Sharma's Blog on Wireless Sensor Networks (WSN)

Wireless Sensor Networks (WSN)

2008-10-19T11:42:00.000-07:00

Introduction

The emerging field of wireless sensor networks combines sensing, computation, and communication into a single tiny device. Through advanced mesh networking protocols, these devices form a sea of connectivity that extends the reach of cyberspace out into the physical world. Here the mesh networking connectivity will seek out and exploit any possible communication path by hopping data from node to node in search of its destination.

The power of wireless sensor networks lies in the ability to deploy large numbers of tiny nodes that assemble and configure themselves. Usage scenarios for these devices range from real-time tracking, to monitoring of environmental conditions, to ubiquitous computing environments, to monitoring the health of structures or equipment, to controlling actuators that extend control from cyberspace into the physical world. The most straightforward application of wireless sensor network technology is to monitor remote environments for low frequency data trends. In addition to drastically reducing the installation costs, wireless sensor networks are the ability to dynamically adapt to changing environments. Adaptation mechanisms can respond to changes in network topologies or can cause the network to shift between drastically different modes of operation.

Current wireless systems only scratch the surface of possibilities emerging from the integration of low-power communication, sensing, energy storage, and computation. Wireless sensor networks use small, low-cost embedded devices for a wide range of applications and do not rely on any pre-existing infrastructure. Unlike traditional wireless devices, wireless sensor nodes do not need to communicate directly with the nearest high-power control tower or base station, but only with their local peers. Peer-to-peer networking protocols provide a mesh-like interconnection of Wireless sensor motes. The flexible mesh architectures evident that the dynamically adaptation to support introduction of new nodes or expand to cover a larger geographic region. Additionally, the system can automatically adapt to compensate for node failures. When nodes are added, the interconnection of a wireless sensor network grows and become stronger. This can be grown up to covering of limit less area.

Hundreds of nodes scattered throughout a field assemble together, establish a routing topology, and transmit data back to a collection point or the coordinator point. The application demands for robust, scalable, low-cost and easy to deploy networks are perfectly met by a wireless sensor network. If one of the nodes should fail, a new topology would be selected and the overall network would continue to deliver data. If more nodes are placed in the field, they only create more potential routing opportunities.

The concept of wireless sensor networks is based on a simple equation:
Sensing + CPU + Radio = Thousands of potential applications [12]
Each individual node must be designed to provide the set of primitives necessary to synthesize the interconnected web that will emerge as they are deployed, while meeting strict requirements of size, cost and power consumption

1. Sensor Network Application Classes
We believe that the majority of wireless sensor network deployments come under one these classes. In general, complete application scenarios contain aspects of all three categories.

1.1 Environmental Data Collection
An environmental data collection application is one where a person wants to collect several sensor readings from a set of points in an environment over a period of time in order to detect trends and interdependencies. He would want to collect data from hundreds of points spread throughout the area and he would be interested in collecting data over several months or years in order to look for long-term and seasonal trends. For the data to be meaningful it would have to be collected at regular intervals and the nodes would remain at known locations.

At the network level, the environmental data collection application is characterized by having a large number of nodes continually sensing and transmitting data back to a set of base stations that store the data using traditional methods. These networks generally require very low data rates and extremely long lifetimes.

Environmental data collection applications typically use tree-based routing topologies where each routing tree is rooted at high-capability nodes that sink data. Data is periodically transmitted from child node to parent node up the tree-structure until it reaches the sink. With tree-based data collection each node is responsible for forwarding the data of all its descendants. Once the network is configured, each node periodically samples its sensors and transmits its data up the routing tree and back to the base station.

The typical environment parameters being monitored, such as temperature, light intensity, and humidity, does not change quickly enough to require higher reporting rates. In addition to large sample intervals, environmental monitoring applications do not have strict latency requirements. Data samples can be delayed inside the network for moderate periods of time without significantly affecting application performance. In general the data is collected for future analysis, not for real-time operation.

The most important characteristics of the environmental monitoring requirements are long lifetime, precise synchronization, low data rates and relatively static topologies. Additionally it is not essential that the data be transmitted in real-time back to the central collection point. The data transmissions can be delayed inside the network as necessary in order to improve network efficiency.

