or internet. Some important protocols of transport layer are STCP (Sensor Transmission Control Protocol), PORT (Price-Oriented Reliable Transport Protocol) and PSFQ (pump slow fetch quick protocol).
  • Network layer: Generally, this layer serves the task of routing. But for WSN, main tasks are in power conserving, data centric sensor network, attribute based addressing and data aggregation. Data Link Layer: Data link layer is used for multiplexing of data stream and medium and error control and data frame detection. Communication link between sensor nodes are created by MAC protocol considering power efficiency as most important.
  • Physical Layer: This layer transfers stream of bits over physical medium. Frequency selection, carrier frequency generation, signal detection, modulation and data encryption are main tasks of this layer. IEEE 802.15.4 is the suggested standard for Wireless sensor network with low cost, low complexity, low power consumption and low range of communication to maximize battery life.
  • Power Management Plane : It is used to manage the power requirement of sensor node. For low power consumption, duplicate messages can be multicast to neighbors instead of broadcast. Node can remain idle when no transmission or reception of messages.
  • Mobility Management Plane: It is responsible for keeping track of exact location of sensor nodes. Tracking of neighboring nodes helps reduce power consumption. This is possible by transmitting message to the nearest possible node and also route back to user.
  • Task Management Plane: All the nodes are not required at the same time to perform the sensing task. So, some sensor nodes perform the task according to their power level. Working of sensor nodes is in power efficient way and share resources among them.
  • Coordination Plane: It determines behavior of a node on the reception of data from both the communication plane and management plane.

Topologies of Wireless Sensor Network

 

Basic networking topologies used in wireless sensor network are: point-to-point, star (point-to- multipoint), or mesh.

 

Point-to-point: In this topology, each node is allowed to communicate directly with another node without the need of a centralized communications hub. Each point is able to function as both a “client” and a “server” with the other points or nodes on the network.

 

Fig. 4. Point-to-Point topology

 

Star topology: In this topology, nodes are connected to a centralized communications hub. A node is not allowed to communicate directly with another node, it has to communicate through the centralized hub. Each node acts as a “client” while the central hub acts as the “server”. It is useful in case of small network area.

 

Fig. 5. Star Topology

 

Mesh topology: This topology allows the network to be self-healing, i.e nodes in the network reconfigure their links and find alternative paths around failed or powered-down nodes. Each node can communicate with each other as data is routed from node to node until it reaches the desired destination. This topology is complex and becomes costly if deployed properly. Useful in case of large scale networks.

 

Fig. 6. Mesh topology

 

Tree topology is also available, which can be called as cascaded star topology. With tree topology, network expansion and error detection is easy. But if the cable breaks through which it is connected, whole network will be collapsed.

 

Applications of Wireless Sensor Network:

 

Sensor nodes are used in diverse applications which require continuous monitoring and detection of specific events. Wireless Sensor Network has wide range of applications.

  • Military: In military, sensor nodes are used for battlefield surveillance and monitoring, guiding intelligent missiles, and detection of attack by weapons of mass destruction such as chemical, biological, or nuclear.
  • Environment: Sensors are used for forest fire, flood detection or any seismic activity detection. Sensors are also used to sense temperature, pressure, light, rainfall etc. and communicate over wireless network. Air pollution can be monitored by deploying WSN in some cities that monitors the concentration of dangerous gases for citizens. The use of many wireless distributed sensors enables the creation of a more perfect layout of the water status. Water quality monitoring is also possible with sensors that involves analyzing water properties in dams, rivers, lakes and oceans, as well as underground water reserves.
  • Medical: For patient diagnosis and monitoring, sensors are frequently used. Sensor devices are attached with patients that monitor their physiological data such as heart rate or blood pressure. Data sensed by sensors can be sent regularly over the wireless network to automated monitoring systems which are designed to alert the concerned doctor on detection of an anomaly. To correctly identify allergies and to prevent wrong diagnosis sensor nodes can be used.
  • Industrial: Industrial plants use a large number of sensors in instruments and controllers. At data centers, high density of servers racks create issues like cabling and IP addresses. To deal with this issue, racks are fitted with wireless temperature sensors which monitor temperature of racks.
  • Home Intelligence: At home, sensor nodes and actuators can be fitted inside appliances like vacuum cleaners, microwave ovens, refrigerators etc. These sensor nodes can interact with each other and with external world through internet. In this way, home devices can also be managed remotely.
  • Agriculture: Wireless network frees the farmer from the maintenance of different instruments in a difficult environment. Sensors are used to monitor water tank level, water usage, controlling pumps and transmits data wirelessly to central control center. Automation in irrigation enables more efficient water usage and reduces waste.

Summary

  • Wireless Sensor Network is a network of sensors, sensor nodes and gateway.
  • WSN is a type of wireless ad-hoc network.
  • Sensor nodes have limited power capacity and fault tolerant.
  • Sensor node contains sensing, processing, power and communication unit.
  • Protocol stack for WSN consists of five layers – application, transport, network, data link and physical layer. Cross layer planes provide necessary support
  • Basic networking topologies used by WSN are point-to-point, star and mesh.
  • WSN has wide range of application in diverse fields.

Suggested Reading:

  1. Mobile Communication 2nd edition by Jochen Schiller, Pearson education
  2. Mobile Computing by Asoke Talukder, Roopa Yavagal (Tata McGraw Hill)
  3. “Wireless communication and networking” by William Stallings
  4. Mobile Cellular Telecommunications — W.C.Y. Lee, Mc Graw Hill
  5. Wireless Communications – Theodore. S. Rapport, Pearson Education
  6. Reza B’Far (Ed), “Mobile Computing Principles”, Cambridge University Press.
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