Wireless Computing PDF

2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 1 2.1. The Network Edge We begin in this section at the edge of the network andlook at the components with which we are most familiar—namely, the computers, smartphones and other devices that we...

2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 1 2.1. The Network Edge We begin in this section at the edge of the network andlook at the components with which we are most familiar—namely, the computers, smartphones and other devices that we use on a daily basis. In the next section, we’ll move from the network edge to the network core and examine switching and routingin computer networks. Recall from the previous section that in computer networking jargon, the computers and other devices connected to the Internet are often referred to as end systems. They are referred to as end systems because they sit at the edge of the Internet,as shown in Figure 1.3. The Internet’s end systems include desktop computers National or Global ISP Datacenter Network Datacenter Network Local or Home Network Regional ISP Content Provider Network Enterprise Network Figure 1.3 ♦ End-system interaction  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 2 (e.g., desktop PCs, Macs, and Linux boxes), servers (e.g., Web and e-mail servers), and mobile devices (e.g., laptops, smartphones, and tablets). Furthermore, an increasing number of non-traditional “things” are being attached to the Internet as end systems (see the Case History feature). End systems are also referred to as hosts because they host (that is, run) application programs such as a Web browser program, a Web server program, an e-mail client program, or an e-mail server program. Throughout this book we will use the terms hosts and end systems interchangeably; that is, host = end system. Hostsare sometimes further divided into two categories: clients and servers. Informally, clients tend to be desktops, laptops, smartphones, and so on, whereas servers tend to be more powerful machines that store and distribute Web pages, stream video, relay e-mail, and so on. Today, most of the servers from which we receive search results, e-mail, Web pages, videos and mobile app content residein large data centers. For example, as of 2020, Google has 19 data centers on four continents, collectively containing several million servers. Figure 1.3 includes two such data centers, and the Case History sidebar describes data centers inmore detail. 1.2.1 Access Networks Having considered the applications and end systems at the “edge of the network,” let’s next consider the access network—the network that physically connects an endsystem to the first router (also known as the “edge router”) on a path from the end system to any other distant end system. Figure 1.4 shows several types of access networks with thick, shaded lines and the settings (home, enterprise, and wide- area mobile wireless) in which they are used. 2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 3 National or Global ISP Datacenter Network Datacenter Network Local or Home Network Regional ISP Content Provider Network Enterprise Network Figure 1.4 ♦ Access networks  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 4 Home Access: DSL, Cable, FTTH, and 5G Fixed Wireless As of 2020, more than 80% of the households in Europe and the USA have Internet access [Statista 2019]. Given this widespread use of home access networks let’s begin our overview of access networks by considering how homes connect to the Internet. Today, the two most prevalent types of broadband residential access are digital subscriber line (DSL) and cable. A residence typically obtains DSL Internet access from the same local telephone company (telco) that provides its wired local phone access. Thus, when DSL is used, a customer’s telco is alsoits ISP. As shown in Figure 1.5, each customer’s DSL modem uses the existing telephone line exchange data with a digital subscriber line access multiplexer (DSLAM) located in the telco’s local central office (CO). The home’s DSL modem takes digital data and translates it to high-frequency tones for transmis-sion over telephone wires to the CO; the analog signals from many such houses are translated back into digital format at the DSLAM. The residential telephone line carries both data and traditional telephone signals simultaneously, which are encoded at different frequencies: A high-speed downstream channel, in the 50 kHz to 1 MHz band A medium-speed upstream channel, in the 4 kHz to 50 kHz band An ordinary two-way telephone channel, in the 0 to 4 kHz band This approach makes the single DSL link appear as if there were three separate links, so that a telephone call and an Internet connection can share the DSL link at Splitter Central Figure 1.5 ♦ DSL Internet access  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 5 the same time. (We’ll describe this technique of frequency-division multiplexingin Section 1.3.1.) On the customer side, a splitter separates the data and telephone signals arriving to the home and forwards the data signal to the DSL modem. On thetelco side, in the CO, the DSLAM separates the data and phone signals and sends the data into the Internet. Hundreds or even thousands of households connect to a single DSLAM. The DSL standards define multiple transmission rates, including downstream transmission rates of 24 Mbs and 52 Mbs, and upstream rates of 3.5 Mbps and16 Mbps; the newest standard provides for