1.2 Security Monitoring
Security monitoring networks are composed of nodes that are placed at fixed locations throughout an environment that continually monitor one or more sensors to detect an anomaly. A key difference between security monitoring and environmental monitoring is that security networks are not actually collecting any data. This has a significant impact on the optimal network architecture. Each node has to frequently check the status of its sensors but it only has to transmit a data report when there is a security violation. Additionally, it is essential that it is confirmed that each node is still present and functioning.

For security monitoring applications, the network must be configured so that nodes are responsible for confirming the status of each other. One approach is to have each node be assigned to peer that will report if a node is not functioning. In security network the optimal configuration would be to have a linear topology that forms a Hamiltonian cycle of the network. The power consumption of each node is only proportional to the number of children it has. A majority of the energy consumption in a security network is spent on meeting the strict latency requirements associated with the signaling the alarm when a security violation occurs.

Once detected, a security violation must be communicated to the base station immediately. The latency of the data communication across the network to the base station has a critical impact on application performance. Users demand that alarm situations be reported within seconds of detection. In security networks, a vast majority of the energy will be spend on confirming the functionality of neighboring nodes and in being prepared to instantly forward alarm announcements. Actual data transmission will consume a small fraction of the network energy.

1.3 Node Tracking Scenarios
There are many situations where one would like to track the location of valuable assets or personnel. Current inventory control systems attempt to track objects by recording the last checkpoint that an object passed through. However, with these systems it is not possible to determine the current location of an object.

With wireless sensor networks, objects can be tracked by simply tagging them with a small sensor node. The sensor node will be tracked as it moves through a field of sensor nodes that are deployed in the environment at known locations. Instead of sensing environmental data, these nodes will be deployed to sense the RF messages of the nodes attached to various objects. The nodes can be used as active tags that announce the presence of a device. A database can be used to record the location of tracked objects relative to the set of nodes at known locations.

Unlike sensing or security networks, node tracking applications will continually have topology changes as nodes move through the network. While the connectivity between the nodes at fixed locations will remain relatively stable, the connectivity to mobile nodes will be continually changing. Additionally the set of nodes being tracked will continually change as objects enter and leave the system. It is essential that the network be able to efficiently detect the presence of new nodes that enter the network.

2. An Example Implementation of a WSN, In-door Temperature Controlling System
A hot or cold work environment can lead to physical discomfort and disrupt work operations. Working in a hot office environment may cause fatigue, headaches and/or stuffiness. A cold office environment may cause a decrease in sensitivity and dexterity of the fingers. Indoor thermal conditions are influenced by air temperature, thermal radiation, air speed (drafts), metabolic rate (sitting versus physical activity), clothing and relative humidity. The recommended living room temperature in tropical countries is in between 20° to 28°. It is also applicable to the office environment also.

Monitoring the environment conditions in a computer room or data center is critical to ensuring uptime and system reliability. Operating expensive IT computer equipment for extended periods of time at high temperatures greatly reduces reliability, longevity of components and will likely cause unplanned downtime. Maintaining an ambient temperature range of 68° to 75°F (20° to 24°C) is optimal for system reliability. This temperature range provides a safe buffer for equipment to operate in the event of air conditioning [13].

Wireless Sensor Network Based Indoor Temperature Controlling System is hybrid module of above mentioned three sensor network application classes. So it is an environmental data collection system. Again it is a kind of security monitoring system and also it’s a sensor node tracking system.

This is some kind of a canonical environmental data collection application which is one coordinator wants to collect several sensor readings from a set of points in an environment over a period of time in order to detect trends and interdependencies. Here we need to adjust the environment temperature in to a precise level. For an example the room temperature of a living room sometime should be not so hotter and also not so cooler. Indoor environment temperature collection is needed to decide the average temperature of the area. For the data to be meaningful it would have to be collected at regular intervals and the nodes would remain at known locations. Then only the control station knows the exact temperature of every corner of the area.

