Wireless sensor networks: a survey I.F. Akyildiz, W. Su *, Y. Sankarasubramaniam, E. Cayirci Broadband and Wireless Networking Laboratory, School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA Received 12 December 2001 accepted 20 December 2001 Abstract This paper describes the concept of sensor networks which has been made viable by the convergence of micro- electro-mechanical systems technology, wireless communications and digital electronics. First, the sensing tasks and the potential sensor networks applications are explored, and a review of factors influencing the design of sensor networks is provided. Then, the communication architecture for sensor networks is outlined, and the algorithms and protocols developed for each layer in the literature are explored. Open research issues for the realization of sensor networks are also discussed. �� 2002 Published by Elsevier Science B.V. Keywords: Wireless sensor networks Ad hoc networks Application layer Transport layer Networking layer Routing Data link layer Medium access control Error control Physical layer Power aware protocols 1. Introduction Recent advances in micro-electro-mechanical systems (MEMS) technology, wireless communi- cations, and digital electronics have enabled the development of low-cost, low-power, multifunc- tional sensor nodes that are small in size and communicate untethered in short distances. These tiny sensor nodes, which consist of sensing, data processing, and communicating components, le- verage the idea of sensor networks based on collaborative effort of a large number of nodes. Sensor networks represent a significant improve- ment over traditional sensors, which are deployed in the following two ways [39]: ��� Sensors can be positioned far from the actual phenomenon, i.e., something known by sense perception. In this approach, large sensors that use some complex techniques to distin- guish the targets from environmental noise are required. ��� Several sensors that perform only sensing can be deployed. The positions of the sensors and com- munications topology are carefully engineered. They transmit time series of the sensed pheno- menon to the central nodes where computations are performed and data are fused. A sensor network is composed of a large num- ber of sensor nodes, which are densely deployed either inside the phenomenon or very close to it. Computer Networks 38 (2002) 393���422 www.elsevier.com/locate/comnet * Corresponding author. Tel.: +1-404-894-5141 fax: +1-404- 894-7883. E-mail addresses: ian@ece.gatech.edu (I.F. Akyildiz), weil- ian@ece.gatech.edu (W. Su), yogi@ece.gatech.edu (Y. Sanka- rasubramaniam), erdal@ece.gatech.edu (E. Cayirci). 1389-1286/02/$ - see front matter �� 2002 Published by Elsevier Science B.V. PII: S1389-1286(01)00302-4

The position of sensor nodes need not be engi- neered or pre-determined. This allows random deployment in inaccessible terrains or disaster relief operations. On the other hand, this also means that sensor network protocols and algo- rithms must possess self-organizing capabilities. Another unique feature of sensor networks is the cooperative effort of sensor nodes. Sensor nodes are fitted with an on-board processor. Instead of sending the raw data to the nodes responsible for the fusion, sensor nodes use their processing abil- ities to locally carry out simple computations and transmit only the required and partially processed data. The above described features ensure a wide range of applications for sensor networks. Some of the application areas are health, military, and se- curity. For example, the physiological data about a patient can be monitored remotely by a doctor. While this is more convenient for the patient, it also allows the doctor to better understand the patient���s current condition. Sensor networks can also be used to detect foreign chemical agents in the air and the water. They can help to identify the type, concentration, and location of pollutants. In essence, sensor networks will provide the end user with intelligence and a better understanding of the environment. We envision that, in future, wireless sensor networks will be an integral part of our lives, more so than the present-day personal computers. Realization of these and other sensor network applications require wireless ad hoc networking techniques. Although many protocols and algo- rithms have been proposed for traditional wireless ad hoc networks, they are not well suited for the unique features and application requirements of sensor networks. To illustrate this point, the dif- ferences between sensor networks and ad hoc networks [65] are outlined below: ��� The number of sensor nodes in a sensor net- work can be several orders of magnitude higher than the nodes in an ad hoc network. ��� Sensor nodes are densely deployed. ��� Sensor nodes are prone to failures. ��� The topology of a sensor network changes