aggregate upstream plus downstream rates of 1 Gbps [ITU 2014]. Because the downstream and upstream rates are dif- ferent, the access is said to be asymmetric. The actual downstream and upstream transmission rates achieved may be less than the rates noted above, as the DSL provider may purposefully limit a residential rate when tiered service (different rates, available at different prices) are offered. The maximum rate is also limitedby the distance between the home and the CO, the gauge of the twisted-pair lineand the degree of electrical interference. Engineers have expressly designed DSLfor short distances between the home and the CO; generally, if the residence is not located within 5 to 10 miles of the CO, the residence must resort to an alternative form of Internet access. While DSL makes use of the telco’s existing local telephone infrastructure, cable Internet access makes use of the cable television company’s existing cable television infrastructure. A residence obtains cable Internet access from the same company that provides its cable television. As illustrated in Figure 1.6, fiber optics Coaxial cable Hundreds Fiber of homes Fiber cable Hundreds of homes Fiber Figure 1.6 ♦ A hybrid fiber-coaxial access network  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 6 connect the cable head end to neighborhood-level junctions, from which tradi- tional coaxial cable is then used to reach individual houses and apartments. Each neighborhood junction typically supports 500 to 5,000 homes. Because both fiber and coaxial cable are employed in this system, it is often referred to as hybrid fiber coax (HFC). Cable internet access requires special modems, called cable modems. As with a DSL modem, the cable modem is typically an external device and con-nects to the home PC through an Ethernet port. (We will discuss Ethernet ingreat detail in Chapter 6.) At the cable head end, the cable modem termination system (CMTS) serves a similar function as the DSL network’s DSLAM— turning the analog signal sent from the cable modems in many downstream homes back into digital format. Cable modems divide the HFC network into two channels, a downstream and an upstream channel. As with DSL, access is typi- cally asymmetric, with the downstream channel typically allocated a higher transmission rate than the upstream channel. The DOCSIS 2.0 and 3.0 standards define downstream bitrates of 40 Mbps and 1.2 Gbps, and upstream ratesof 30 Mbps and 100 Mbps, respectively. As in the case of DSL networks, the maximum achievable rate may not be realized due to lower contracted data ratesor media impairments. One important characteristic of cable Internet access is that it is a shared broad-cast medium. In particular, every packet sent by the head end travels downstream onevery link to every home and every packet sent by a home travels on the upstream channel to the head end. For this reason, if several users are simultaneously down- loading a video file on the downstream channel, the actual rate at which each user receives its video file will be significantly lower than the aggregate cable down- stream rate. On the other hand, if there are only a few active users and they are all Web surfing, then each of the users may actually receive Web pages at the full cabledownstream rate, because the users will rarely request a Web page at exactly the same time. Because the upstream channel is also shared, a distributed multiple accessprotocol is needed to coordinate transmissions and avoid collisions. (We’ll discuss this collision issue in some detail in Chapter 6.) Although DSL and cable networks currently represent the majority of residentialbroadband access in the United States, an up-and-coming technology that provides even higher speeds is fiber to the home (FTTH) [Fiber Broadband 2020]. As the name suggests, the FTTH concept is simple—provide an optical fiber path fromthe CO directly to the home. FTTH can potentially provide Internet access rates in the gigabits per second range. There are several competing technologies for optical distribution from the CO to the homes. The simplest optical distribution network is called direct fiber, with one fiber leaving the CO for each home. More commonly, each fiber leaving the central office is actually shared by many homes; it is not until the fiber gets rela- tively close to the homes that it is split into individual customer-specific fibers. There are two competing optical-distribution network architectures that perform  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 7 Central office Optical splitter Optical Figure 1.7 ♦ FTTH Internet access this splitting: active optical networks (AONs) and passive optical networks (PONs). AON is essentially switched Ethernet, which is discussed in Chapter 6. Here, we briefly discuss PON, which is used in Verizon’s FiOS service. Figure 1.7 shows FTTH using the PON distribution architecture. Each home hasan optical network terminator (ONT), which is connected by dedicated opticalfiber to a neighborhood splitter. The splitter combines a number of homes (typi- cally less than 100) onto a single, shared optical fiber, which connects to an optical line terminator (OLT) in the telco’s CO. The OLT, providing conversion between optical and electrical signals, connects to the Internet via a telco router. At home, users connect a home router (typically a wireless router) to the ONT and access the Internet via this home router. In the PON architecture, all packets sent from OLT tothe splitter are replicated at the splitter (similar to a cable head end). In addition to DSL, Cable, and FTTH, 5G fixed wireless is beginning to be deployed. 