At the network level, the entire indoor environment is consist of a large number of nodes continually sensing and transmitting data back to a set of base stations that collect the temperature data and calculate the temperature values for all the end stations. Then at the end stations for an example we are having a decentralized air conditioner controlling system with number of air conditioning machine here and there inside the our desired area. So those machines get the controlling message to adjust the air conditioner temperature to the calculated value. These networks generally require very low data rates and extremely long lifetimes. In typical usage scenario, the nodes will be evenly distributed over the indoor environment. This distance between adjacent nodes will be minimal yet the distance across the entire network will be significant.

The key difference between security monitoring and environmental monitoring is that security networks are not actually collecting any data. When it comes to indoor temperature controlling system as a security monitoring systems, we can remove all the traditional wired fire detections systems and those can be replaced by the wireless fire detection system. Here when we have a pre established wireless sensor network based indoor temperature controlling system, nothing specially needed to be done and that system itself can be used as a fire detection and fire alarming system. Since the monitoring networks are composed of nodes that are placed at fixed locations throughout the indoor environment that continually monitor more sensors to detect the temperatures, wireless sensor system has the basic requirements needed for fire detection.

Node tracking scenarios comes in to play when it is needed to track any locations of the mesh network which has been configured for the indoor temperature controlling system. For a fire detection system it is needed to locate the exact location of fire. For an example if any person needs to control the temperature of his/her area according to his/her desire. There should be a mechanism to allow that person to manually control the air conditioner and other locations should be under control of the established system. Then also it is needed to have the node tracking capability within the system and then only we can allow the system to manually remove the mote from auto controlling mode. Motes can be simply tracked by configuring an ID for each and every node separately. Then all the nodes can be identified and tracked by the ID.

The routing strategy can then be used to route data to a central collection points. That’s the coordinator node of the indoor temperature controlling system. In this application, it is not essential that the nodes develop the optimal routing strategies on their own. Instead, it may be possible to calculate the optimal routing topology of the network and then communicate the necessary information to the nodes as required. This is possible because the physical topology of the network is relatively constant. Here for this application we can use tree based routing topologies. Temperature values captured at end stations is periodically transmitted through routing nodes to parent node of the tree-structure until it reaches the sink node. For many scenarios, the interval between these transmissions can be on the order of minutes. Because the rapid changes of the temperature level can’t be expected within less than one minute, it can be vary up to two three minutes also. When it comes to the fire detection system the nodes must be configured so that nodes are responsible for confirming the status of each other. Otherwise some intruder can light a cigarette and keep it near the temperature sensor. If the nodes doesn’t have the knowledge of node’s status of the nearest, this scenario can be alarmed and warned as a fire.
In contrast, with a security network the optimal configuration would be to have a linear topology that forms a Hamiltonian cycle of the network.

3. Further Modifications of Indoor Temperature Controlling System
3.1 Fire Detection with Smoke Sensors
Fire detection with smoke sensors (using wireless sensor networking technology) come under the application class of security monitoring using wireless sensor networking. Thermal sensor of nodes of the indoor temperature controlling system should be replaced with the smoke detecting sensors. Then the network topology should also be optimized by having linear topology rather than having hierarchical network topology. One of the major fact in this system is majority of the energy consumption is spent on meeting the strict latency requirements associated with the signaling the alarm when a smoke detection is occurs. The latency of the data communication across the network to the base station has a critical impact on application performance and it should be optimized by the routing protocol. This means that network nodes must be able to respond quickly to requests from their neighbors to forward data to the base station. In the indoor temperature controlling system, reducing the energy consumption for availability of long lasting purposes is the matter. But in smoke detection system reducing the latency of an alarm transmission is significantly more important than reducing the energy cost of the transmissions. Here the routing nodes must monitor the network more frequently than the indoor temperature controlling system.

