Wireless Sensor Systems and Networks

Wireless Sensor Systems and Networks: Technologies, Applications, Implications and Impacts

David J. Nagel Professor of Engineering and Applied Science The George Washington University

This topical review draws upon presentations and discussions at a workshop convened by the Center for Strategic and International Studies on May 19, as part of the “Technology Futures and Global Power, Wealth and Conflict” project supported by the National Intelligence Council. A workshop agenda is appended.

Executive Summary The generation, communication, processing, storage, and use of information in digital form define the current age. Progress is continually being made for all these functions. In the recent past, major advances in sensors, which produce new kinds of information, and wireless links, which communicate that information to users, have enabled new capabilities and a nascent industry. The new industry is largely built on the foundation of micromachining processes for manufacture of microelectromechanical systems (MEMS) that grew out of the microelectronics industry. It also exploits the increasing capabilities of wireless technologies for voice and image transmission in the cell-phone and related industries. Wireless links, which offer highperformance communication of sensor or any information, are usually cheaper to buy and use compared to wired links, and can be reconfigured much easier. More significant, they enable retrieving information from mobile sensors, as well as delivering it to users on the move. That is, they serve both the "first mile" and the "last mile" in acquiring and delivering sensor information. Ubiquitous and embedded, often invisible and silent, sensing technologies will be one of the hallmarks of this century. Pervasive sensing is now in its early stages of development and adoption. Distributed computing is a growing reality. The communications of any information

to or from anywhere at anytime is another new and remarkable capability. The combination of these individually important technologies will have even greater effects. The separate and integrated capabilities will be increasingly a part of the fabric of life in technological societies, with consequences on many levels from the solely technical to the broadly social. Wireless sensor technologies are expected to have effects that range from common objects being "sentient" all the way to changes, which impact so many individuals that they are cultural. It is thought that wireless sensor systems and networks will become as noteworthy in this decade as did microcomputers in the 1980s and the Internet in the 1990s. The combination of very widely distributed sensors and the ability to interrogate them leads to the ability to "browse reality" in close to real-time, in addition to information stored at earlier times. "Reality mining" will be possible, similar in many ways to data mining. The essential point is the melding of the "real" world with the information sphere by virtue of being able to monitor many kinds of parameters and events in many places over long periods, often times with high spatial and temporal resolution. Wireless sensor systems, which are often called wireless sensors, use pre-determined, sometimes-single-hop radio-frequency links to transfer information from the originating sensor node through other nodes, routers, and gateways to users. Wireless sensor networks, with many sensor and communication nodes, provide situation-determined multi-hop paths for the information to flow from the source to the destination. Such networks have two important features. They can be ad hoc, that is, self-organizing, which greatly simplifies both their deployment and employment. Also, their configuration in two-dimensional arrays called meshes enables information to travel over alternative available paths. This greatly increases the reliability of information delivery. It makes possible many industrial applications for which the precise control of processes is critical. Wireless sensor technologies enable two primary functions. The first is monitoring, for which information flows from the field to the user. The second is control, that is, management of the sensor system itself or the environment in which it is embedded. Such distributed monitoring and control within a local region, based on the sensors and the nodes into which they are incorporated, are qualitatively new capabilities. Because wireless sensor systems can be integrated with the cellular-, satellite-, and Internet-communication systems and networks, information both from the monitoring function and for control can span the globe, even if the wireless sensor technology is local in its reach. The more specific functions that wireless sensor technologies will perform are distributed and collaborative sensing, detection, and tracking; location determination and event recording; computing and signal processing; data and information management, aggregation and storage; and query processing. Their behavior will be collaborative, and it may also evolve in response to the conditions they encounter. The integration of multiple miniature technologies is fundamental to short-range wireless sensor systems and networks. The convergent technologies include the micro-machined sensors, application-specific and other integrated circuits, small but very capable computers, flash memories and chip-scale radios. The MEMS and other sensors being used in short-range, generally line-of-sight systems and networks can measure a wide variety of physical, chemical and biological parameters. Small imagers, including both visible and temperature-sensitive infrared cameras, are incorporated into wireless systems and networks. Forwarding their information usually requires greater transmission rates (bandwidth). Short-range radars and other sensors are now being integrated into wireless technologies.

Nagel 2

Just as individual sensors evolved to become "smart" by being mated with computers, smart sensors are now becoming wireless. Given their small size and low power, multiple sensors can share a given power source, computer, memory, and wireless link in order to perform an entire function, such as monitoring atmospheric conditions at the position of a node. In a fashion similar to the evolution of sensors, networks have gone from being entirely wired, to point-topoint wireless to peer-to-peer mesh arrangements. Moore's Law about the increase in microelectronics capabilities leads to continual advances in component capabilities. Metcalf's Law on the increasing utility of networks as the number of users grows indicates the value of mesh networks that may have thousands of nodes. These trends are related because advances in microelectronics bring down the costs of nodes, making the deployment and employment of very large networks cost effective. It costs money both to have and not to have information. The high-level aim of wireless sensor technologies is to make the value of acquiring old types of information better and cheaper, as well as getting entirely new information, significantly outweighing the cost of their installation and operation. The "value chain" starts with individual components and goes through the network level to the applications and their enterprise-wide payoffs. The flow is from data to information to knowledge, understanding, and wisdom. Because of the intellectual challenges in the design, simulation, fabrication, testing, and operation of wireless sensor networks, and because of their many applications, there has been an explosion of interest in distributed sensing technologies. The subject of wireless sensor networks is already recognized as a discipline, with centers and institutes that were created at several universities in the recent past. The literature on wireless sensor networks is already large and is growing rapidly. Books on the topic have appeared in the past few years and their rate of publication is increasing. Many universities and companies have developed prototype wireless sensor nodes, systems, and networks in the past decade. Networks with over one thousand sensor nodes have been demonstrated, and networks with about one million nodes are being modeled. There are many advantages and challenges within the complexity of wireless sensor technologies. These generate both enthusiasm and concerns within companies that might employ wireless sensor systems and networks. Since the turn of the millennium, increasingly many usually small companies sell wireless sensors. They are finding that customers want the lower costs and flexibility that go with wireless connectivity. Over a dozen companies now sell generally low-cost wireless sensor networks, or the communications hardware and software for such networks. Most of them were founded in the past few years. Some companies started with a sensor focus and others with wireless communications as their major expertise. Wireless-sensor-network companies have usually integrated available chips into their nodes, although some have designed integrated circuits specifically for wireless sensor usage. Some network products come with a suite of commonly used sensors already integrated with the communication nodes; others provide for the integration of user-specific sensors for particular applications. Roughly 100,000 wireless sensor nodes have been sold to date. Some large companies, notably Intel, are involved in both the development and use of wireless sensor technologies. Others, such as BP, with its sensor network initiative, and Royal Philips Electronics, which seeks to automate lighting and other building functions, are among the early adopters of wireless sensor networks. Barriers to adoption of wireless sensor technologies are decreasing both because of competition within user industries and the increasing ease of acquisition, installation, and operation. Industrial expositions devoted to wireless sensor systems and networks are now being scheduled. Some of them focus on the low-power radios and wireless-transmission standards (the "chips and stacks") Nagel 3

that are fundamental to making widely distributed sensors highly valuable. It remains to be seen if the wireless sensor industry will become large and distinct enough to have its own association, magazines, and conferences. Dozens of applications for wireless sensor networks have been described, and hundreds more are expected. They are causing a great increase in the variety and amount of information that can be obtained in real time from areas of interest. The applications can be grouped into several major areas that are largely aligned with major industries. The categories overlap somewhat because some specific applications apply to more than one area. However, the categories provide a useful way to organize the current and prospective applications of wireless sensor networks. The areas, and some of the more noteworthy applications, are noted here. This is but a sampler of the applications described in the text, which are themselves only part of the picture for the utility of wireless sensors. • Weather, Environment, and Agriculture. Monitoring environmental parameters, including weather and pollutants, has many applications. Assessment of the state of hydration is important for agriculture, and for control of lawn sprinklers. The ability to track and measure the activities of large animals in herds will be important. •

Factories, Facilities, Buildings, and Homes. Industrial automation should be one of the largest applications of wireless sensor technologies. The control of the manufacturing and processing steps that are distributed within large factories should be a major market. Control systems for the heating, ventilation, and air conditioning within most structures might lead to significant energy savings.