very frequently. ��� Sensor nodes mainly use broadcast communica- tion paradigm whereas most ad hoc networks are based on point-to-point communications. ��� Sensor nodes are limited in power, computa- tional capacities, and memory. ��� Sensor nodes may not have global identification (ID) because of the large amount of overhead and large number of sensors. Since large number of sensor nodes are densely deployed, neighbor nodes may be very close to each other. Hence, multihop communication in sensor networks is expected to consume less power than the traditional single hop communication. Fur- thermore, the transmission power levels can be kept low, which is highly desired in covert opera- tions. Multihop communication can also effec- tively overcome some of the signal propagation effects experienced in long-distance wireless com- munication. One of the most important constraints on sensor nodes is the low power consumption requirement. Sensor nodes carry limited, generally irreplaceable, power sources. Therefore, while traditional net- works aim to achieve high quality of service (QoS) provisions, sensor network protocols must focus primarily on power conservation. They must have inbuilt trade-off mechanisms that give the end user the option of prolonging network lifetime at the cost of lower throughput or higher transmission delay. Many researchers are currently engaged in de- veloping schemes that fulfill these requirements. In this paper, we present a survey of protocols and algorithms proposed thus far for sensor networks. Our aim is to provide a better understanding of the current research issues in this field. We also at- tempt an investigation into pertaining design constraints and outline the use of certain tools to meet the design objectives. The remainder of the paper is organized as follows: In Section 2, we present some potential sensor network applications which show the use- fulness of sensor networks. In Section 3, we discuss the factors that influence the sensor network design. We provide a detailed investigation of current proposals in this area in Section 4. We conclude our paper in Section 5. 394 I.F. Akyildiz et al. / Computer Networks 38 (2002) 393���422

2. Sensor networks applications Sensor networks may consist of many different types of sensors such as seismic, low sampling rate magnetic, thermal, visual, infrared, acoustic and radar, which are able to monitor a wide variety of ambient conditions that include the following [23]: ��� temperature, ��� humidity, ��� vehicular movement, ��� lightning condition, ��� pressure, ��� soil makeup, ��� noise levels, ��� the presence or absence of certain kinds of ob- jects, ��� mechanical stress levels on attached objects, and ��� the current characteristics such as speed, direc- tion, and size of an object. Sensor nodes can be used for continuous sens- ing, event detection, event ID, location sensing, and local control of actuators. The concept of micro-sensing and wireless connection of these nodes promise many new application areas. We categorize the applications into military, environ- ment, health, home and other commercial areas. It is possible to expand this classification with more categories such as space exploration, chemical processing and disaster relief. 2.1. Military applications Wireless sensor networks can be an integral part of military command, control, communications, computing, intelligence, surveillance, reconnaissance and targeting (C4ISRT) systems. The rapid de- ployment, self-organization and fault tolerance characteristics of sensor networks make them a very promising sensing technique for military C4ISRT. Since sensor networks are based on the dense deployment of disposable and low-cost sensor nodes, destruction of some nodes by hostile actions does not affect a military operation as much as the destruction of a traditional sensor, which makes sensor networks concept a better approach for battlefields. Some of the military applications of sensor networks are monitoring friendly forces, equipment and ammunition bat- tlefield surveillance reconnaissance of opposing forces and terrain targeting battle damage as- sessment and nuclear, biological and chemical (NBC) attack detection and reconnaissance. Monitoring friendly forces, equipment and am- munition: Leaders and commanders can constantly monitor the status of friendly troops, the condi- tion and the availability of the equipment and the ammunition in a battlefield by the use of sen- sor networks. Every troop, vehicle, equipment and critical ammunition can be attached with small sensors that report the status. These reports are gathered in sink nodes and sent to the troop leaders. The data can also be forwarded to the upper levels of the command hierarchy while being aggregated with the data from other units at each level. Battlefield surveillance: Critical terrains, ap- proach