3.2 Intruder Detection with IR Sensors at Door/ Window openings
Wireless sensor network based intruder detection with IR sensors at door/ Window openings and perimeters is also comes under application class of security monitoring. Here thermal sensors of each node should be replaced by the IR sensors. This IR sensor should be capable of detecting the IR raise emitted by the humans. All the other network topology related matters are modified as mention under the system modifications to the fire detection with smoke sensor. As a consequence of this special network architecture, from the application point of view, a position in the nodes is really under the surveillance of the WSN if and only if this position is within the sensing range of at least one of the sensor nodes connected to the coordinator (base station). When this comes to the intruder detection at perimeter level, the sensor node placement is a really big task which should comes under special surveillance. The system should function during specified time period of the day. For example intruder detection with IR sensor at door/window most probably works during the night time. But intruder detection at the perimeter level should works all the time.

3.3 Location Detection of Equipments
Location detection of equipments comes under the category of node tracking system using wireless sensor networks. So the indoor temperature controlling system can also be modified as a location detection system. Then this is not limited to the indoor environment. Here the nodes should be modified by removing the thermal sensor part and modifying the routing algorithm from data collection scenario to node tracking scenario.
With wireless sensor networks, each moving objects can be tracked by simply tagging them with a small sensor node. Then the exact location can be noted without having any difficulty. The Moving object which has been tagged with a sensor node will be tracked as it moves through a field of sensor nodes that are deployed in the environment at known locations. Instead of sensing temperature variations, these nodes will be deployed to sense the RF messages of the nodes attached to various moving objects. With this system, it becomes possible to detect where an object is currently, not simply where it was last scanned.

Ad-hoc Wireless Networks (AWN)

2008-10-19T11:31:00.000-07:00

Introduction

In recent years mostly due to the growing prevalence of mobile computing devices, the popularity of wireless networking technology has increased dramatically,. Due to its convenience, wireless networking is becoming more popular in traditional desktop computers as well. Increasing trend is in the near future most computing devices will come with some form of wireless technology.
There are many situations which static infrastructure is either inconvenient or impractical, but nonetheless communication is desired. Users with mobile computers might want to collaborate on a group project in an outdoor area where there are no wireless access points. Or, users in a neighborhood might want to share a doc through the network, but without having to pay for broadband Internet access. Lastly the disaster scenarios are where the infrastructure may have been destroyed. Then the communication without infrastructure would be the desired solution. If the users of wireless networking devices wish to communicate with other networked devices, typically a static infrastructure such as a wireless access point or base station must be set up ahead of time to provide the wireless devices with connectivity. In these cases, the wireless devices can arrange themselves into an Ad Hoc network.
An Ad Hoc network is a collection of wireless nodes that form a network without the use of any static infrastructure such as a wireless Access Point. Instead of communicating via a centralized access point, the nodes in the network cooperate to allow information to be exchanged between them. In particular, each node acts as a router, forwarding packets for other nodes in the network.
The IEEE 802.11 specification uses the term Ad Hoc to describe a network composed solely of stations within mutual communication range of each other" [1]. In some cases Ad Hoc refers to multi-hop self-configuring wireless networks. Wikipedia define this as “A wireless ad hoc network is a decentralized wireless network”.

1. Ad-hoc Wireless network technologies
Most of the wireless network technologies are supported to create Ad-hoc networks while implementing those technologies in to practical situations. Some of those technologies are;

Infrared
Bluetooth
WiFi
WiMax
WiBro

1.1 Infrared
Recently, it becomes popular to use small size computers such as notebook computers or PDAs in the mobile environment. It sometimes happens that several computers meet at the same place such as meeting rooms or conference sites. In such environment, there are demands to make a direct communication among mobile computers. Route maintenance and host enumeration are key requirement for such an ad hoc network.
Infrared technology allows computing devices to communicate via short-range wireless signals. With infrared, computers can transfer files and other digital data bidirectionally. The infrared transmission technology used in computers is similar to that used in consumer product remote control units. Infrared communications span very short distances. Place two infrared devices within a few feet (no more than 5 meters) of each other when networking them. Unlike Wi-Fi and Bluetooth technologies, infrared network signals cannot penetrate walls or other obstructions and work only in the direct "line of sight."

