Disappearing Hardware
R E A C H I N G F O R W E I S E R ’ S V I S I O N
Disappearing Hardware
In Mark Weiser’s vision of ubiquitous computing, computers
disappear from conscious thought. From a hardware perspective, the
authors examine how far we’ve succeeded in implementing this vision
and how far we have to go.
For many tasks today, the use of comput- (such as spell checkers, calculators, electronic trans-
ers is not entirely satisfactory. The inter-
lators, electronic books, and Web pads). These
actions take effort and are often difficult.
devices have a specialized interface and address the
The traditional, and still prevalent, com-
desired goal of ease of use. In contrast, a PC is a gen-
puting experience is sitting in front of a
eralized machine, which makes it attractive to pur-
box, our attention completely absorbed in the dia-
chase—the one-time investment having the potential
log required to complete the details of a greater task.
for many different uses. However, in other ways it
Putting this in perspective, the real objective is the
adds a level of complexity and formalism that hin-
task’s completion, not the interaction with the tools
ders the casual user.
we use to perform it.
Mark Weiser wanted to explore whether we could
To illustrate the point further, if somebody asks
design radically new kinds of computer systems.
you for an electric drill, do they want to use a drill,
These systems would allow the orchestration of
or do they really want a hole? The answer is prob-
devices with nontraditional form factors that lend
ably the latter—but computers are currently very
themselves to more natural, tacit interaction. They
much a drill, requiring knowl-
would take into account the space in which people
edge, training, effort, and skill to
worked, allowing positional and manipulative1—
Roy Want and Trevor Pering
use correctly. Creating a hole is
rather than just keyboard and mouse—interactions.
Intel Research, Santa Clara
relatively simple, and many hid-
Along with specialization and the use of embedded
Gaetano Borriello
den computers invisibly accom-
computers, support for mobile computing and wire-
University of Washington and
plish what seem to be simple
less data networks is an important facet of this
Intel Research, Seattle
tasks, such as regulating our cars’
vision—in other words, invisible connectivity. A goal
Keith I. Farkas
brakes. But unlike other inani-
of this exploration is that we would learn to build
Compaq Western Research
mate objects, a computer system
computer systems that do not distract the user; ide-
Laboratory
might be able to infer the result
ally, the user might even forget the hardware is pres-
autonomously and affect the
ent.2 In essence, Weiser was proposing that well-
desired outcome, increasing both
designed computer systems would become invisible
its potential and end-user complexity. Realizing this
to the user and that our conscious notion of com-
potential while managing the complexity is the fun-
puter hardware would begin to disappear. Some
damental challenge facing computer system
years later, Don Norman popularized this concept
researchers.
in his book The Invisible Computer.3
An important trend over the last decade is the
In this article, we survey the progress toward
emergence of specialized, task-specific hardware
Weiser’s vision from a hardware viewpoint. Where
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1536-1268/02/$17.00 © 2002 IEEE
Figure 1. Hardware improvement over
the last decade: (a) the Xerox ParcTab,
the first context-sensitive computer
(1992). The design shows the limited
display available at that time—a 128- ×
64-pixel, monochrome LCD. (b) a typical
PDA available today with a color 240- ×
320-pixel VGA (transreflective) screen.
have we been, where are we, and where are
we headed? What characteristics will make
hardware disappear from our conscious-
ness, and what will it take to achieve them?
Where we’ve been
The research community embraced
Weiser’s call to explore ubiquitous com-
puting. For example, his vision inspired
the work at the Xerox Palo Alto Research
(a)
(b)
Center (PARC) in the early 1990s and
such projects as ParcTab,4 Mpad,5 and
Liveboard.6 Olivetti Research’s Active
a pen and paper. Most of these products
use: if we notice it’s there, it’s distracting
Badge7 and Berkeley’s InfoPad8 projects
have fallen by the wayside: Momenta and
us from our real task. For example, if we
also embraced this research direction, as
EO, IBM’s early ThinkPad, and later the
notice that we are using a slow wireless net-
did other notable centers of excellence,
Apple Newton, the Casio Zoomer, and Gen-
work connection instead of just editing our
such as at Carnegie Mellon University,
eral Magic’s pad. For these designs, the ben-
files, then the action of accessing the files is
IBM, Rutgers University, Georgia Tech,
efit-to-cost ratio was just not large enough.