5G fixed wireless not only promises high-speed residential access, but will do so without installing costly and failure- prone cabling from the telco’sCO to the home. With 5G fixed wireless, using beam-forming technology, datais sent wirelessly from a provider’s base station to the a modem in the home.A WiFi wireless router is connected to the modem (possibly bundled together), similar to how a WiFi wireless router is connected to a cable or DSL modem.5G cellular networks are covered in Chapter 7. Access in the Enterprise (and the Home): Ethernet and WiFi On corporate and university campuses, and increasingly in home settings, a local area network (LAN) is used to connect an end system to the edge router. Although there are many types of LAN technologies, Ethernet is by far the most preva-lent access technology in corporate, university, and home networks. As shown in  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 8 Ethernet Institutional To Institution’s ISP Server Figure 1.8 ♦ Ethernet Internet access Figure 1.8, Ethernet users use twisted-pair copper wire to connect to an Ethernet switch, a technology discussed in detail in Chapter 6. The Ethernet switch, or a network of such interconnected switches, is then in turn connected into the larger Internet. With Ethernet access, users typically have 100 Mbps to tens of Gbps access to the Ethernet switch, whereas servers may have 1 Gbps 10 Gbps access. Increasingly, however, people are accessing the Internet wirelessly from lap- tops, smartphones, tablets, and other “things”. In a wireless LAN setting, wireless users transmit/receive packets to/from an access point that is connected into the enterprise’s network (most likely using wired Ethernet), which in turn is connected to the wired Internet. A wireless LAN user must typically be within a few tens of meters of the access point. Wireless LAN access based on IEEE 802.11 technol- ogy, more colloquially known as WiFi, is now just about everywhere—universities,business offices, cafes, airports, homes, and even in airplanes. As discussed in detailin Chapter 7, 802.11 today provides a shared transmission rate of up to more than 100 Mbps. Even though Ethernet and WiFi access networks were initially deployed in enterprise (corporate, university) settings, they are also common components of home networks. Many homes combine broadband residential access (that is, cable modems or DSL) with these inexpensive wireless LAN technologies to create pow-erful home networks Figure 1.9 shows a typical home network. This home network consists of a roaming laptop, multiple Internet-connected home appliances, as well as a wired PC; a base station (the wireless access point), which communicates with the wireless PC and other wireless devices in the home; and a home router that con-nects the wireless access point, and any other wired home devices, to the Internet. This network allows household members to have broadband access to the Internet with one member roaming from the kitchen to the backyard to the bedrooms.  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 9 Home Network Figure 1.9 ♦ A typical home network Wide-Area Wireless Access: 3G and LTE 4G and 5G Mobile devices such as iPhones and Android devices are being used to message, share photos in social networks, make mobile payments, watch movies, stream music, and much more while on the run. These devices employ the same wireless infrastructureused for cellular telephony to send/receive packets through a base station that is oper-ated by the cellular network provider. Unlike WiFi, a user need only be within a fewtens of kilometers (as opposed to a few tens of meters) of the base station. Telecommunications companies have made enormous investments in so-called fourth-generation (4G) wireless, which provides real-world download speeds of up to60 Mbps. But even higher-speed wide-area access technologies—a fifth-generation (5G) of wide-area wireless networks—are already being deployed. We’ll cover the basic principles of wireless networks and mobility, as well as WiFi, 4G and 5G tech-nologies (and more!) in Chapter 7. Physical Media In the previous subsection, we gave an overview of some of the most important network access technologies in the Internet. As we described these technologies,we also indicated the physical media used. For example, we said that HFC uses a combination of fiber cable and coaxial cable. We said that DSL and Ethernet use copper wire. And we said that mobile access networks use the radio spectrum. In this subsection, we provide a brief overview of these and other transmission media that are commonly used in the Internet. In order to define what is meant by a physical medium, let us reflect on the brief life of a bit. Consider a bit traveling from one end system, through a seriesof links and routers, to another end system. This poor bit gets kicked aroundand transmitted many, many times! The source end system first transmits the  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 10 bit, and shortly thereafter the first router in the series receives the bit; the firstrouter then transmits the bit, and shortly thereafter the second router receives thebit; and so on. Thus our bit, when traveling from source to destination, passes through a series of transmitter-receiver