•

Transportation Systems and Vehicles. Smart highways, which facilitate better traffic routing, will employ wireless sensor technologies. Many vehicles will have wireless tirepressure monitors. Robotic systems are made possible by wireless sensor systems and networks. They are expected to have many applications, including near-term uses for housekeeping and lawn mowing, and to longer-range, less-certain uses.

•

Safety, Health and Medical. Monitoring unsafe working conditions and the routine living of people with health problems are large and imminent applications. Wireless sensor technologies will find growing use for monitoring the activities of elderly people, especially with the aging population in the U.S.

•

Security, Crisis Response, and Military Operations. Wireless communications will have major applications for monitoring secure areas with both cameras and point sensors. Wireless sensor systems are being put into shipping containers now to insure their integrity during transits. Both installed and deployable wireless sensor systems and networks will be useful in crises varying from fires in large buildings to terrorist attacks. There are many military applications of wireless sensor networks. Some are closely related to civilian uses, such as machinery monitoring. Others are more uniquely military, such as the use of unattended ground sensors on the battlefield.

•

Infrastructure and Other Applications. Some fixed facilities, notably government buildings, air and sea ports, hospitals, and sources of food, water, and power, are critical to the functioning of a complex society. Local wireless sensor technologies will be used increasingly to operate and protect them. Transportation systems, pipelines for water, gas, and oil and electrical- power-distribution systems are also fundamentally important.

Nagel 4

Wireless sensor technologies will be used to monitor such distributed facilities. A strong synergy is developing between wireless sensor systems and radio-frequency identification (RFID) technologies. The nodes in wireless sensor systems can serve as either active (powered) RFID tags or as readers for small and cheap passive (unpowered) RFID tags. This will increase the utility and sales of wireless sensor systems and networks. Many opportunistic applications of wireless sensor systems and networks can be expected as they are more widely used for targeted applications, come down in price, and become more user friendly. The technologies are already being used for creative student research projects. Several current implications of local wireless sensor systems and networks are getting attention. Possible future impacts of the technologies are also under discussion. Both the nearand far-term effects cut across policy, regulatory, and legal regimes. Some regulatory issues are important to wireless sensor systems and networks, but they are now driven by other technological and market forces. Major implications under active consideration are spectrum allocation and control of information, especially information security and personal privacy. The large, contentious, and overlapping areas of security and privacy are of widespread concern. • Spectrum Allocation. The radio-frequency bands, which can be employed for wireless transmission of information from sensors, and the emitted powers, which are used for the communication links, are specified by governments. This is fundamentally necessary because unregulated transmissions would lead to unacceptable and costly interference between signals in various wireless systems and networks. However, it leads to two problem areas. Within one country, there is strong competition for commercially useful frequencies. And, there are instances of different bands being available for the same purpose in different countries. This leads to balkanization of commercial technologies, and hampers the international utilization of cell phones and shipping containers with wireless sensor systems embedded within them. •

Information Security. The control of information flowing from sensor nodes is a critical concern with many facets. Because wireless signals are easy to intercept, information security is a subject of special concern for sensor systems. Both message confidentiality (secrecy) and message integrity (correctness) are beginning to get necessary study. The limited resources for power, calculations, and information storage on small sensor nodes greatly constrain the security services that can be embedded within wireless sensor networks.

•

Personal Privacy. The control of information about individuals is an issue closely related to information security. It also causes a diversity of viewpoints. The individual is pitted against two groups regarding privacy. The first is organizations, including especially employers, who want information on people for reasons ranging from physical security to the costs of health care. There is a further need for balance between individual privacy and societal protection against terrorism, an arena that is still in a state of flux following the 9/11 attacks on the U.S. Examination of these implications of local wireless sensor systems and networks shows that they all fall within the continuum from cooperation to competition and on to combat. Spectrum allocation is a balance between cooperation and competition. Security involves cooperation between legitimate partners on a communication link, and combat between people who want to

Nagel 5

access information and those trying to restrict such access. Privacy requires cooperation among individuals and larger groups, even as it inescapably involves competition over the rights of the same entities. Many future impacts of wireless sensor technologies are worth considering. The impacts that will actually develop are uncertain because they will depend both on the degree of use of the new wireless sensor technologies and on decisions made about the near-term implications. Local wireless sensor networks will impact individuals, organizations, and nations in both foreseeable and unexpected ways. Whether the advantages will greatly outweigh the drawbacks for personal use, industrial applications and national security will not be until local wireless sensor networks come into true widespread use. The contemplated impacts are: • Individuals. Given the many applications for safety, health, and medicine, it is very likely that the impacts of wireless sensor systems and networks on individuals will be both early and favorable. If adoption of the technologies also leads to more energyefficient homes, there will also be monetary advantages to individuals and families. These two positive impacts might be balanced by the potentially negative impact of lessened privacy due to sensor technologies. •

Industrial. Companies are also likely to benefit greatly and soon from the commercial availability of local wireless sensor technologies. The chance to perform current functions with higher reliability at lower costs is a great inducement to adoption of wireless sensor technologies. The possibility of being able to do things, both for monitoring and control that are not feasible now, is also attractive. Having information from more points of manufacturing or processing functions will improve understanding, enable better control, and reduce costs.

•

Education. There are two potential educational impacts of wireless sensor technologies. The first is the need for education of people who will work in companies that provide the technologies and of people in user organizations. The required skills range from advanced design, simulation, fabrication, and testing skills to technical credentials for operation and maintenance of the technologies. If wireless sensor technologies do become pervasive, then there might also be a need to educate the populace quite broadly about their characteristics and issues that derive from their use. The current discontent over genetically modified foods provides an example of how not to introduce a new technology to society at large.

•

Society and Government. There is a possibility that local wireless sensor technologies could become part of the fabric of life in advanced societies. It is very unlikely that sensors will be anywhere nearly as widely adopted as cars, television, computers, and the Internet. However, it seems possible to some people that small sensors will be embedded in most manufactured objects. The good and bad impacts of pervasive sensing remain to be determined. It is a responsibility of government to monitor, and influence by policy, regulations and laws, the effects of wireless sensor technologies, in order to improve the positive aspects and mitigate negative effects.