getting in the way of the real task, which is
and the University of Washington. Unfor-
To be successful, these new devices had to
contained in the files themselves. If the link
tunately, many of the early systems were
either be better than the traditional pencil-
is fast and robust, we will not notice it and
based on technologies that were barely
and-paper technology they were replacing
can focus on the content. Likewise, if a dis-
adequate for the task, so they fell short of
or provide desired new functionality. The
play can present only a poor representa-
designer expectations.
physical hardware was the dominating fac-
tion of a high-quality underlying image, we
Figures 1a and 1b illustrate the extent
tor, and almost every design aspect affected
see a bad display. A high-quality display
of hardware improvement over the last
acceptance: size, weight, power consump-
suspends our belief that the image is only
decade. In 1990, no Wireless Local Area
tion, computation speed, richness of inter-
a representation.
Network standards existed; the processors
face, and simplicity of design.
The four most notable improvements in
suitable for mobile devices operated at only
We started to cross the acceptability
hardware technology during the last decade
a few megahertz, while PCs were typically
threshold only in the latter half of the
that directly affected ubiquitous comput-
shipping with up to 50-MHz processors.
decade with the Palm Pilot. It was smaller
ing are wireless networking, processing
The early electronic organizers (pen-based
and lighter, focused on simple applications,
capability, storage capacity, and high-qual-
PDAs had not been invented) proudly
and incorporated a novel one-button
ity displays. Furthermore, the current pop-
claimed 128 Kbytes of memory, while PCs
approach to data synchronization. Finally,
ular adoption of emerging technology, such
shipped with 30-Mbyte disks. The displays
an electronic organizer was useful for a sig-
as cell phones and PDAs, strongly indicates
were also quite crude: laptops used mono-
nificant number of people and had real
that the market is generally ready for
chrome VGA, and the few handheld
advantages over the more traditional paper
advanced new technology. This adoption,
devices available mainly used character-
products such as Day-Timers. The com-
however, requires common standards
based displays.
puter industry was beginning to move in
across many products and locales.
Industry soon responded to the challenge
the right direction.
with a tighter focus on mobile computing.
Wireless networking
A flurry of early products hit the market,
Where we are
Although progress in wireless connec-
particularly in the tablet style, that tried to
For hardware to disappear from our
tivity was initially slow, it has increased.
make using computers feel more like using
consciousness, we require transparency of
This area has witnessed two distinct devel-
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opment trends. The first is in short-range
The emergence of the latter IEEE wire-
means that we can operate these devices at
connectivity standards, such as Bluetooth
less standards allows for communication
higher speeds, increasing their effective per-
(IEEE 802.15) and the IrDA (Infrared Data
cells that span many hundreds of feet with
formance. Additionally, reduced transistor
Association) standards, which are primar-
sufficient bandwidth to make us feel as if
sizes decrease power consumption, allevi-
ily for simple device-to-device communi-
we were connected to a wired LAN, but
ating some of the perpetual problems sur-
cation. Bluetooth, which will get its first
without a physical connection’s con-
rounding energy storage technologies.
real test in the marketplace in 2002, was
straints. IEEE 802.11b has already been
The combination of more transistors on
designed as a short-range cable replace-
widely adopted, and 802.11a is expected
a given area of silicon and a reduced power
ment, allowing for proximate interaction
to follow with higher bandwidths. Next-
budget has brought us the capabilities of
and the discovery of resources in the user’s
generation digital cellular networks such
mid-1980’s desktop computers in today’s
locality. IrDA had a similar aim. But
as 2.5G (for example, General Packet
battery-operated, handheld PDAs. Two
because infrared signaling requires a line
Radio Service and NTT’s DoCoMo—with
examples are the Motorola Dragonball
of sight, users had to physically place
greater than 24 million users) and the com-
and Intel StrongARM processors, the most
devices next to each other, often an incon-
ing 3G networks will extend these capa-
common processors used by today’s PDAs.
venience. This technology, which predates
bilities to cover entire metropolitan areas.
