THE CHALLENGES OF WEARABLE COMPUTING: PART 2 WEARABLE COMPUTING PURSUES AN INTERFACE IDEAL OF A CONTINUOUSLY WORN, INTELLIGENT ASSISTANT THAT AUGMENTS MEMORY, INTELLECT, CREATIVITY, COMMUNICATION, AND PHYSICAL SENSES AND ABILITIES. MANY CHALLENGES AWAIT WEARABLE DESIGNERS. PART 2 BEGINS WITH THE CHALLENGES OF NETWORK RESOURCES AND PRIVACY CONCERNS. THIS SURVEY DESCRIBES THE POSSIBILITIES OFFERED BY WEARABLE SYSTEMS AND, IN DOING SO, DEMONSTRATES ATTRIBUTES UNIQUE TO THIS CLASS OF COMPUTING. Challenges throughput. Another serious issue is open The most immediately striking challenge standards to enable interoperability between in designing wearable computers is creating different services. For example, only one long- appropriate interfaces. However, the issues of range radio should be necessary to provide power use, heat dissipation, networking, and telephony, text messaging, Global Positioning privacy provide a necessary framework in System (GPS) correction signals, and so on. which to discuss interface. Part 1 of this arti- For wearable computers, networking cle covers the first two of these issues; Part 2 involves communication off body to the fixed begins with the networking discussion. network, on body among devices, and near Thad Starner body with objects near the user. Each of these Networking three network types requires different design Georgia Institute of As with any wireless mobile device, the decisions. Designers must also consider pos- amount of power and the type of services sible interference between the networks. Technology available can constrain networking. Wearable computers could conserve resources through Off-body communications. Wireless commu- improved coordination with the user inter- nication from mobile devices to fixed infra- face. For example, the speed at which a given structure is the most thoroughly researched of information packet is transferred can be bal- these issues. On the consumer side, analog cel- anced against latency, energy costs, and finan- lular phones and digital amateur-radio cial costs. Often, bits per second per watt is a repeaters provided the first glimpse of future more meaningful measure of a particular wire- problems; these systems would often drop less networking technology than maximum connections as the user moved. Communica- 54 0272-1732/01/$10.00 2001 IEEE tions researchers developed systems based on several standards—cellular digital packet data Cellular phones (CDPD), Global System for Mobile Com- Although not wearable computers, the original analog cellular phones provide a study in munications (GSM), time-division multiple- design constraints related to networking and interfaces of devices. Initially, one of the most access (TDMA), and code-division important features of these phones was a user interface that resembled, as closely as possi- multiple-access (CDMA)—to help this prob- ble, a push button telephone. This feature provided a key improvement over the half-duplex, lem. Today, next-generation communications push-to-talk walkie-talkies of the time. systems (2.5G and 3G) will further improve As the technology matures, a variety of fundamentally different interfaces are becoming connection reliability and aggregate through- popular, such as instant messaging. Alternative networking paradigms will also appear because put. The “Cellular phones” sidebar discusses of the expense of deploying cellular towers, upgrading infrastructure continuously, and shrink- networking and interface issues applicable to ing available bandwidth. These new paradigms will take advantage of asynchronous com- wearable computers and their design. munication and other modes of human-to-human interaction, such as those used in Internet No matter what technologies finally dom- store-and-forward networks. Even today, cellular phones are becoming more like wearable com- inate, some challenges will remain. First, none puters. Higher-end microprocessors, multitasking operating systems, and bitmapped displays of the current networking systems will be now exist in cellular phones. Cellular phone manufacturers embed sensors in these devices ubiquitous. Users will always face situations to determine if the user is in a meeting or walking on a noisy street. The cellular phone uses in which mobile devices will not be in range this information to determine if the user is interruptible and the mode to use as an incoming- of a network cell. Although additional cell call alert. Indeed, researchers are developing some systems in which the cellular phone com- deployment and satellite use is slowly address- municates a worker’s current task to a remote expert who can provide advice. ing this problem, it will remain unprofitable to provide coverage for some areas. However, an interesting concept is to employ automo- bandwidth. When the user is working offline, biles as repeaters for the wearable user’s wire- the system employs this cache and updates any less data traffic.1 Although a wearable changes when network connectivity becomes computer has a relatively small battery and available.2 But what happens when a conflict antenna, cars can carry much larger equip- occurs? For example, suppose a businessman ment. In addition, drivers rarely stray further updates his calendar while disconnected, and, than a few miles from their automobiles. Fur- during the same time, his assistant also sched- thermore, even when driven into a remote ules an appointment. In practice, such conflicts location, a car is often within communication are rare, but this problem raises the issue of range of other cars. For example, imagine a where to locate a file’s correct, or master, copy. series of cars along rural highways acting as Most wireless mobile devices by nature repeaters to route wireless data to a local, fixed adhere to a thin-client approach to comput- networking center, such as a US post office. ing. In other words, the device provides just This wireless service does not need to be real enough processing power, user interface, and time to provide value; store-and-forward net- data storage to access services that are based working has been used successfully with the on a fixed-infrastructure server located else- Internet for decades. where. Mobile devices without wireless access, For such a wireless participatory network- such as the original PalmPilot, provide asyn- ing scheme to succeed, developers must chronous services where docking the device address standardization, security, quality of ser- with the fixed network updates the file’s mas- vice, and synchronization. This style of ad hoc ter copy. However, the rapid increase in networking of mobile devices complicates tra- mobile mass-storage capacity, increasing at a ditional issues of resource discovery and rout- rate that surpasses that of Moore’s law, makes ing. Furthermore, mobile nodes traveling at a strong case for the mobile device maintain- variable speeds create difficulties for many ing the master copy. Today’s ruggedized, pock- wireless systems. Fortunately, this area of et-size hard drives can store 48 Gbytes. Soon, research has become very active in recent years. mobile users will maintain a terabyte on body, Another way to alleviate coverage issues is to making local storage space a nonissue. In fact, employ aggressive caching. By observing the users might prefer keeping all their data with wearer’s network use, the wearable computer them, making the file data physically secure, can speculate about what the user will access accessible at any time, and always the author- next and cache material using spare network itative copy. JULY–AUGUST 2001 55 WEARABLE COMPUTING Caching, revision control, and intelligent will also need standards for resource discov- agents can emulate remote access to the user’s ery and node arbitration to enable communi- on-body personal directories when wireless cation. Solving this problem for wearables connectivity is unavailable. Although this strat- could prove easier than for ubiquitous com- egy could prove inconvenient for the occasional puting because a wearer will only occasional- third-party user, it gives the wearer the most ly add or remove a device from his body convenient access. For applications like calen- network. dars, the wearer knows her copy is definitive. Although on-body communication requires Retaining information locally could also significantly less energy than off-body net- help conserve battery power. In terms of works, energy use becomes critical because power, wireless network access requests are each device must have its own, relatively small generally more expensive than local access battery. This is a current challenge for the requests. Thus, if the user is responsible for the Bluetooth and IEEE 802.15 communities. majority of the data accesses, maintaining a Some experimental systems, such as BBN’s local copy will conserve power. Modeling user BodyLAN, require as little as 4-nJ/bit to trans- activities can also conserve power. Many wire- mit, while maintaining moderate bandwidth.4 less networks scale transmission power to the With such energy conservation, low-band- minimum necessary to maintain a connection. width, on-body sensors and interface devices Thus, transmitting a 10-Mbyte e-mail mes- could last for a year on one charge. As a side sage when the wearer is near a receiver could benefit, such low transmission power makes require significantly less power than when only US Federal Communications Commission a weaker signal is available. If the wearable approval significantly easier. computer understands a particular