pairs. For each transmitter- receiver pair,the bit is sent by propagating electromagnetic waves or optical pulses across a physical medium. The physical medium can take many shapes and forms anddoes not have to be of the same type for each transmitter-receiver pair alongthe path. Examples of physical media include twisted- pair copper wire, coaxial cable, multimode fiber-optic cable, terrestrial radio spectrum, and satellite radio spectrum. Physical media fall into two categories: guided media and unguided media. With guided media, the waves are guided along a solid medium, such asa fiber-optic cable, a twisted-pair copper wire, or a coaxial cable. With unguided media, the waves propagate in the atmosphere and in outer space, such as in a wireless LAN or a digital satellite channel. But before we get into the characteristics of the various media types, let us say afew words about their costs. The actual cost of the physical link (copper wire, fiber-optic cable, and so on) is often relatively minor compared with other networking costs. In particular, the labor cost associated with the installation of the physical linkcan be orders of magnitude higher than the cost of the material. For this reason, many builders install twisted pair, optical fiber, and coaxial cable in every room in a build-ing. Even if only one medium is initially used, there is a good chance that another medium could be used in the near future, and so money is saved by not having to layadditional wires in the future. Twisted-Pair Copper Wire The least expensive and most commonly used guided transmission medium istwisted-pair copper wire. For over a hundred years it has been used by telephone networks. In fact, more than 99 percent of the wired connections from the telephonehandset to the local telephone switch use twisted-pair copper wire. Most of us have seen twisted pair in our homes (or those of our parents or grandparents!) and work environments. Twisted pair consists of two insulated copper wires, each about 1 mmthick, arranged in a regular spiral pattern. The wires are twisted together to reduce theelectrical interference from similar pairs close by. Typically, a number of pairs are bundled together in a cable by wrapping the pairs in a protective shield. A wire pair constitutes a single communication link. Unshielded twisted pair (UTP) is com- monly used for computer networks within a building, that is, for LANs. Data rates for LANs using twisted pair today range from 10 Mbps to 10 Gbps. The data rates that can be achieved depend on the thickness of the wire and the distance between transmitter and receiver. When fiber-optic technology emerged in the 1980s, many people dispar- aged twisted pair because of its relatively low bit rates. Some people even felt  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 11 that fiber-optic technology would completely replace twisted pair. But twistedpair did not give up so easily. Modern twisted-pair technology, such as category 6a cable, can achieve data rates of 10 Gbps for distances up to a hundred meters.In the end, twisted pair has emerged as the dominant solution for high-speed LAN networking. As discussed earlier, twisted pair is also commonly used for residential Inter- net access. We saw that dial-up modem technology enables access at rates of up to 56 kbps over twisted pair. We also saw that DSL (digital subscriber line) technologyhas enabled residential users to access the Internet at tens of Mbps over twisted pair(when users live close to the ISP’s central office). Coaxial Cable Like twisted pair, coaxial cable consists of two copper conductors, but the two con-ductors are concentric rather than parallel. With this construction and special insula-tion and shielding, coaxial cable can achieve high data transmission rates. Coaxial cable is quite common in cable television systems. As we saw earlier, cable televi- sion systems have recently been coupled with cable modems to provide residential users with Internet access at rates of hundreds of Mbps. In cable television and cableInternet access, the transmitter shifts the digital signal to a specific frequency band, and the resulting analog signal is sent from the transmitter to one or more receivers. Coaxial cable can be used as a guided shared medium. Specifically, a number of end systems can be connected directly to the cable, with each of the end systems receiving whatever is sent by the other end systems. Fiber Optics An optical fiber is a thin, flexible medium that conducts pulses of light, with each pulse representing a bit. A single optical fiber can support tremendous bit rates, up to tens or even hundreds of gigabits per second. They are immune to electromagnetic interference, have very low signal attenuation up to 100 kilometers, and are very hardto tap. These characteristics have made fiber optics the preferred long-haul guided transmission media, particularly for overseas links. Many of the long-distance tele- phone networks in the United States and elsewhere now use fiber optics exclusively.Fiber optics is also prevalent in the backbone of the Internet. However, the high costof optical devices—such as transmitters, receivers, and switches—has hindered theirdeployment for short-haul transport, such as in a LAN or into the home in a resi- dential access network. The Optical Carrier (OC) standard link speeds range from 51.8 Mbps to 39.8 Gbps; these specifications are often referred to as OC-n, where the link speed equals n × 51.8 Mbps. Standards in use today include OC-1, OC-3, OC-12, OC-24, OC-48, OC-96, OC-192, OC- 768.  