Besides providing low power consumption
Bluetooth by many years, has been con-
The wireless networking of today and
and high performance, these processors
sidered a market failure. (The sidebar lists
the immediate future thus enables portable
integrate their DRAM and LCD con-
URLs for Bluetooth, IrDA, and other areas
ubiquitous hardware that remains con-
trollers and a host of other interface I/O
of interest in this article.)
nected to the global infrastructure. How-
capability on the same die.
These trends directly affect ubiquitous
A truly ubiquitous computing experience
computing for mobile devices in two ways.
First, we can better match algorithmic com-
requires high-quality displays to let us see
plexity and execution speed to real-world
problems. Second, the resulting power con-
through the display process and effortlessly
sumption allows for a reasonable operating
time before batteries fail. These properties
acquire the underlying information.
have let us build ubiquitous computing hard-
ware that we can adapt to a greater range of
The second trend is in wireless LAN
ever, to date, wireless networks have lagged
task-specific activities. Nevertheless, further
technology, such as the 11-Mbit-per-sec-
behind the bandwidth capabilities of the
improvement is still required to satisfy the
ond IEEE 802.11b standard and the more
equivalent wired networks, leaving both
complete ubiquitous computing vision.
recent 54-Mbps IEEE 802.11a standard.
the opportunity and the user desire for
Wireless technologies provide for two
improved wireless hardware.
Storage capacity
basic needs: the ability to detect location
A less visible industry trend is the rate
and the more basic ability to communicate.
Processing capability
at which storage capacity is improving for
In many cases, the ubiquitous computing
For 30 years, processing capability has
rotating magnetic storage and solid-state
vision can to some degree be implemented
basically followed Moore’s law, which can
devices. For 25 years, the capacity of
by interpreting simple context information,
be summarized as “The number of active
rotating disks has been roughly doubling
such as a user’s location.9 Accurately deter-
devices we can place on a given area of sil-
every year, a rate of improvement faster
mining in-building locations is difficult.
icon doubles every 18 months”10 (This is
than Moore’s Law! The current storage
The Global Positioning System (GPS), for
actually a revision of Gordon Moore’s
density found on a disk drive is approxi-
the most part, can locate an object only to
1965 estimate of doubling per year and the
mately 10 Gbits per square inch.11 Today,
within 10 meters and does not work in
later 1995 estimate of doubling every two
IBM markets the Microdrive, a one-
buildings. Short-range wireless standards
years.) This trend’s obvious consequence
square-inch device with 1 Gbyte of stor-
such as Bluetooth let us more easily discern
is that we can continue to increase the
age in a compact flash-card format. If this
location and context within a limited area
capability of devices fabricated in a given
trend continues, in another decade we will
without having to support sophisticated
area of silicon. Instead of designing systems
be able to carry 1 Tbyte of data in a sim-
wide-area location technologies. Further-
built from separate board-level compo-
ilar form factor.
more, short range implies lower power and
nents, we can integrate diverse functional-
From a ubiquitous computing view-
the ability to build a smaller device from a
ity onto a single chip, resulting in remark-
point, storage is becoming so inexpensive
more compact energy source, making it
ably compact consumer electronics.
and plentiful that, for many devices, inter-
more attractive for many human-centric
Similarly, the reduced capacitance result-
nal storage capacity is not a limiting fac-
applications.
ing from smaller transistor dimensions
tor for basic operation. Moreover, we can
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http://computer.org/pervasive
Related URLs
begin to use storage in extravagant ways
I Aibo: www.aibo.com
by prefetching, caching, and archiving data
I Blackberry: www.blackberry.net
that might be useful later, lessening the
I Bluetooth: www.bluetooth.com
need for continuous network connectivity.