1.2 Bluetooth
Bluetooth uses radio waves to transmit wireless data over short distances. Bluetooth can support many users in any environment. Eight devices can communicate with each other in a small network known as piconet and it create small ad-hoc network here. At one time, ten of these piconets can coexist in the same coverage range of the Bluetooth radio and then it becomes a large ad-hoc network. A Bluetooth device can act both as a client and a server. A connection must be established to exchange data between any two Bluetooth devices. In order to establish a connection a device must request a connection with the other device. Bluetooth was based on the idea of advancing wireless interactions with various electronic devices. Devices like mobile phones, personal digital assistants, and laptops with the right chips could all communicate wirelessly with each other. However, later it was realized that a lot more is possible [15].

1.3 WiFi
Wi-Fi is the wireless technology to handle the networking/communication. Wi-Fi allocates internet connection globally and to be transmitted by the radio waves. Radio waves are the main cause of Wi-Fi. Radio waves are transmitted from antenna and Wi-Fi receivers pick them up. When a user receives the Wi-Fi signals, a wireless internet connection is produced and a user is prompted to provide the user name and password if required to establish a wireless connection.
Wi-Fi maintains certain security issues. WEP or Wired Equivalent Privacy is used in the physical and data link layers. It was planned to provide the wireless security by protecting the data, while it transmits from one point to another. Wi-Fi networking usually maintained inside a building premises. The data transmission in the Wi-Fi is protected by Wireless LANs but due to the fact that data travels over the radio waves so there are chance that data can be exposed and capture.

1.4 WiMax
WiMax is a wireless digital communications system, also known as IEEE 802.16, that is intended for wireless "metropolitan area networks". WiMax can provide broadband wireless access up to 30 miles for fixed stations, and 5 - 15 km for mobile stations. WiMax can be used for wireless networking in much the same way as the more common WiFi protocol. WiMax is a second-generation protocol that allows for more efficient bandwidth use, interference avoidance, and is intended to allow higher data rates over longer distances.

1.5 WiBro
Developed in South Korea, WiBro (Wireless Broadband) is the newest variety of mobile wireless broadband access. WiBro is based on the same IEEE 802.16 standard as WiMax, but is designed to maintain connectivity on the go, tracking a receiver at speeds of up to 37 miles per hour (60 km/h). WiMax is the current standard in the United States, offering wireless Internet connectivity to mobile users at fixed ranges of up to 31 miles (50 km) from the transmitting base. However, WiMax is not designed to be used while the receiver is in motion. WiBro can be thought of "mobile WiMax," though the technology and its exact specifications will change as it undergoes refinements throughout its preliminary stages.

2. Ad Hoc Routing Protocols
A number of different routing protocols have been proposed and evaluated in various work. We have mentioned a few of these protocols here.

Dynamic Destination-Sequenced Distance-Vector (DSDV)
Ad-hoc On-Demand Distance Vector (AODV)
Dynamic Source Routing (DSR)
Grid routing protocol

Most of the Ad-hoc routing protocols use hops count as their link metric when choosing routes in the network. That is, if a node has two candidate routes that it can use to send packets to a particular destination, it will always choose the route with the smallest number of hops.

2.1 Dynamic Destination Sequenced Distance (DSDV)
Dynamic Destination-Sequenced Distance-Vector (DSDV) routing protocol [2]. As its name suggests, DSDV is a distance vector routing protocol and operates by having each node in the network maintain a table of destination nodes along with a first hop and distance for each destination. Route updates are tagged with sequence numbers to avoid routing loop problems, and routing updates are broadcast on a periodic basis whether routing changes have occurred or not.

2.2 Ad-hoc On-Demand Distance Vector (AODV)
The Ad-hoc On-Demand Distance Vector (AODV) routing protocol [3] is similar to DSDV, but does not broadcast route updates periodically. Instead, routes are updated on an as-needed basis and each node only maintains entries in its routing table for nodes that it is actually communicating with. This reduces the amount of network traffic required for routing updates, and also reduces the complexity of routing information stored per node.

2.3 Dynamic Source Routing (DSR)
The Dynamic Source Routing (DSR) [4] protocol is somewhat similar to AODV in that it also avoids periodic routing updates. However, as its name implies, it uses source routing rather than distance vector routing, and the protocol authors claim that DSR responds more quickly to routing changes than do the distance vector protocols.