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 12 Terrestrial Radio Channels Radio channels carry signals in the electromagnetic spectrum. They are an attrac- tive medium because they require no physical wire to be installed, can penetrate walls, provide connectivity to a mobile user, and can potentially carry a signalfor long distances. The characteristics of a radio channel depend significantly on the propagation environment and the distance over which a signal is to be carried. Environmental considerations determine path loss and shadow fad-ing (which decrease the signal strength as the signal travels over a distance and around/through obstructing objects), multipath fading (due to signal reflection off of interfering objects), and interference (due to other transmissions and electro- magnetic signals). Terrestrial radio channels can be broadly classified into three groups: those that operate over very short distance (e.g., with one or two meters); those that operate in local areas, typically spanning from ten to a few hundred meters; and those that oper-ate in the wide area, spanning tens of kilometers. Personal devices such as wireless headsets, keyboards, and medical devices operate over short distances; the wireless LAN technologies described in Section 1.2.1 use local-area radio channels; the cel- lular access technologies use wide-area radio channels. We’ll discuss radio channelsin detail in Chapter 7. Satellite Radio Channels A communication satellite links two or more Earth-based microwave transmitter/ receivers, known as ground stations. The satellite receives transmissions on one frequency band, regenerates the signal using a repeater (discussed below),and transmits the signal on another frequency. Two types of satellites are usedin communications: geostationary satellites and low-earth orbiting (LEO) satellites. Geostationary satellites permanently remain above the same spot on Earth.This stationary presence is achieved by placing the satellite in orbit at 36,000 kilo- meters above Earth’s surface. This huge distance from ground station throughsatellite back to ground station introduces a substantial signal propagation delay of 280 milliseconds. Nevertheless, satellite links, which can operate at speeds of hundreds of Mbps, are often used in areas without access to DSL or cable-based Internet access. LEO satellites are placed much closer to Earth and do not remain permanently above one spot on Earth. They rotate around Earth (just as the Moon does) and may communicate with each other, as well as with ground stations. To provide continuouscoverage to an area, many satellites need to be placed in orbit. There are currently many low-altitude communication systems in development. LEO satellite technology may be used for Internet access sometime in the future. 2.2. WSN Hardware Design Issues In a generic sensor node (Figure 3), we can identify a power module, a communication block, a processing unit with internal and/or external memory, and a module for sensing and actuation. Power Using stored energy or harvesting energy from the outside world are the two options for the power module. Energy storage may be achieved with the use of batteries or alter- native devices such as fuel cells or miniaturized heat engines, whereas energy-scavenging opportunities [D37] are provided by solar power, vibrations, acoustic noise, and piezoelectric effects [D38]. The vast majority of the existing commercial and research platforms relies on bat- teries, which dominate the node size. Primary (non- rechargeable) batteries are often chosen, predominantly AA, AAA and coin-type. Alkaline batteries offer a high ener- gy density at a cheap price, offset by a non-flat discharge, a large physical size with respect to a typical  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 13 sensor node, and a shelf life of only 5 years. Voltage regulation could in principle be employed, but its high inefficiency and large quiescent current consumption call for the use of compo- nents that can deal with large variations in the supply volt- age [A5]. Lithium cells are very compact and boast a flat discharge curve. Secondary (rechargeable) batteries are typically not desirable, as they offer a lower energy densi- ty and a higher cost, not to mention the fact that in most applications recharging is simply not practical. Fuel cells [D39] are rechargeable electrochemical ener- gy-conversion devices where electricity and heat are pro- duced as long as hydrogen is supplied to react with oxygen. Pollution is minimal, as water is the main byproduct of the reaction. The potential of fuel cells for energy storage and power delivery is much higher than the one of traditional battery technologies, but the fact that they require hydro- gen complicates their application. Using renewable energy and scavenging techniques is an interesting alternative. Communication Most sensor networks use radio communication, even if alternative solutions are offered by laser and infrared. Nearly all radio-based platforms use COTS (Commercial Off-The-Shelf) components. Popular choices include the TR1000 from RFM (used in the MICA motes) and the CC1000 from Chipcon (chosen for the MICA2 platform). More recent solutions use industry standards like IEEE 802.15.4 (MICAz and Telos motes with CC2420 from Chipcon) or pseudo-standards like