routes, paths and straits can be rapidly covered with sensor networks and closely watched for the activities of the opposing forces. As the operations evolve and new operational plans are prepared, new sensor networks can be deployed anytime for battlefield surveillance. Reconnaissance of opposing forces and terrain: Sensor networks can be deployed in critical ter- rains, and some valuable, detailed, and timely in- telligence about the opposing forces and terrain can be gathered within minutes before the oppos- ing forces can intercept them. Targeting: Sensor networks can be incorporated into guidance systems of the intelligent ammuni- tion. Battle damage assessment: Just before or after attacks, sensor networks can be deployed in the target area to gather the battle damage assessment data. Nuclear, biological and chemical attack detec- tion and reconnaissance: In chemical and biological warfare, being close to ground zero is important for timely and accurate detection of the agents. Sensor networks deployed in the friendly region and used as a chemical or biological warning sys- tem can provide the friendly forces with critical reaction time, which drops casualties drastically. We can also use sensor networks for detailed I.F. Akyildiz et al. / Computer Networks 38 (2002) 393���422 395

reconnaissance after an NBC attack is detected. For instance, we can make a nuclear reconnais- sance without exposing a recce team to nuclear radiation. 2.2. Environmental applications Some environmental applications of sensor networks include tracking the movements of birds, small animals, and insects monitoring environ- mental conditions that affect crops and livestock irrigation macroinstruments for large-scale Earth monitoring and planetary exploration chemical/ biological detection precision agriculture biolog- ical, Earth, and environmental monitoring in ma- rine, soil, and atmospheric contexts forest fire detection meteorological or geophysical research flood detection bio-complexity mapping of the environment and pollution study [2,6���8,10,11,14, 31,35,39,40,42,61,81,88,89]. Forest fire detection: Since sensor nodes may be strategically, randomly, and densely deployed in a forest, sensor nodes can relay the exact origin of the fire to the end users before the fire is spread uncontrollable. Millions of sensor nodes can be deployed and integrated using radio frequencies/ optical systems. Also, they may be equipped with effective power scavenging methods [12], such as solar cells, because the sensors may be left unat- tended for months and even years. The sensor nodes will collaborate with each other to perform distributed sensing and overcome obstacles, such as trees and rocks, that block wired sensors��� line of sight. Biocomplexity mapping of the environment [11]: A biocomplexity mapping of the environment re- quires sophisticated approaches to integrate in- formation across temporal and spatial scales [26,87]. The advances of technology in the remote sensing and automated data collection have en- abled higher spatial, spectral, and temporal reso- lution at a geometrically declining cost per unit area [15]. Along with these advances, the sensor nodes also have the ability to connect with the Internet, which allows remote users to control, monitor and observe the biocomplexity of the environment. Although satellite and airborne sensors are useful in observing large biodiversity, e.g., spatial complexity of dominant plant species, they are not fine grain enough to observe small size biodiver- sity, which makes up most of the biodiversity in an ecosystem [43]. As a result, there is a need for ground level deployment of wireless sensor nodes to observe the biocomplexity [29,30]. One example of biocomplexity mapping of the environment is done at the James Reserve in Southern California [11]. Three monitoring grids with each having 25��� 100 sensor nodes will be implemented for fixed view multimedia and environmental sensor data loggers. Flood detection [7]: An example of a flood de- tection is the ALERT system [90] deployed in the US. Several types of sensors deployed in the ALERT system are rainfall, water level and weather sensors. These sensors supply information to the centralized database system in a pre-defined way. Research projects, such as the COUGAR Device Database Project at Cornell University [7] and the DataSpace project at Rutgers [38], are investigating distributed approaches in interacting with sensor nodes in the sensor field to provide snapshot and long-running queries. Precision Agriculture: Some of the benefits is the ability to monitor the pesticides level in the drink- ing water, the level of soil erosion, and the level of air pollution in realtime. 2.3. Health applications Some of the health applications for sensor net- works are providing interfaces for the disabled integrated patient monitoring diagnostics drug administration in hospitals monitoring the move- ments and internal processes of insects or other small animals telemonitoring of human physio- logical data and tracking and monitoring doctors and patients inside a hospital [8,42,60,71,88]. Telemonitoring of human physiological data: The physiological data collected by the sensor net- works can be stored for a long period of time [41], and can be used for medical exploration [62]. The installed sensor networks can also monitor and detect elderly people���s behavior, e.g., a fall [9,16]. These small sensor nodes allow the subject a 396 I.F. Akyildiz et al. / Computer Networks 38 (2002) 393���422