2.4 Grid Routing Protocol
The Grid routing protocol [5] is a position-based protocol. With this protocol, each node must know its own geographic coordinates, via GPS or some other mechanism. To send a packet to a particular destination, a node looks up the position of the destination node and forwards the packet to whichever of its neighbors is geographically nearest to the final destination. The Grid protocol includes a lookup service that allows nodes to learn the positions of other nodes in a scalable way.

3. Wireless Network Projects
3.1 Monarch Project Testbed
A testbed network originally developed at CMU as part of the Monarch project [10, 11] shares some of the goals of the Grid Roofnet, in particular the goal of providing an outdoor physical testbed for evaluating the performance and properties of Ad Hoc networking protocols. However, the Grid Roofnet has a number of major differences both in its goals and implementation. The Monarch testbed used mobile nodes mounted on cars to evaluate the performance of the DSR [13] protocol under dynamic network topologies. This differs from the Roofnet's static node locations, and goal of providing usable Internet access to users in a neighborhood. Probably the most signiffcant difference between this testbed and the Grid Roofnet is the wireless networking hardware used, which operated in the 900 Mhz range rather than the 2.4 Ghz range used by the Roofnet. Also, radios using the 802.11 standard were unavailable when the Monarch testbed was being built, and as a result their radios did not use 802.11. This probably turned out to be rather fortuitous for their system, as we have found the 802.11 layer to be less than ideal for building a multi-hop wireless network.

3.2 Nokia Rooftop
The Nokia Rooftop [6] is a commercial system capable of providing wireless broadband Internet access to residential areas. This is a commercial system and not a research platform. The Nokia system uses a variety of specialized proprietary hardware and methods in order to make their system perform well. Nokia Rooftop uses proprietary 2.4 GHz wireless routers. These routers use frequency-hopping radios which operate at variable power levels between 16 mw and 500 mw, which at the high end is five times the power output of the 802.11b radios used in the Grid Roofnet. The Nokia Rooftop uses a proprietary routing protocol. This protocol uses super nodes referred to as “Air Head routers" to route traffic between the mesh network and the Internet. The system is structured to have in most cases no more than two hops from a regular node to a super node. The routing protocol uses a common radio channel to cooperatively schedule non-interfering data burst transmissions between pairs of nodes and uses multiple data channels simultaneously to maximize overall network throughput. The system uses these multiple channels along with dynamic power control to avoid unnecessary interference between transmissions.

3.3 Ricochet MCDN
The Ricochet Micro Cellular Data Network (MCDN) System [7] is a commercial network providing Internet access to mobile users. The system provides Internet service via a network architecture in which end users use a radio modem which communicates with microcells. Packets from the user then pass from microcell to microcell until they reach a wired backbone, at which point the packets are routed to the Internet. The system uses proprietary routing algorithms to route packets from the microcells to the wired backbone and back, and has special provisions to ensure that nodes that are moving continue to stay in touch with microcells that are in radio range.
Although the Grid Roofnet and MCDN both share the very general goal of providing Internet access to end users via a multi-hop wireless network, the specific goals and implementation of the MCDN system differ significantly from the Grid Roofnet.

3.4 WINGS and DAWN
The Wireless Internet Gateways (WINGS) [8] and the Density and Asymmetry adaptive Wireless Network (DAWN) [9] projects are both part of the DARPA Global Mobile (GloMo) Information Systems program. Both address a number of issues in wireless networking, but in general tackle problems more related to mobility and military goals. The WINGS project extends the Internet to multi-hop wireless networks using an extension to the IP routing protocol. WINGS use a MAC layer protocol called FAMA-NCS that is somewhat similar to 802.11. A network layer protocol called WIRP (Wireless Internet Routing Protocol) that uses Dijkstra's shortest-path algorithm over a hierarchical graph of the network is used for packet routing. The WINGS project uses 900 MHz radios which are not 802.11. The goal of the DAWN project is to provide topologies for multi-hop wireless networks which have a number of properties that are of particular importance in tactical military networks.