•

Homeland and National Security. If sensors do become a significant new source of information, and because information security is now basic to homeland security, the protection of distributed sensor networks becomes an issue. The direct use of wireless sensor technologies for homeland security is likely to be very important. Applications

Nagel 6

vary from monitoring the atmosphere with fixed sensors within public places, like subway stations, to the mobile instruments taken to the scene of a chemical spill or attack. The protection of major public events, ranging from conventions to sport contests, is another area in which wireless sensor technologies will promote security. However, the use of local wireless sensor networks by terrorists to determine the timing for the most-damaging attacks is a worrisome eventuality. Military applications of local wireless sensor technologies are likely to be very significant. The Department of Defense has already adopted "network-centric warfare" as both policy and practice. The Department of Homeland Security is being urged to follow suit. •

Economic. The economic impacts of new local wireless sensor systems and technologies will fall into two categories. The first is the new and growing industry to develop and sell the technology. There are projections asserting that there will be tens of millions of nodes in use by the year 2010. If they cost about $100 on average, the industry might sell some $1B of hardware and associated software in the coming years. It should be noted that there are also projections that say the cost of nodes, exclusive of sensors, will be below $10 by 2010. The situation might well be like the history for many electronic products, from hand calculators to cell phones, where the volume increases steeply and the unit cost plummets at the same time. The second impact of wireless sensor technologies is on organizations, especially companies, which can either save money performing current functions or do new things that make money. The dollar volume for replacement or new functions is quite inscrutable now. Looking further into the future, local wireless sensor technologies might result in unexpected, possible emergent effects. Whether or not that occurs, there are at least four historic possibilities. One deals with large numbers of robots with many sensor and wireless communication links amongst themselves. Will they exhibit new behaviors? The second has to do with the nature of computing in the coming decades. Can distributed sensors lead to proactive, that is, anticipatory computing? The third centers on the question of whether or not the Internet can become conscious. If it did, how would it interact with humans? Finally, the melding of humans and machines is considered. What if our thoughts could control systems that are able to wirelessly communicate with others’ brains? Could thoughts be shared, much as files are now transferred over the Internet? • Robot Swarms. The same miniature sensors, communications, and other technologies that make possible and go into wireless sensor nodes can be used to manufacture large numbers of small, mobile robots. Many people now think that groups of hundreds of such robots could display emergent behavior, that is, functions that the individual units cannot do by themselves. Ants, bees, termites, birds, and fish are among the natural entities that exhibit emergent swarm behaviors. They provide impetus to current research on robot swarms. Such collective behavior might have military utility before it becomes important in society at large. •

Proactive and Trusted Computing. The first prospective effect of local wireless sensor systems and networks is evolutionary, but would be really important. Computing has evolved from mainframe machines to desktop and hand-held systems. Embedded, ubiquitous computing is coming soon. Having many sensors in the environment might lead to, or at least, facilitate proactive and trusted computing. Anticipatory actions on the part of computers, and their ability to recognize individuals, are in prospect. Nagel 7

•

Conscious Internet. While some people believe that man-made systems cannot become consciousness, others are working toward the development of such a capability. Progress is limited by the current lack of understanding of the basic nature of consciousness. However, systems, especially the Internet, already exhibit many of the characteristics of consciousness. Prime among them is self-awareness. The Internet is designed to monitor its conditions and traffic. Adding large numbers of sensors to the world would make it possible for the Internet to know the status of its environment. The development of an Internet consciousness, and even more, its interactions with humans, however speculative now, would change the world.

•

Melding Systems and Humans. Another historic change, which is much more likely and still very profound, is the integration of artificial systems with people. First, mechanical, and then electrical systems have been implanted within individuals. Electrodes embedded in the brain now enable control of exterior systems by "merely" thinking. That is, the human-computer interface is already within the human. The shrinking of semiconductor-based electronics will lead to having significant storage media within people for identification and medical records. Sensors will be increasingly implanted to measure glucose levels in diabetics and for other reasons. The development of organic electronic materials might overcome biocompabibility problems that now restrict the use of some materials within the body. The sequence from science to technology to engineering to manufacture and sale of products, with its ultimate impacts on society, is relevant to wireless sensor technologies. Academic and other researchers focus on the science and technology. The business interests of companies span technology, engineering, and the development and sale of products. Societies are impacted by both the commercial aspects and actions of their governments. Wireless sensor systems and networks are complex at all levels from the material to the cultural. Hence, they offer many challenges that might be overcome and turned into opportunities. The wide variety of technical challenges dominantly includes the longevity of the network nodes. Reduction in the power required and development of better power sources are both getting much attention. The development of software for everything from design and simulation of sensor nodes to the optimization of network architecture, as well as efficient security and embedded network monitoring, will continue to require major attention. Software for the codesign and co-simulation of wireless sensor technologies, which is coupled across all levels from the material to the economic, is a clear possibility, but a daunting challenge. Many other technical and higher-level challenges will influence or limit the technologies, applications, implications, and impacts of local wireless sensor systems and networks.

Nagel 8

Outline 1. Introduction to Wireless Sensor Systems and Networks A. Types of Sensors B. Wireless Communications C. Large-Area Wireless Sensor Systems and Networks D. Organization of the Report 2. Local Wireless Sensor Systems and Networks A. Foundational Technologies and Resultant Opportunities B. Comparison of Wireless Sensor Systems and Networks C. Primary Functions: Monitoring and Control D. Advantages and Challenges 3. Technologies for Current Wireless Sensors System and Networks A. Prototype Wireless Sensor Nodes B. Commercial Wireless Sensor Systems C. Commercial Wireless Sensor Networks 4. Applications of Wireless Sensor Systems and Networks A. Weather, Environment, and Agriculture B. Factories, Facilities, Buildings, and Homes C. Transportation Systems and Vehicles, including Mobile Robots D. Safety, Health, and Medical E. Security, Crises, and Military F. Infrastructure and Other Applications 5. Implications of Wireless Sensor Technologies and Their Applications A. Spectrum Allocation B. Control of Information: Security C. Control of Information: Privacy D. Cooperation, Competition, and Combat 6. Impacts on Individuals, Organizations and Nations A. Individuals at Home, Work, and Elsewhere B. Industrial C. Education D. Society and Government E. Homeland and National Security F. Economic 7. Conclusion A. Robot Swarms B. Proactive and Trusted Computing C. The Conscious Internet D. Melding Systems and Humans Nagel 9

Acknowledgments Appendix A: Workshop Agenda and Speakers' Contact Information Appendix B: Component and Integrated Technologies A. B. C. D. E.

Wireless Sensor and Other Nodes Wireless Communication Links Network Architectures and Deployment Design and Simulation of Wireless Sensor Nodes, Systems, and Networks Technical Challenges

Nagel 10

1. Introduction to Wireless Sensor Systems and Networks Information has always been crucial to people, however it is obtained and used. So, why is the current era called the Information Age? The most fundamental reason is the widespread hardware and software capabilities that now exist for the acquisition, communication, processing, storage, and utilization of information. Each of these five basic information functions has been growing dramatically because of advances in micro-electronics and micromagnetics, and in the radio-frequency and optical communications that they enable. The five functions are indicated in Figure 1. The general status of each of them now and in the future deserves attention. B y P e o p le , C o m p u te r s & Sensors

P R O C E S S IN G

G E N E R A T IO N

C O M M U N IC A T I O N

B y P e o p le , C o m p u te r s & D iv e r se S y s te m s

U T IL I Z A T I O N

STOR AGE

Figure 1. The five basic information functions. All of these are growing rapidly because of improvements in technology, and the introduction of new modes of operation. They are each dependent on reliable hardware and software, and on security services, such as confidentiality, authentication and message integrity.

The generation of information was, until recently, dominated by inputs from people and computers. Now, the ready availability of small and highly capable, inexpensive and low-power sensors is causing rapid growth in sensors as a source of information. The employment of sensors in large numbers as a means of generating information is a comparatively recent phenomenon. Ten years ago, relatively little information came from sensors. Ten years from now, a very significant fraction of information will originate from sensors, especially because of the growing commercial importance of sensor systems. Communication is truly central to information technology. The recent dual explosions in fiber-optic networks and in wireless systems, plus improvements in satellite communications, have made the communication of information incredibly faster and better than was possible even 15 years ago. Development and utilization of the Internet are redefining the Information Revolution. The ever-shrinking integrated circuit has led relentlessly to better computers for the processing of information. Even the prospect of physical limits to smaller line widths in semiconductor ICs, which could be reached in 10-15 years, might not slow increasing computer capabilities. The double impact of greater capabilities and lower prices is leading to ubiquitous computing, as well as to ever-more-capable high-performance computing. Dramatic improvements in information storage within hard drives and optical discs may be somewhat less heralded, but they are also fundamental to the Information Revolution. Relentless