Bluetooth. Typically, the transmit power ranges between −25 dBm and 10 dBm, while the receiver sensitivity can be as good as −110 dBm. Power Sensors ADC Communication Hardware (Actuators) Processor Memory Spread spectrum techniques increase the channel reliabil- ity and the noise tolerance by spreading the signal over a wide range of frequencies. Frequency hopping (FH) is a spread spectrum technique used by Bluetooth: the carrier frequency changes 1600 times per second on the basis of a pseudo-random algorithm. However, channel synchro- nization, hopping sequence search, and the high data rate increase power consumption; this is one of the strongest caveats when using Bluetooth in sensor network nodes. In Direct Sequence Spread Spectrum (DSSS), communication is carried out on a single carrier frequency. The signal is multiplied by a higher rate pseudo-random sequence and thus spread over a wide frequency range (typical DSSS radios have spreading factors between 15 and 100). Ultra Wide Band (UWB) is of great interest for sensor networks since it meets some of their main requirements. UWB is a particular carrier-free spread spectrum tech- nique where the RF signal is spread over a spectrum as large as several GHz. This implies that UWB signals look like noise to conventional radios. Such signals are pro- duced using baseband pulses (for instance, Gaussian monopulses) whose  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 14 length ranges from 100 ps to 1 ns, and baseband transmission is generally carried out by means of pulse position modulation (PPM). Modulation and demodulation are indeed extremely cheap. UWB provides built-in ranging capabilities (a wideband signal allows a good time resolution and therefore a good location accu- racy) [D40], allows a very low power consumption, and performs well in the presence of multipath fading. Radios with relatively low bit-rates (up to 100 kbps) are advantageous in terms of power consumption. In most sensor networks, high data rates are not needed, even though they allow shorter transmission times thus permitting lower duty cycles and alleviating channel access contention. It is also desirable for a radio to quickly switch from a sleep mode to an operational mode. Optical transceivers such as lasers offer a strong power advantage, mainly due to their high directionality and the fact that only baseband processing is required. Also, security is intrinsically guaranteed (intercepted sig- nals are altered). However, the need for a line of sight and precise localization makes this option impractical for most applications. Processing and Computing Although low-power FPGAs might become a viable option in the near future [D41], microcontrollers (MCUs) are now the primary choice for processing in sensor nodes. The key metric in the selection of an MCU is power consumption. Sleep modi deserve special atten- tion, as in many applications low duty cycles are essen- tial for lifetime extension. Just as in the case of the radio module, a fast wake-up time is important. Most CPUs used in lower-end sensor nodes have clock speeds of a few MHz. The memory requirements depend on the application and the network topology: data storage is not critical if data are often relayed to a base station. Berke- ley motes, UCLA’s Medusa MK-2 and ETHZ’s BTnodes use low- cost Atmel AVR 8-bit RISC microcontrollers which consume about 1500 pJ/instruction. More sophisticated platforms, such as the Intel iMote and Rockwell WINS nodes, use Intel StrongArm/XScale 32-bit processors. Sensing The high sampling rates of modern digital sensors are usually not needed in sensor networks. The power effi- ciency of sensors and their turn-on and turn-off time are much more important. Additional issues are the physi- cal size of the sensing hardware, fabrication, and assem- bly compatibility with other components of the system. Packaging requirements come into play, for instance, with chemical sensors which require contact with the environment [D42]. Using a microcontroller with an on- chip analog comparator is another energy-saving tech- nique which allows the node to avoid sampling values falling outside a certain range [D43]. The ADC which complements analog sensors is particularly critical, as its resolution has a direct impact on energy consump- tion. Fortunately, typical sensor network applications do not have stringent resolution requirements. Micromachining techniques have allowed the minia- turization of many types of sensors. Performance does decrease with sensor size, but for many sensor network applications size matters much more than accuracy. Standard integrated circuits may also be used as temperature sensors (e.g., using the temperature- dependence of subthreshold MOSFETs and pn junc- tions) or light intensity transducers (e.g., using photodi- odes or phototransistors) [D44]. Nanosensors can offer promising solutions for biological and chemical sensors while concurrently meeting the most ambitious minia- turization needs. 