04-05-2011, 12:23 PM
Abstract
Wireless sensor networks – networks of smalldevices equipped with sensors, microprocessor and wirelesscommunication interfaces – are a technology that has gaineda lot of interest lately. The broad spectrum of new andinteresting applications, ranging from personal health-care toenvironmental monitoring and military applications, isproposed for such networks.Various wireless technologies, like simple RF, Bluetooth,UWB or infrared might be used for communication betweensensors. In this paper the main principles, applications andissues of Bluetooth based wireless sensor networks, as wellas an implementation of a simple Bluetooth based sensornetwork are described. The main problems experiencedduring the implementation and applied solutions arepresented.
1. Introduction
In a vision, called ubiquitous computing, of the worldwhere humans and computers were seamlessly united isdescribed. The essence of the vision was the creation ofenvironments saturated with computing and communicationin an unobtrusive way. Recently, WWRF (Wireless WorldResearch Forum) and ISTAG (IST Advisory Group) releasedtheir visions of the future communication networks [2], [3].Both institutions envisage a vast number of variousintelligent devices, embedded in the environment, sensing,monitoring and actuating the physical world, communicatingwith each other and humans.Today, we are witnessing a rapid proliferation of variouswireless devices with considerable computing power andfairly small size. These devices are still not, to paraphraseMarc Weiser [1], indistinguishable from the fabric ofeveryday life, but are becoming a part of that fabric(Bluetooth headset built as a part of jewellery for example).Various sensors are already in a broad use today as part ofdifferent devices (temperature sensors in home or car heatingsystem, smoke alarms, etc.) or as standalone devicesconnected to a network, usually to monitor industrialprocesses, equipment or installations.The advancements in MEMS (micro-electrical-mechanicalsystems) technology, wireless communications and electriccomponents have enabled development of small, low-powerand low-cost devices, called smart sensor nodes, capable ofperforming various sensing tasks, processing data andcommunicating over wireless connections. Such devices,when organized into a network, present a powerful platformthat can be used in many interesting applications, like healthmonitoring, security systems, detection of chemical agents inair and water etc. During the last few years significantresearch efforts were focused on development of requiredhardware resources [4], [5], [6], [7] and efficientcommunication protocols [8], [9], [10], [11], [12], [13], [14],to enable networking and collaboration of smart sensornodes.In the next section the main principles of wireless sensornetworks are given and research issues are explained. Section3 presents Bluetooth issues related to its use in sensornetworks. An implementation of a Bluetooth based sensornetwork is described in section 4 along with some of theimplementation issues and solutions. Section 5 concludes the paper
2. Wireless sensor networks
Wireless sensor networks comprise number of small devicesequipped with a sensing unit, microprocessor, wirelesscommunication interface and power source. In contrast to thetraditional sensor networks that are carefully planned anddeployed to the predetermined positions, wireless sensornetworks can be deployed in an ad-hoc manner.Of course, such deployment requires adequatecommunication protocols that are able to organize thenetwork automatically, without the need for humanintervention.Beside self-organization capability, another important featureof wireless sensor networks is collaboration of network nodesduring the task execution. In contrast to the traditional sensornetworks where all sensor data is gathered at a server andthen analysed and fused, data processing and fusion is nowperformed by smart nodes themselves. Each node processesraw measurement data in order to decrease amount of datasent over wireless links and forwards only relevant parts tonodes responsible for data fusion.Data-centric nature of the network is yet another specificcharacteristic of wireless sensor networks. As deployment ofsmart sensor nodes is not planned in advance and positions ofnodes in the field are not determined, it could happen thatsome sensor nodes end in such positions that they eithercannot perform required measurement or the error probabilityis high. That is why a redundant number of smart nodesobserving the same phenomenon is deployed in the field.These nodes then communicate, collaborate and share data,thus ensuring better results (each sensor has its own view ofthe phenomenon – when these views are combined a betterpicture of the phenomenon is obtained). Having this in mind,it is more reasonable for a user to send a data request to allsensors monitoring the phenomenon than to send it to onespecific sensor node. Using a multicast routing protocol tosend messages to all relevant nodes would require uniqueaddressing scheme in the network. However, due to the sheernumber of sensors and user requirements (user needsinformation about the phenomenon, does not needinformation about the phenomenon from a particular sensor),data-centric approach is used where sensors are designatedusing description of data they can provide instead of usingunique IDs. Messages are directed to nodes using routingprotocols that can find the route based on the data descriptioncontained in the message.Power efficiency is one of the main requirements for allprotocols and algorithms used in these networks. As powerresources of each node are limited and required lifetime formany scenarios is measured in months and even years, it is ofparamount importance to design system in such a way toensure power savings whenever possible