greater freedom of movement and allow doctors to identify pre-defined symptoms earlier [56]. Also, they facilitate a higher quality of life for the sub- jects compared to the treatment centers [5]. A ������Health Smart Home������ is designed in the Faculty of Medicine in Grenoble������France to validate the feasibility of such system [60]. Tracking and monitoring doctors and patients inside a hospital: Each patient has small and light weight sensor nodes attached to them. Each sensor node has its specific task. For example, one sensor node may be detecting the heart rate while another is detecting the blood pressure. Doctors may also carry a sensor node, which allows other doctors to locate them within the hospital. Drug administration in hospitals: If sensor nodes can be attached to medications, the chance of getting and prescribing the wrong medication to patients can be minimized. Because, patients will have sensor nodes that identify their allergies and required medications. Computerized systems as described in [78] have shown that they can help minimize adverse drug events. 2.4. Home applications Home automation: As technology advances, smart sensor nodes and actuators can be buried in appliances, such as vacuum cleaners, micro-wave ovens, refrigerators, and VCRs [67]. These sensor nodes inside the domestic devices can interact with each other and with the external network via the Internet or Satellite. They allow end users to manage home devices locally and remotely more easily. Smart environment: The design of smart envi- ronment can have two different perspectives, i.e., human-centered and technology-centered [1]. For human-centered, a smart environment has to adapt to the needs of the end users in terms of input/ output capabilities. For technology-centered, new hardware technologies, networking solutions, and middleware services have to be developed. A sce- nario of how sensor nodes can be used to create a smart environment is described in [36]. The sensor nodes can be embedded into furniture and appli- ances, and they can communicate with each other and the room server. The room server can also communicate with other room servers to learn about the services they offered, e.g., printing, scanning, and faxing. These room servers and sensor nodes can be integrated with existing em- bedded devices to become self-organizing, self- regulated, and adaptive systems based on control theory models as described in [36]. Another example of smart environment is the ������Residen- tial Laboratory������ at Georgia Institute of Techno- logy [21]. The computing and sensing in this environment has to be reliable, persistent, and transparent. 2.5. Other commercial applications Some of the commercial applications are mon- itoring material fatigue building virtual key- boards managing inventory monitoring product quality constructing smart o���ce spaces environ- mental control in o���ce buildings robot control and guidance in automatic manufacturing envi- ronments interactive toys interactive museums factory process control and automation moni- toring disaster area smart structures with sensor nodes embedded inside machine diagnosis trans- portation factory instrumentation local control of actuators detecting and monitoring car thefts vehicle tracking and detection and instrumenta- tion of semiconductor processing chambers, ro- tating machinery, wind tunnels, and anechoic chambers [2,8,14,23,24,42,63,69���71,77,88]. Environmental control in o���ce buildings: The air conditioning and heat of most buildings are cen- trally controlled. Therefore, the temperature inside a room can vary by few degrees one side might be warmer than the other because there is only one control in the room and the air flow from the central system is not evenly distributed. A dis- tributed wireless sensor network system can be installed to control the air flow and temperature in different parts of the room. It is estimated that such distributed technology can reduce energy consumption by two quadrillion British Thermal Units (BTUs) in the US, which amounts to saving of $55 billion per year and reducing 35 million metric tons of carbon emissions [71]. I.F. Akyildiz et al. / Computer Networks 38 (2002) 393���422 397