Nagel 11

gains in magnetic-storage densities may also come up against physical limits. However, micromechanical storage devices, which could offer densities of hundreds of gigabits per square inch, are under development. The utilization of information has also undergone explosive growth. Both the amount and timeliness of information that is now available to a large and growing fraction of humanity is remarkable. Further, the ability to store vast amounts of information and to search it quickly enables data mining and the discovery of important latent relationships. In general, new ways to produce, transfer, process, store, and use information are appearing and growing rapidly nowadays. This is highly likely to remain the case for the coming decades. The ability to deploy large numbers of small sensors in most environments to obtain new information, and to employ wireless communications to forward that information, is expected to be widely exploited and to have important ramifications. Wireless sensor technologies are an important new aspect of the Information Age. Hence, a workshop entitled, "The Internet 'Gets Physical': Wireless Sensor Networks: Technology Forecasts, Applications and Impact,” was held on May 19, 2004, by the Center for Strategic and International Studies (CSIS) in Washington, DC. The agenda for the review and the speakers’ list constitute Appendix A. The "Gets Physical" part of the workshop title concerns the ability of wireless sensors to pull information out of the physical (and chemical and biological) world, and to distribute that information over the Internet. This report is a summary of that workshop. It is also a review of the field of local, that is, short-range wireless sensor technologies. Topics range from the materials that make up the basis of sensor systems and networks, through their components to their characteristics and performance, and then on to the numerous applications, implications, and impacts of the technologies. The organization of the report is dictated by the characteristics of the topic of local wireless sensor networks. Major points from the CSIS Workshop are interwoven where appropriate. Avoiding wires for communication links has two immense advantages. The less- important advantage is the fact that wireless links cost about half of what wired connections cost. Wires are expensive to install, protect, maintain and reconfigure. But, stationary wireless systems can be modified relatively easily. The more-important advantage is mobility, the ability for the sending and receiving sides of a link to move. Such movement might be infrequent, such as reconfiguring an office network. It might be slow, such as following the motion of a glacier. Or, it might be much faster, for example, with nodes in a car. But the essential quality is motion without loss of the ability to communicate. Daniela Rus spoke of the characteristics and advantages of mobile sensors at the CSIS Workshop. There are several definitions of wireless sensor systems and networks. Robert Poor, the Chief Technology Officer of Ember Corporation, gave the essence of each aspect of a wireless embedded network at the CSIS Workshop as: (a) wireless: communication is via radio frequency links; (b) embedded: communication is built into, and not added onto, manufactured goods; and (c) network: devices form an interconnected communication system to share information and resources. He also provided a list of attributes of such networks, which are optimized for widespread, autonomous, unattended operation. They include self-organization and self-healing, low cost, low power, and scalability in size and density, generally without the requirement for

Nagel 12

high-speed operation. A recent report1 defines such networks as two or more wireless communication nodes that consist of processors, wireless transceivers, sensors and batteries with at least one gateway using the seven-layer Open Systems Interconnection (OSI) model of network communication protocols.2 Other definitions also emphasize the amalgam of technologies that enable wireless sensor systems and networks, and dictate their performance characteristics. A gateway, also called an access point, is a device, which might also contain sensors, that forms a bridge between the wireless sensor network and conventional networks. There are many visions for the use of wireless sensor technologies. Some of them derive from the emergence of ubiquitous computing and communications. The sheer numbers of computers made annually, about 8.5 B, according to remarks at the CSIS workshop by David Tennenhouse, Vice President in the Corporate Technology Group and Director of Research at Intel, and the fact that around 98% of them are embedded processors,3 speaks to the current state of widely available computing. Similarly, the sale of about 600M wireless phones each year indicates the extent to distributed wireless technologies. Together, these facts are the basis of the viewpoint that wireless sensor technologies will also become pervasive. A recent book deals with the expectation that many common objects will have embedded into them both computers and wireless communications.4 Another book addresses the linkage of things.5 Drivers for the growth of sales of wireless sensor technologies are: (a) inclusion in more and more products; (b) reduced barriers to adoption for that and other reasons, such as convenience of setup and use; and (c) the wide range of applications, as well as current rapid improvements. Some very large-scale aspects of wireless sensor technologies give perspective to their importance. Paul Saffo of the Institute for the Future was quoted as saying, "Sensors will be to this decade what microprocessors were to the 1980s and the Internet to the 1990s."6 In a recent address, Gary Boone of the Accenture Technologies Laboratory, asserted that "browsing reality" will prove to be the killer application for wireless sensor networks, similar to how browsing stored information has become the primary use of the Internet.7 There may also be an analog to data mining, which Boone terms "reality mining." The essential idea in both cases is to find valuable connections that are not otherwise known. The linking of the physical (and chemical and biological) worlds with the ever-expanding information world is at the basis of the projected utility, indeed, criticality of wireless sensor networks, whether if be for personal interests or major businesses. At the CSIS Workshop, John Stankovic listed both hardware and software advances, which enable wireless sensor technologies. The hardware technologies include new, smaller, and moreaccurate sensors, lower-power computers, low-power wireless radios, and specialized hardware

1

C. Chi and M. Hatler, "Industrial Wireless Sensor Networking; A Market Dynamics Study," ON World, www.onworld.com, 2004 2 W. Feibel, "The Encyclopedia of Networking," The Network Press, Sybex, p. 871, 2000 and http://www2.rad.com/networks/1994/osi/layers.htm 3 http://www.stanford.edu/class/ee392s/presentations/040203Zhao.pdf 4 N. Gershenfeld, "When Things Start to Think," Henry Holt and Company, 1999 5 A.-L. Barabasi , "Linked: How Everything Is Connected to Everything Else and What It Means," Plume Books, 2003 6 B. G. Goode, "Wireless for Industry,” Supplement to Sensors Magazine, p. S6, June 2004 7 G. Boone, "Reality Online Tomorrow: Ubiquitous, Networked and Extremely Cheap Sensors,” Wireless Sensor Solutions Conference, Rosemont IL, 22 September 2004

Nagel 13

for security. The software technologies are self-organizing and reorganizing middleware, wireless networking protocols for real situations and for autonomic operation, including selfmonitoring, -control and -healing. A recent report cited the need to conquer system complexity as one of five grand challenges.8 It envisions large-scale information systems as being able to configure, optimize, maintain, protect, heal, and form differentiated behavior from similar component parts. Distributed sensor networks with many nodes are among those information systems. Greg Pottie, Deputy Director of the Center for Embedded Networked Sensing at the University of California at Los Angeles, provided the mission statement for the Center at the CSIS Workshop. It is essentially a statement of purpose for much of the field of wireless sensor systems and networks: • To address scientific issues of national and global priority through pioneering research and education in Embedded Networked Sensing technology, •

To develop and demonstrate architectural principles and methodologies for deeply embedded, massively distributed, sensor-rich systems,

•

To apply and disseminate these systems in support of scientific research critical to social and environmental concerns, and

•

To create meaningful inquiry-based science instruction using embedded networked sensing technology, for a diverse grade 7-12 population; and to disseminate education materials and technology through outreach and professional-development networks. We will examine the applications of wireless sensor systems and networks in some detail later. Here it can be noted that these technologies perform a variety of functions that cut across applications. They include distributed and collaborative sensing; detection and tracking; location determination and event recording; computing and signal processing; data and information management; aggregation and storage; and query processing, among others. Wireless sensor systems and networks can be programmed to be reactive, so they adapt to circumstances. Fundamentally, it is the ability of wireless sensor technologies to perform these functions, often in a responsive manner, which makes them so useful for a wide variety of applications. Wireless sensor systems and networks have grown quickly in their variety and applications since the turn of the millennium. The rapidly growing number of research projects, publications and products evidences this growth. There is already an extensive literature on the component and system-level technologies. It provides valuable information on many topics that could not be treated adequately in this review. Books on self-organizing networks are on the market.9 A few recent books devoted to wireless sensor technologies have appeared10 and others are in 8