2.1. Querying and tasking
From the user point of view, querying and tasking are twomain services provided by wireless sensor networks [14].Queries are used when user requires only the current value ofthe observed phenomenon. As wireless sensor networks aredata-centric networks, the user does not query a specific nodefor the information it might provide, but defines data (type,location, accuracy, time, etc.) he/she is interested in andrequests it from all nodes that can provide the answer. Forexample, user can look for “temperature in the north regionof the observed area” or for “location of all sensors wherechemical agents are present and their level is above thethreshold”.Tasking is a more complex operation and is used when aphenomenon has to be observed over a longer period of time.For example, a user can ask a sensor network to detect aspecific type of vehicle in the area and to monitor itsmovement. In order to execute the task, different types ofsensors have to collaborate: seismic to detect motion, videoand audio to detect type of vehicle etc. Information about thevehicle trajectory is forwarded to the user.Both queries and tasks are injected to the network by thegateway which also collects replies and forwards them tousers.
2.2. Gateway functionality
Smart sensor nodes scattered in the field collect data and sendit to users via gateway using multiple hop routes (Figure 1).
1. A Wireless Sensor Network
The main functions of a gateway are the following:− Communication with sensor network - Short-rangewireless communication is used (Bluetooth, UWB, RF,IR, etc.). Provides functions like discovery of smartsensor nodes, generic methods for sending and receivingdata to and from sensors, routing, etc.;− Gateway logic – Controls gateway interfaces and dataflow to and from sensor network. It provides anabstraction level with API that describes the existingsensors and their characteristics; provides functions foruniform access to sensors regardless of their type,location or network topology, inject queries and tasksand collect replies;− Communication with users – Gateway communicateswith users or other sensor networks over the Internet,wide area networks (GPRS, UMTS), satellite or someshort-range communication technology.It is possible to build a hierarchy of gateways, i.e. to connectgateways described above to a backbone and then to providea higher-level gateway that is used as a bridge to othernetworks and users
2.3. Applications
The features described above ensure a wide range ofapplications for sensor networks. Some of the possiblescenarios are given below:
• Health monitoring - Wireless sensor networks can beused in various ways to improve or enhance health careservices. Monitoring of patients, health diagnostics, drugadministration in hospitals, telemonitoring of humanphysiological data and tracking and monitoring doctorsand patients inside a hospital are some of the possiblescenarios [16], [17], [18], [19].Various sensors (blood pressure, heart monitoring, etc.)can be attached to the patient’s body to collectphysiological data that can be either stored locally (on aPDA or home PC) or forwarded directly to the hospitalserver or to the doctor in charge. There are severaladvantages of such monitoring: it is more comfortablefor patients, doctors can have 24 hours access to patientsand can better understand the patient’s condition andlast, but not the least, incurred expenses are much lessthan when such tests are done in hospitals.Wearable sensors can also be used to track patients anddoctors in the hospital or to monitor and detect behaviourand health condition of elderly persons and children.
• Environmental monitoring - Fire detection, waterpollution monitoring, tracking movements of birds,animals or insects, detection of chemical and biologicalagents are some of the examples of environmentalapplications of wireless sensor networks [20], [21], [22],[23].For example, numerous smart sensor nodes withtemperature sensors on board can be dropped from anairplane over a remote forest. After successful landing,these devices will self-organise the network and willmonitor temperature profile in the forest. As soon as thefire starts, that information along with the location of fireis transferred to the command centre that can act beforethe fire spreads to cover a large area.
• Military and security - The initial push towards wirelesssensor network research came from military agencies.Military applications are various and vary frommonitoring soldiers in the field, to tracking vehicles orenemy movement.Sensors attached to soldiers, vehicles and equipment cangather information about their condition and location tohelp planning activities on the battlefield.
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