2.3. Applications of Sensor Networks Possible applications of sensor networks are of interest to the most diverse fields. Environmental monitoring, war- fare, child education, surveillance, micro-surgery, and agriculture are only a few examples [A4]. Through joint efforts of the University of California at Berkeley and the College of the Atlantic, environmental monitoring is car- ried out off the coast of Maine on Great Duck Island by means of a network of Berkeley motes equipped with var- ious sensors [B6]. The nodes send their data to a base  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 15 station which makes them available on the Internet. Since habitat monitoring is rather sensitive to human presence, the deployment of a sensor network provides a non- invasive approach and a remarkable degree of granularity in data acquisition [B7]. The same idea lies behind the Pods project at the University of Hawaii at Manoa [B8], where environmental data (air temperature, light, wind, relative humidity and rainfall) are gathered by a network of weather sensors embedded in the communication units deployed in the South-West Rift Zone in Volcanoes National Park on the Big Island of Hawaii. A major concern of the researchers was in this case camouflaging the sen- sors to make them invisible to curious tourists. In Prince- ton’s Zebranet Project [B9], a dynamic sensor network has been created by attaching special collars equipped with a low-power GPS system to the necks of zebras to monitor their moves and their behavior. Since the net- work is designed to operate in an infrastructure-free environment, peer-to-peer swaps of information are used to produce redundant databases so that researchers only have to encounter a few zebras in order to collect the data. Sensor networks can also be used to monitor and study natural phenomena which intrinsically discourage human presence, such as hurricanes and forest fires. Joint efforts between Harvard University, the University of New Hampshire, and the University of North Carolina have recently led to the deployment of a wireless sensor network to monitor eruptions at Volcán Tungurahua, an active volcano in central Ecuador. A network of Berkeley motes monitored infrasonic signals during eruptions, and data were transmitted over a 9 km wireless link to a base station at the volcano observatory [B10]. Intel’s Wireless Vineyard [B11] is an example of using ubiquitous computing for agricultural monitoring. In this application, the network is expected not only to collect and interpret data, but also to use such data to make deci- sions aimed at detecting the presence of parasites and enabling the use of the appropriate kind of insecticide. Data collection relies on data mules, small devices carried by people (or dogs) that communicate with the nodes and collect data. In this project, the attention is shifted from reliable information collection to active decision- making based on acquired data. Just as they can be used to monitor nature, sensor networks can likewise be used to monitor human behav- ior. In the Smart Kindergarten project at UCLA [B12], wirelessly-networked, sensor-enhanced toys and other classroom objects supervise the learning process of chil- dren and allow unobtrusive monitoring by the teacher. Medical research and healthcare can greatly benefit from sensor networks: vital sign monitoring and accident recognition are the most natural applications. An impor- tant issue is the care of the elderly, especially if they are affected by cognitive decline: a network of sensors and actuators could monitor them and even assist them in their daily routine. Smart appliances could help them organize their lives by reminding them of their meals and medications. Sensors can be used to capture vital signs from patients in real-time and relay the data to handheld computers carried by medical personnel, and wearable sensor nodes can store patient data such as identification, history, and treatments. An interesting application to civil engineering is the idea of Smart Buildings: wireless sensor and actuator net- works integrated within buildings could allow distributed monitoring and control, improving living conditions and reducing the energy consumption, for instance by con- trolling temperature and air flow. Military applications are plentiful. An intriguing example is DARPA’s self-healing minefield [B15], a self- organizing sensor network where peer-to-peer communi- cation between anti-tank mines is used to respond to attacks and redistribute the mines in order to heal breaches, complicating the progress of enemy troops. Urban warfare is another application that distributed sensing lends itself to. An ensemble of nodes could be deployed in a urban landscape to detect chemical attacks, or track enemy movements. PinPtr is an ad hoc acoustic sensor network for sniper localization devel- oped at Vanderbilt University [B16]. The network detects the muzzle blast and the acoustic shock wave that origi- nate from the sound of gunfire. The arrival times of the acoustic events at different sensor nodes are used to esti- mate the position of the sniper and send it to the base sta- tion with a special data aggregation and routing service. Going back to peaceful applications, efforts are under- way at Carnegie Mellon University and Intel for the design of IrisNet (Internet-scale Resource-Intensive Sensor Net- work Services) [B17], an architecture for a worldwide sen- sor web based on common computing hardware such as Internet-connected PCs and  2. THE TRANSMIAAION, CHANLLAGES & APPLICATIONS OF WIRELESS COMPUTING 16 low-cost sensing hardware such as webcams. The network interface of a PC indeed senses the virtual environment of a LAN or the Internet rather than a physical environment; with an architecture based on the concept of a distributed database [B18], this hardware can be orchestrated into a global sensor system that responds to queries from users.