"Grand Research Challenges in Information Systems," Computing Research Association, 2002 C. E. Perkins, "Ad Hoc Networking," Addison-Wesley, 2000; C.-K. Toh, "Ad Hoc Mobile Wireless Networks; Protocols and Systems," Prentice Hall, 2001; M. Ilyas (Editor), "The Handbook of Ad Hoc Wireless Networks,” CRC Pres, 2002; C. S. R. Murthy and B.S. Manoj, "Ad Hoc Wireless Networks: Architectures and Protocols," Prentice Hall PTR, 2004 10 E. H.Callaway Jr., "Wireless Sensor Networks: Architectures and Protocols," Auerbach Publications, 2003; A. Hac, "Wireless Sensor Network Designs," John Wiley and Sons, 2003; F. Zhao and L. J. Guibas, "Wireless Sensor Networks: An Information Processing Approach," Morgan Kaufman, 2004; C. S. Raghavendra, K. M. Sivalingam and T. F. Znati , "Wireless Sensor Networks," Kluwer Academic Publishers, 2004; M. Ilyas and I. Mahgoub, "Handbook of Sensor Networks: Compact Wireless and Wired Sensing Systems," CRC Press, 2004; H. 9

Nagel 14

prospect.11 One commercial study specifically on wireless sensor networking for industrial applications is on the market.1 Several workshops on the subject have been held in the past two years and reports are available.12 The report edited by Estrin, Michener, and Bonito is particularly useful. At least two special issues of a journal were devoted to "Fundamental Performance Limits of Wireless Sensor Networks."13 A search with Google in October of 2003 using "wireless sensor networks" produced 16,800 hits. The same search in September of 2004 yielded 39,000 hits. Some of the sites have compilations of papers,14 other information,15 conferences,16 and materials from meetings17 on wireless sensor networks. Materials from one university course entitled "Smart Sensor Network Systems"18 and another called "Recent Advances in Wireless Sensor Networks"19 are on the web. The Center for Embedded Networked Sensing at the University of California at Los Angeles was already mentioned.20 A sensor network consortium joins three universities in Massachusetts.21 Another joins three universities in Michigan to develop and use Wireless Integrated MicroSystems (WIMS).22 Sensors magazine has a Web site specifically on wireless sensor technologies.23 Journal articles and web sites will be referred to frequently in the rest of this report.

Karl, A. Willig and A. Wolisz (Editors), "Proceedings of the Wireless Sensor Networks: First European Workshop," Berlin, Germany, Springer-Verlag, Inc., 2004 11 T. F. Znati et al (Editors), "Wireless Sensor Networks,” Acacemic Press, in press; J. Wu, "Handbook on Theoretical and Algorithmic Aspects of Sensor, Ad Hoc Wireless, and Peer-To-Peer Network," Auerbach Publications, in press 12 G. Markowsky et al (Editors), "Anywhere, Anytime, Any Size any Signal: Scalable Remote Information Sensing & Communication Systems," NSF Workshop, Jan 2002, http://homeland.maine.edu/anywhere.htm; S. D. Glaser and D. Pescovitz, " National Workshop on Future Sensing Systems: Living, Nonliving, and Energy Systems,” August 2002, http://www.ce.berkeley.edu/Programs/Geoengineering/sensors/NSFSensorWorkshop.pdf; "Industrial Wireless Technology for the 21st Century," 2002, www.oit.doe.gov/sens_cont/pdfs/wireless_technology.pdf ; R. Loewenstein and G. Markowsky, "Polar Science and Advanced Networking," April 2003, http://www.polar.umcs.maine.edu/OPP-CISE-Workshop-Final.pdf; and D. Estrin, W. Michener and G. Bonito, Environmental Cyberinfrastructure Needs for Distributed Sensor Networks," August 2003, http://eee.lternet.edu/sensor_report 13 SIGBED Review, vol.1 (2), "Special Issue on Embedded Sensor Networks and Wireless Computing," http://www.cs.virginia.edu/sigbed/vol1_num2.html and IEEE Journal of Selected Areas in Communications, to be published 14 http://www.research.rutgers.edu/~mini/sensornetworks.html and http://www.ee.ucla.edu/~kansal/reporting/survey/biblio.html 15 S. Mererian, http://www.cs.ucla.edu/~seapahn/academic.htm; B. Krishnamachari, http://ceng.usc.edu/~bkrishna/teaching/SensorNetBib.html, http://www.hackandsecure.com/4/wireless-sensornetwork.html and http://www-2.cs.cmu.edu/~sensing-sensors/S2004/L2004-03-data_acquisition/L2004-03-motesJen_Morris.pdf 16 B. Krishnamachari, http://ceng.usc.edu/~bkrishna/confs.html 17 http://today.cs.berkeley.edu/retreat-1-02/ 18 http://www.cs.wmich.edu/~wsn/cs691_sp03/ 19 http://www.cs.virginia.edu/~qc9b/fall03cs851/cs851_03.html 20 http"//www.cens.ucla.edu 21 The Center for Information and Systems Engineering, http://www.bu.edu/systems 22 http://www.wimserc.org/ 23 http://www.sensorsmag.com/wireless

Nagel 15

One measure of the level of interest in wireless technologies, with or without sensors, for industrial applications is the number of technology and market-analysis firms working on them. A compilation of such companies became available recently, and constitutes Table 1.24 FocalPoint Group www.thefpgroup.com 415-601-0649 Harbor Research, Inc. www.harborresearch.com 415-435-7033 ON World www.onworld.com 858-259-2397 Venture Development Corp. www.vdc-corp.com 508-653-9000 West Technology Research www.westtechresearch.com 650-940-1196 Wireless Data Research www.wirelessdataresearch.com 650-524-1689 Table 1. Market-research firms monitoring wireless sensor technologies. The remainder of this section concerns the key technologies that enable local wireless sensor systems and networks, namely sensors and short-range wireless communications. Before getting to them individually in the following two sub-sections, it is worthwhile to relate sensors and wireless communications technologies to each other. One way to do this is to consider the factors given in the matrix of Figure 2, which permits setting wireless sensor systems and networks in a context. Sensors

System or Network

Local

Regional

Biometrics

Several Technologies

RFID Tags

Passive and Active

Localizers

RF, Optical and Acoustic

Radar & Sonar GPS, etc.

Global

Many Technologies GPS, Glonass and Other Location Systems

Point Sensors

Local Systems & Networks

Imagers

Visible and IR Cameras

Cell Phone Communications

Satellite Communications

Weather and Surveillance Satellite Imagers

Figure 2. The types of items commonly referred to as sensors (left column) and the scope of the wireless networks with which they are associated (top row). The shaded boxes highlight useful combinations of existing and emerging sensor and communication technologies. The heavy solid line indicates the local wireless sensor systems and networks that are the focus of this report. The heavy dashed lines encompass other combinations also treated in this survey. The trend toward integrating two or more of the sensor technologies into single products is noteworthy.

24

B. G. Goode, "Wireless for Industry," Supplement to Sensors Magazine, p. S8, June 2004

Nagel 16

The wireless part of wireless sensor networks is quite clear because of our familiarity with cell phones. Information, which originates at sensors and is sent over a network through free space and not over electrical wires, optical fibers or other conduits, is the focus of this report. However, the situation is not as clear for either the sensors or the networks. So, we must consider what falls under the heading of sensors and, also, the diverse characteristics of networks. A very great variety of technologies are called sensors, as listed in the left-hand column of Figure 2. They are surveyed in the next sub-section. Information from sensors can be transmitted over wireless and other systems and networks on local, regional, and global scales, as also indicated in Figure 2. Local networks are usually line-of-sight and have ranges less than about 1 km. Cell-phone communications are the best example of regional networks. Global wireless networks employ communications satellites as relays. Wireless communications technologies are reviewed in the second sub-section. Figure 2 shows that all types of sensors are used locally, and some of them are employed on regional and global scales. Large-area wireless sensor systems are considered at the end of this section, before turning to the details of local wireless sensor technologies.