3.5 Community Wireless Networking
In recent years a large number of non-profit community wireless networking projects have sprung up around the world. The goals of these projects vary, but generally include sharing Internet access in communities, providing free Internet access in public areas, and providing alternative networks that can be used to share information outside of the Internet. In the simplest cases they may consist simply of a number of individual users who have 802.11 access points connected to an Internet connection and share their connection freely, while some more ambitious projects are working to create metropolitan area wireless networks intended to augment the Internet.

Wireless Sensor Networks (WSN) based Air Conditioner Temperature Controlling System

2008-08-25T23:26:00.000-07:00

Introduction

This Intelligent Air Conditioner Temperature Control System will be implemented by using a Wireless Sensor Network (WSN). WSN is a wireless network that has distributed autonomous devices that use sensors to cooperatively monitor physical or environmental conditions, such as temperature, humidity, sound, vibration, pressure, motion or pollutants at different locations. Those are used in gathering the information needed by smart environments, whether in buildings, utilities, industrial automation, shipboard, transportation systems automation, habitat monitoring, healthcare applications, home automation, and traffic control systems etc. Sometimes we want to change the locations of sensors time to time. In such applications, running wires or cabling is usually impractical or difficult. A sensor network is required that is fast and easy to install and maintain [1]. How a WSN differs from a typical wireless network is a WSN is distributed and the devices that form the sensor network communicate with each other collaboratively to achieve a common objective. AWSN has one or more base-stations which collect data from all sensor devices. These base stations are the interface through which the WSN interacts with the outside world.

A wireless sensor device is typically called a mote. A mote is a battery-operated device, capable of sensing physical quantities. In addition to sensing, it is capable of wireless communication, data storage, and a limited amount of computation and signal processing as well.

Some of the leading manufacturers of wireless sensor networks are Crossbow Technology, Inc and Digi International. Crossbow Technology Inc. is the leading end-to-end solutions supplier in wireless sensor networks and inertial sensor systems.

Project Objective

When air conditioning larger size office spaces which contains more than one AC machine it is difficult to maintain a constant temperature through out the building. Mostly places where the air conditioners are on operation are cooler than other places. And also it is less cool where the entrance doors are situated and on occasions where more people are getting together in small areas. As a solution for this problem we will built a wireless sensor network attached to the air conditioners by which the air conditioners will communicate with each other and maintain a constant temperature throughout the building.

Problem Areas

Power Consumption

In a wireless sensor system, each node has a short-range transmission due to low radio frequency (RF) transmit power. Short-range transmission minimizes the possibility of the transmitted signals being eavesdropped; also, it helps in prolonging the lifetime of the battery. In some sensor system applications, the nodes are hard to reach and it is impossible to replace their batteries. In other applications, the nodes must operate without battery replacement for a long time. Such conditions make the system power consumption a very crucial parameter [2].

Frequency

Selection of the operating frequency for wireless sensor systems must comply with government regulations and wireless standards. Currently, frequencies used for wireless sensor systems include 315 MHz, 433 MHz, 868 MHz (Europe), 915 MHz (North America), and the 2.45-GHz Industrial-Scientific-Medical (ISM) band. The 2.45-GHz band provides implementation flexibility due to the abundance of commercially available RF devices in that band.

Using lower frequencies would help in extending the communications range due to low path loss attenuation. If the density of deployment allows for few meters spacing between the nodes, the choice of lower frequencies would be a good choice [2]. In contrast, for future wireless sensor systems that require very small size nodes and very high density of deployment, the best suited operating frequency might be the millimeter wave bands (70 GHz or above). The advantage of these high frequencies include small size antennas, frequency reuse, and low power consumption. However, these very short-range wireless links may involve routing issues.

Security Considerations

A key requirement from both the technological and commercial point of view is to provide adequate security capabilities. Realising privacy and security requirements in an appropriate architecture for WSNs offering pervasive services is essential for user acceptance. Three key research areas for developing secure and reliable WSNs: “Security & Reliability”, “Routing & Transport” and “In-network Processing” [3].

The current wireless sensor networks suffer from one or more of the limitations below:

• Specific mission dependent- requires pre-determined locations of sensors and nodes.