A. Types of Sensors Humans contain many sensors that transduce physical, chemical, and biological conditions into perceptions within our central nervous system. Combinations of sensory organs, nerves for signal conduction, and the brain, which processes, stores, and acts upon information from the sensors, respond to both internal and external stimuli. We have many kinds of internal sensors. They provide information on our positions, movements, blood chemistry, the degree of filling of various organs and other factors. These internal sensors generally provide threshold responses. Also, they are slow, that is, they have long response times (low bandwidth). External Human Senses Temperature Force and Vibration Taste and Smell Hearing Vision Human-Designed Sensors Many Stimuli

Max. Range for Receipt of Stimuli Contact Contact Contact Kilometers (Depends on Loudness) Utility Over 10 Kilometers

Bandwidth of Response to Changes Less than 1 Hz Less than 1 Hz Less than 1 Hz Greater than 1 Hz About 30 Hz

Interplanetary (Comms Over 1 GHz in Extreme Cases Limited) Table 2. The range and bandwidth for natural human senses and for designed sensors in systems. The external sensors respond to several well-known stimuli, as listed in Table 2. The thermo-mechanical responses of the skin to temperature, forces, and vibrations require contact with the factor being sensed, and they have low bandwidths in response to variable stimuli. The same is true for the chemical senses of taste and smell. The spectroscopic senses of sight and hearing have both relatively long reaches and faster responses. Our external senses react to

Nagel 17

continuously varying inputs over wide ranges, but do not provide quantitative measures of the various stimuli. There are many conditions to which we are not intrinsically sensitive. In most cases, artificial sensors provide quantitative information over their ranges of utility. As indicated in Table 1, designed sensors can obtain information over long distances, depending on available communications. They respond to rapidly changing inputs in many cases. The emergence of designed sensors to compliment our natural senses has given us important new tools. We can now respond to many more quantities and to harsh conditions that would be unhealthy or lethal for humans. The availability of wired connections (notably the Internet) and wireless communications give our "new" senses much greater reach and bandwidth than our natural senses. The entire field of sensors is evolving rapidly due both to scientific developments (the push) and very diverse needs (the pull). At the CSIS Workshop, Sharon Smith of Lockheed Martin Corporation emphasized the promise of nanotechnology for both sensor materials and devices. There are many books25 on sensors, including a coming encyclopedia26 and useful general Web sites on sensors.27 A flood of information from sensors around the world can come to us, and it must be coupled into our brains primarily through our vision and hearing. The limitations of our ability to assimilate information, and the fact that we must pay attention to our surroundings, even while focused on receipt of information from sensors, have two important implications. First, information from sensors has to be "down-sized" as much as possible as soon as possible during its flow from sensors to humans. Consider our vision system as a megapixel imager that receives color and brightness information from each pixel at rates exceeding 30 Hz. It must shunt unimportant information, or we could not process the stream of input data. So, our vision system has capabilities, such as motion and edge detection, which permit us to focus on a small but important subset of the inputs to our eyes. In a similar fashion, it must be possible to automatically shed relatively useless information from high-bandwidth wired or wireless sensor networks. Second, because information can be received from many sensors, there is the challenge of melding that information into forms that can be consumed by humans without undo effort. This challenge is more than "data fusion." It is really about "information fusion" in ways that match the physiological, psychological, and varied conditions and capabilities of a wide spectrum of people. If sensor information goes to computers for storage or use in control systems, it can be assimilated at rates exceeding what people can handle. However, requirements for discarding useless information and for presenting it in a proper fashion are also germane for both storage of information and its autonomic use in control systems. Melding information from natural human senses and from artificial sensors is one of the challenges of wireless sensor technologies. There are many components and systems that are sometimes referred to as sensors. They fall generally into three categories: (a) those that identify people and things; (b) those that give the position of something; and (c) those that report on conditions at a location or within a volume. Biometrics is the term used for a collection of mostly recent technologies that can be used to

25

J. Fraden, "Handbook of Modern Sensors: Physics, Design and Applications," AIP Press, 2003. A search under "books" on www.amazon.com in Sept 2004 yielded over 19K hits 26 C. A. Grimes, E. C. Dickey and M. V. Pishko, "Encyclopedia of Sensors," American Scientific Publishers, to be published. 27 http://www.sensorsmag.com/ (click on Buyer's Guide) and http://www.sensorsportal.com/

Nagel 18

identify a particular person.28 Besides the time-honored fingerprint identification, biometric identifiers include hand prints, scans of the irises, retinas, and vein patterns in the back of hands, and techniques for voice, facial, and motion recognition, as well as for DNA "finger printing." Biometrics sensors often require contact, and are always local, even if they do not require contact. Radio-frequency identification (RFID) tags are essentially electronic barcodes, although they can store and provide more information than common optical bar codes29. RFID systems provide the identity of things, including animals. Passive tags on the object or subject of interest can be probed by a reader, which provides the energy for the identification transaction, or they can broadcast their identity, if they have batteries and are "active." RFID systems are also good only over a short range. RFID technology is expected to have immense impacts in the coming years on the production of goods, their flow to points of sale, and the automation of retail, service, and other functions. Where people and things are located is hardly less important than their identity. Hence, devices based on various principles have been developed to provide position information. They are sometimes called localizers. Passive infrared (PIR) sensors have long been used to detect the nearby presence of people. Recent additions to the arsenal of methods for determining location exploit RF, optical, and even acoustic signals. They operate with different precisions over diverse ranges. Rosum, Inc. has developed a technology that uses synchronization information within television signals to provide the location of objects with an accuracy of less than one meter indoors and in urban canyons, where GPS signals are either not available or not reliable.30 Aether Wire and Location, Inc. employs broadband RF transceiver technology, which can give position information to about one centimeter over 30 to 60 meters.31 Triangulation using line-ofsight optics, which is essentially surveying, can still be used to locate positions. Trilon Technology makes the Shot Spotter acoustic system for determination of the location of gunfire in cities.32 With microphones about 800 meters apart, it provides location information to 10 meters. Acoustic communication systems and triangulation are also used to locate objects underwater.33 Radar (radio detection and ranging) and sonar (sound navigation and ranging) systems for locating objects can be active or passive. In active systems, an RF or sound pulse is transmitted from a system that can determine the time of the return to get range, and detect the angle of arrival to get direction to the object being located. In passive systems, radiation from the object is sensed by an array of receivers to give the direction to the object. Both radars and sonars can provide location information within a region to ranges far in excess of 100 kilometers, although they are generally used for local areas with ranges of less than 10 kilometers. Michael Horton, CEO of CrossBow, Inc., noted at the CSIS Workshop that a small ultra-wideband radar with a range of 50 feet had been developed by the Lawrence Livermore National Laboratory. It was

28

J. D. Woodward Jr., "Biometrics," McGraw-Hill Osborne Media, 2002 K. Finkenzeller, "RFID Handbook : Fundamentals and Applications in Contactless Smart Cards and Identification," John Wiley and Sons, 2003 30 http://www.rosum.com/rosum_tv-gps_indoor_location_technology_overview.html 31 http://www.aetherwire.com/ 32 http://www.shotspotter.com/index.shtml 33 I. F. Akyildiz, D. Pompili, T. Melodia, "Challenges for Efficient Communication in Underwater Acoustic Sensor Networks," ACM Sigbed Review, July 2004 and http://users.ece.gatech.edu/~dario/Dario's%20Publications.htm 29

Nagel 19

commercialized by Advantac, Inc.34 The device can be integrated into the nodes of local wireless sensor systems and networks. The Global Positioning System (GPS) provides worldwide-position information to within less than a meter by use of three-dimensional triangulation of signals from satellites with precise clocks. The Glonass system, built by the former Soviet Union and still operated by Russia, also gives precise positions. A comparison of the technical characteristics of the GPS and Glonass systems is available.35 The GPS and Glonass systems are basically for navigation, that is, the determination of a position and route, although they are being used for other purposes, notably targeting. There are also electronic-warfare systems for determination of the positions of RF emitters. CrossBow, Inc. now sells boards with multiple sensors for wireless networks, which have optional GPS capabilities. Sensors that are used to determine conditions, such as temperature, humidity, and vibrations, at a specific location are termed point sensors. One Web site lists over 100 types of point sensors.36 Each type of sensor is linked by the site to companies that sell that kind of sensor. Generally, there are 10 to 100 companies offering sensors in each class. However, the most popular sensors are sold by more than 100 companies, as indicated in parentheses: flow (147), gas (105), level (109), pressure (286), proximity (124) and temperature (391). There are over $1B worth of sensors sold annually. Figure 3 shows the distribution by sensor types for the year 2001.37 Point sensors are a central technology in wireless sensor systems and networks. Their operation depends on one or more physical, chemical, or biological mechanisms, integrated with electronic functions. The study of sensors is complex, and their development and use are challenging.

Figure 3. The demand for sensors of the indicated types within the U. S. for the year 2001.

Imagers are particularly useful sensors because of the dominant importance of human vision. Cameras and other imagers are positioned at a specific location, but they can acquire pictures from the volume of space within their view. That is, they get information from a threedimensional region rather than at just a point. In the last decade, there has been a remarkable decrease in the size and cost of visible cameras. The cameras in cell phones occupy a volume of

34

http://www.advantaca.com/radar.htm

35

http://www.oso.chalmers.se/~geo/gg_comp.html

36

http://www.motionnet.com/cgi-bin/search.exe?a=cat&no=2075 P. l. Fuhr and W. Manges, "Why Wireless?" in B. G. Goode (Editor), "Wireless for Industry," Supplement to Sensors Magazine, p. S9, June 2004, quoted from the Freedonia Group Report on Sensors, August 2002. 37

Nagel 20

about one cubic centimeter and cost only a few dollars. These factors have led to an explosion in the number of cameras and imagers employed in terrestrial wired and wireless monitoring systems. It is estimated that there are currently 31 million surveillance cameras in use worldwide.38 The plummeting cost of imagers, the efficacy of monitoring large areas with few people, and improvements in related computer technology for detecting movements contribute to the growth. Satellite-borne optical and IR imagers have also continued to grow in importance for weather, surveillance, and many other applications. Charge-coupled devices (CCDs) have diversified in opposite ways in the past decade. Megapixel CCD chips have been produced in large numbers for instrumentation and, especially, digital photography. Meanwhile, low-end CCDs have proliferated in cameras for computers, cell phones and other systems. It is now possible to buy a color and sound video system with a CCD imager and related electronics, a transmitter, two antennas, and a receiver for the 2.4 GHz wireless link (good to 30 meters), plus an interface box to a TV or VCR, for $80.39 The imager and its electronics can be bought for half that price. In the past few years, visible imagers based on CMOS technology have burst onto the market.40 They offer overall performance comparable to state-of-the-art CCDs, but require significantly lower power. One application of CMOS imagers has received much attention, namely their use in a wireless imaging system for examination of the gastrointestinal tract in humans.41 Figure 4 shows the pill in relation to a U.S. quarter dollar, and a cross-sectional drawing of the pill. It is noteworthy that the batteries occupy about half of the volume of the system.

Figure 4. Photograph and schematic of the endoscopic pill from Given Imaging in Israel. The CMOS imager is shown in the inset on the left near a U.S. dime.

Infrared imagers to detect radiation from warm objects also have changed significantly in the recent past. Cryogenic-imager arrays based on narrow-bandgap semiconductors, notably mercury cadmium telluride (HCT), have been developed for decades, primarily for military uses. IR imagers based on micro-machining technologies, which do not require cooling to suppress noise, have come on the market. They still cost about $5K, but that compares well with the $50K costs of cooled IR systems. While they do not have the sensitivity of HCT systems, they are substantially simpler, besides being about one-tenth the cost of cooled systems. Such imagers

38

J. Kumagai and S. Cherry, "Sensors and Sensibility," IEEE Spectrum, 22-28, July 2004 http://www. X10.com 40 http://www.photobit.com 41 http://www.givenimaging.com 39

Nagel 21

have come on the market in the past few years.42 One possible application of uncooled infrared imagers is in luxury cars for the detection of the body heat of humans and animals beyond the range of the headlights.43 Such cameras will decline in price greatly as they come into widespread use, possibly to below $1K each. They will find applications in home security, and for the monitoring of machinery and other systems for hot spots that are indications of current or imminent mechanical or thermal problems. While point sensors and imagers may seem quite different, there is an interesting commonality. The picture elements in visible and infrared cameras are arranged in regular arrays. The point sensors that are distributed in a region can also be put out in a regular grid. The information that they provide, such as temperatures, can be used to make a threedimensional plot of signal amplitude (temperature) versus the X and Y coordinates of the region sampled. Such a plot is very similar to an image, which is essentially a plot of light intensity over a two- dimensional space. Like a video image, the distribution of point sensor readings can be time dependent. Of course, there are differences, such as the ability to capture color information as well as intensity, and the fact that point sensors are usually not deployed in a regular pattern. However, the ability to gather real-time point sensor information from an area permits construction of a three-dimensional distribution, or surface, even for sensors in irregular and moving distributions. More nodes in a network will mean either coverage of a larger area, or higher sensor density, that is, better spatial resolution. Faster nodes and wireless communications will increase the temporal resolution of the information obtained from the sensors in the network. The term "macroscope" is being applied to distributed sensor networks.44 It is also germane to satellite photographs, which can cover even larger areas than those probed by local sensor networks. It is expected that real-time information from sensor networks will be melded with geographical information systems (GIS) containing satellite and other images in the coming years. Whether the geographical data is stored or comes from cameras near the earth or in orbit, it should be possible to see the locations of point sensors in the GIS presentation. The sensor data could be on all the time, similar to airplane identifications on flight-controller's screens, or be made available by clicking on the location indicator for a sensor. There can be a close integration of point sensors and imagers in a short-range wireless network. In general, imagers produce more information and, hence, require faster communication links than point sensors. The focus in this report is on point sensors for the very wide variety of physical, chemical, and biological quantities to which they are sensitive. The basic function of such sensors is to provide information. Now, we turn to the means by which the information gets from the point of origin to the users, be they people or systems.

B. Wireless Communications One of the hallmarks of the current era is the remarkable expansion of wireless technologies within the communications industry, and the penetration of such technologies into other

42

Courtesy of T. Schimert, Raytheon Corporation, Dallas TX http://www.cadillac.com/tech/index.htm 44 D. E. Culler and W. Hong (Editors), "Wireless Sensor Networks," Comm. of the CAM, V. 47,30-33, June 2004 43