• Low level programming and tasking tools

• Sensor and node level manual tasking – applicable to small scale systems only

• Nodes and sensors loss vulnerability to the loss

• Operation and inter-sensor cooperation must be defined in advance and is limited by the topology

• Not adaptable to dynamic changes in the operating environment

• Very limited capabilities for expansion and for large scale deployments

• Operation schemes are not capable to operate in a context triggered mode inter-sensor cooperation.

Why people go to WSN

Wireless sensor network applications have enormous potential benefit for scientific communities and society as a whole. There is a tremendous potential for Wireless sensor networks (WSN) to become the future most ubiquitous tool of the industry and society. The wide adoption of WSN technology depends on affordability, standardization, the development of generic building blocks and on the availability of user-friendly customization tools.

Limitation

Fundamental limitations of wireless sensor networks impose a barrier to implement it in critical mission applications as well as in harsh outdoors conditions and applications (such as the monitoring of natural resources and the environment). Presently WSN are designed and utilized for very specific applications with limited number of sensors and nodes, each of these systems is Unique and specially tailored to its application. There is no affordable and adjustable system, available today for a generic range of applications. Currently available WSNs have limited scalability, as each sensor and node should manually be programmed, located and managed. Large scale expansion in these systems would require tremendous effort in planning the deployment, in tasking and in managing.

Project implementation Plan

The following resources will be required for the implementation of this project,

Hardware

• XBEE series 2 OEM RF Modules.

• XBEE Development kit.

• Thermal Sensing Devices, Thermometers

• Air Conditioners, Their Remote Controller circuit.

• Multi-meters, Project Boards, Power supplies

Software

• nesC programming language

• TinyOS operating system

• TOSSIM wireless sensor network simulator

• IEEE 802.15.4 and Zigbee protocol stack

Other Resources

• Product Manual v1.x.1x – Zigbee protocol

• IEEE 802.15.4 Documentation

A wireless sensor network for communication among the Air Conditioners will be developed. This will be developed using the zigbee protocol stack.

Zigbee is a communication protocol that has been developed to use on low power digital radios based on the IEEE 802.15.4 standard for wireless personal area networks (WPANs). This protocol is simpler than other WPANs such as Bluetooth and it is focused at providing applications that require a low data rate, long battery life, and secure networking [5].

Zigbee Protocol stack,

There are three different types of Zigbee devices,

1. Zigbee Coordinator - This device starts and controls the network. The coordinator stores information about the network, which includes acting as the Trust Center and being the repository for security keys.

2. Zigbee Router - These devices extend network area coverage, dynamically route around obstacles, and provide backup routes in case of network congestion or device failure. They can connect to the coordinator, other routers and end devices.

3. Zigbee End Devices - These devices can transmit or receive a message, but cannot perform any routing operations. They must be connected to either the coordinator or a router but they cannot connect to other end devices. Deliverables

• Stand alone application for controlling the overall temperature of the Air Conditioners of the building.

• Motes which communicate with each other and can control a temperature of an Air Conditioner remotely.

References

[1] “Wireless sensor network”, Wikipedia, Wikimedia Foundation, Inc., 2008. http://en.wikipedia.org/wiki/Wireless_sensor_network

[2] Roshdy Hafez, Ibrahim Haroun, Ioannis Lambadaris, “Building Wireless Sensor Networks”, Microwaves & RF, Penton Media, Inc., 2005. http://www.mwrf.com/Articles/ArticleID/11071/11071.html

[3] Dirk Westhoff, Joao Girao, Amardeo Sarma, “Security Solutions for Wireless Sensor Networks”, NEC Technical Journal, NEC Corporation, 2006. http://www.nec.co.jp/techrep/en/journal/g06/n03/060322.html

[4] Anna Parnes, “Self Configurable Automatic learning Large-scale WSN and High level application tool”, 2007. http://www.kpk.gov.pl/7pr/pp/i.html?id=1048

[5] “Zigbee”, Wikipedia, Wikimedia Foundation, Inc., 2008. http://en.wikipedia.org/wiki/ZigBee