Nagel 22

industries. There is, in fact, a Wireless Industry Networking Alliance. It is "a coalition of companies, industry organizations, technology suppliers, software developers, system integrators, and others interested in the advancement of wireless solutions for industry."45 A recent review of wireless technologies listed many of the companies in the wireless industry.46 The complexity of wireless RF communications, even without sensors integrated into networks, is very great, indeed. The physical factors, which enable and limit such communications, and the performance of wireless links, are the subject of many conferences, books, journals, and other technical media. The underlying physics does not change, but the means of exploiting it and pressing to the practical limits of what is possible are very dynamic. Means for measuring the performance of links, their capacity, margins, and error rates, are well developed both in principle and in commercial instrumentation. Given the great and growing importance of the Internet, it is natural to question the relative importance of wired and wireless sensor networks. The case for wireless sensor networks has two primary facets. The first is the combination of greater capabilities and lower costs. The derivative advantage is the growing availability of wireless connectivity. We are in the early stages of what has been termed the "Wireless Revolution." WiFi, Bluetooth, ZigBee, cellular networks, and satellite systems are among the competitive and complementary technologies. The growing use of cellular systems is especially noteworthy. Figure 5 shows the increases in wireless connectivity for both voice and data communications.47 It is only a matter of the market before sensors, computers, and wireless communications are all integrated into units smaller than the size of cell phones. Single-chip sensors and computers are widely available. Complete radios on a single chip are appearing, some of them with multiple standards. The integration of all three functions on the same substrate can be foreseen. Such devices would be one type of a "system on a chip." It must be noted that power sources will not become chip-scale in the near future. Antennas are critical to wireless links, of course. They vary in size from chip-scale devices to wires that are about the same size as the node. Antenna geometries and sizes determine the efficiency and radiation patterns, which greatly influence the performance of a distributed network. The total size and weight of sensor nodes, routers, and gateways are restricted by the largest components. MILLIONS OF PEOPLE

WIRELESS SUBSCRIBERS

& WIRELESS INTERNET SUBSCRIBERS

1 out of every 6 people on earth!

1000 800 600 400 200

1998

1999

2000

2001

2002

2003

Figure 5. The growth of wireless usage worldwide.

45

http://www.wina.org R. Gecker, "A Walk on the Wireless Side," Inbound Logistics, 62-72 March 2004 and http://www.inboundlogistics.com/articles/features/0304_feature03.shtml 47 The Industry Standard Magazine, p. 72, January 10-17 2000 46

Nagel 23

The area over which the sensors of any size are deployed and connected is a major aspect of a wireless sensor system or network. Network descriptions such as WANs (wide area), LANs (local area), and even PANs (personal area) are familiar. Bluetooth technology is designed to form spontaneously what are called "piconets" within areas about 10 meters in diameter. A relatively natural categorization of the spatial extent of networks is given in Table 3. Size Scale Terminology Examples Technologies 1-10 meters Personal An Individual's Electronic Devices Many 10-1000 meters Neighborhood Office Buildings & Factories Many 1-100 kilometers Regional Between Buildings & Within Cities Cellular Phones 100-10K kilometers Country Countries to Continents Satellites > 10K kilometers Global Primarily Satellite Networks Satellites Table 3. Networks of various sizes and communication technologies. Networks that span the personal and neighborhood domains are termed "local" in this report. Those that include countries and continents are termed "global" here, as in Figure 2. The boundaries between these classes of systems are not sharp. The radio frequencies employed for satellite, cellular, and line-of-sight communications are allocated by international and national organizations. The bands of primary interest for local wireless sensor systems and networks are given in Table 4.48 Each of these has technical and other advantages and disadvantages. The two highest-frequency bands are most germane to local wireless sensor systems. That is, most commercial activity is in the 900 MHz and 2.4 GHz bands. Those are two of the Industrial, Scientific and Medical (ISM) bands. Transmissions in the bands given in Table 3 are usually narrow-band, that is, they use well-defined frequencies for communications. It should be noted that there is now great interest in the possible use of wideand ultra-wide-band (UWB) communications for sensor and other information. UWB transmissions spread over a range of frequencies that is a large fraction of the center frequency can essentially hide the signals in the noise at any specific frequency.49

Frequency 418 MHz 433.9-868 MHz 902-928 MHz 2.4 GHz Typical Range 30-300 km 700-1000 m 300-500 m 50-100 m Applicability U. S. Europe North America Global Table 4. The frequencies, typical ranges and regions of use for the four primary bands employed for wireless sensor communications. The short ranges at these frequencies constrain wireless sensor technologies, which use them, to a local area. This is good for purposes of security because the sensor-information signals do

48

http://www.cs.berkeley.edu/~adj/cs294-1.s98/wireless_lan/sld008.htm M. Ghavami, L. Michael and R.Kohno, "Ultra Wideband Signals and Systems in Communication Engineering," John Wiley and Sons, 2004 49

Nagel 24

not expend far into regions where they might be intercepted or otherwise compromised. However, those ranges are for commonly used transmission powers, which are usually the major factor in determining the lifetime of batteries that power sensor nodes. The ranges fall off with higher frequency, as shown in Table 4. They are also heavily dependent on the environment. In free space, signal strength falls off with the inverse square power of the distance from a transmitter. The decline within buildings is steeper. Outdoors, the decline in available strength can fall off as the inverse fourth power of distance if both the transmitter and receiver are close to the earth. This is an important limitation on many applications. A paper on factors affecting wireless-transmission ranges is available.50 A magazine addresses the short-range wireless industry. 51 A conference on the topic was held recently.52 The available frequencies are only the starting point for operation of wireless links. It is also necessary to have adopted standards, protocols, and conventions, so equipment, software, protocols, and applications made by different manufacturers will be interoperable. The most fundamental standard is the seven-layer OSI model of network communication protocols, which is the basis of the operation of the Internet and most other networks. There are several standards specific to wireless communications.53 The importance of standards was emphasized by Edgar Callaway, Distinguished Member, Technical Staff at Motorola Laboratories, at the CSIS Workshop. Open standards, and architectures based on them, reduce costs and risks and, hence, accelerate the adoption of technologies. Proprietary protocols are less flexible and interoperable because they are (a) offered by fewer manufacturers, and (b) more costly because of the limited competition. There are many examples of the beneficial impact of industrial standards. There is one failure to achieve standards that is relevant to sensor systems. Wired industrial sensor and other systems have suffered from a proliferation of standards in what was called the "bus wars." The inability to converge on a dominant set of standards has been complex and costly. It is possible that the standards emerging in the wireless sensor arena will solve the problem of multiple industrialcommunications standards as wireless technologies increasingly penetrate the industrialcommunications market. Three wireless standards are most germane to sensor systems and networks. Their features are summarized in Table 5.54 Wi-Fi (IEEE 802.11 a, b and now g) Standard Designation

Wi-Fi IEEE 802.11.b

BlueTooth IEEE 802.15.1

50

ZigBee IEEE 802.15.4

http://www.savewithaccutech.com/pdf/Tech%20Note%20215%20Wireless%20Range.pdf http://www.srw-magazine.com/ 52 http://www.srw-magazine.com/srw_conf_index.htm 53 G. Karayannis, "Wireless Networking Alternatives," Sensors Magazine, December 2003, http://www.sensorsmag.com/articles/1203/26/main.shtml and J. A. Gutierrez at al, "IEEE 802.15.4 Low Rate Wireless Personal Area Networks: Enabling Wireless Sensor Networks," IEEE, 2003 54 G. Martin, "The Sensor Network Value Chain," presented at the Wireless Sensing Solutions meeting, Rosemont IL, 21-22 Sept 04 and at http://www.wssconference.com/live/images/other/Sensor_Network_Martin.Chipcon_20040927.pdf; B. Heile, "Emerging Standards: Where do ZigBee and UWB Fit," June 2004 and at http://www.zigbee.org/resources/documents/WiCon_zigbee_standards_presentation.pdf ; G. Karayannis, "Emerging Wireless Standards,” AMRA 2003 International Symposium and http://www.palowireless.com/zigbee/resources.asp 51

Nagel 25

Application Focus

Web, Video, Email

Goals

Speed, Flexibility

Cable Replacement Cost, Convenience

Monitoring & Control Reliability, Power, Cost 100+ 250 @2.4 GHz

Range (meters) 100 10+ Band Width > 10,000 1000 (kb/sec) Latency < 3 sec < 10 sec < 30 msec Stack Size (kBytes) 1000+ 250+ 4-64 Network Size Many 7 10 cm3 RF rate (kbps) < 50 < 100 < 500 < 500 MIPS