I DoCoMo: www.nttdocomo.co.jp (in Japanese)
Flash memory, DRAM, and Static RAM
I Electronic ink: www.eink.com
have all benefited from progress in inte-
I IEEE 802.11 Wireless Local Area Networks:
gration, making large capacities at low
http://grouper.ieee.org/groups/802/11/index.html
prices possible. A typical DRAM chip has
I Infrared Data Association (IrDA): www.irda.org
approximately 32 Mbytes of capacity;
I Intel StrongARM processors:
flash memory has 16 Mbytes. Top-end
http://developer.intel.com/design/pca/applicationsprocessors/index.htm
CompactFlash cards, with 512 Mbytes of
I Power MEMs research:
capacity, closely rival the IBM Microdrive’s
http://web.mit.edu/aeroastro/www/labs/GTL/research/micro/micro.html
capacity.
I Universal Plug and Play: www.upnp.org
Storage is fundamental to most ubi-
I Versus Technology: www.versustech.com
quitous storage applications. Storage
limitations for common tasks hinder the
ubiquitous computing experience, forc-
ing us to leave behind information and
At the smaller end, PDA displays have
convergence of previously separate tech-
to carefully select what must be mobile.
also improved. In the past year, PDAs
nologies. Devices such as cell phones,
Abundant storage negates these issues,
such as the Compaq iPAQ have used trans-
PDAs, and digital cameras are beginning
letting us focus on the important under-
reflective color LCDs. These displays can
to merge. Their combined capabilities
lying tasks.
take natural light entering their surface
reduce the number of devices a user must
and reflect it back through the LCD stack,
carry or own. Such convergent devices will
High-quality displays
yielding considerable energy savings.
likely succeed because numerous devices
Because vision is one of our most impor-
Although all these displays look promis-
will no longer burden users.
tant and acute senses, the need to render
ing, they still have scope for improvement:
In contrast, the problems that a unified
information at a very high quality cannot
the contrast ratio is lower than most
device’s size and complexity create have
be overestimated. Low-quality displays
printed magazines, and the resolution also
given rise to single-function information
distract us by drawing our attention to
needs to increase to the level of print
appliances. These appliances are easily
pixelation, granularity, and poor repre-
media.
adopted because of their simplicity and
sentations.
Ultimately, the need for improvement in
low cost. When personal mobility is not
The last decade has seen a remarkable
display technology is bounded by the
the main motivating factor, such divergent
improvement in display technology: most
human eye’s visual acuity, but we still have
design approaches become attractive—for
commercial laptop PCs now have a 13-
some way to go. A truly ubiquitous com-
example, customizing a computer to be
inch color TFT (thin film transistor) LCD
puting experience requires high-quality dis-
solely a spell checker. Such a device is
display at XGA resolution with a viewing
plays to let us see through the display
physically better suited to its task, but then
angle of at least 140 degrees. However,
process and effortlessly acquire the under-
we have many diverse devices to keep
these are mainly transmissive displays
lying information.
track of and maintain. Even so, organized
requiring a backlight that accounts for
users can “accessorize” their devices for a
approximately one-third of the device’s
Adoption trends
particular task. There is an ongoing ten-
total power consumption. So, from a sys-
For the year 2000, the annual sales of
sion in ubiquitous-hardware design
tem viewpoint, these displays are far from
PCs were approximately 150 million
between convergent and divergent devices.
ideal. Some LCDs have a resolution that
units,13 a remarkable statistic about the
We can see that our first taste of ubiqui-
exceeds 300 dpi, making them suitable for
rate of adoption of computer technologies
tous computing is already in the PC’s two
x-ray-quality pictures. Quality has also in-
into all aspects of modern life. However,
strongholds—the office and home—and a
creased for very large displays, such as the
in that year 8 billion embedded processors
unique third domain, the automobile. The
60-inch diagonal plasma display that Sam-
made their way into the infrastructure of
office has benefited from an integration of
sung showed at Comdex 2001. Such com-
industry and electronic consumer devices.
applications and services driven by the
mercially available displays provide the
So, the fraction of PC sales is a mere 2 per-
need for coordinated enterprise solutions.
means for shared display workspaces,
cent of the total processors sold. From this
For example, the Blackberry, a wireless
which have been the subject of research
statistic, it is obvious that processors are
device that integrates with corporate email,
for some time (for example, the Stanford
beginning to be deployed ubiquitously.
has created a considerable following
I-Room12).
Similarly, another market trend is the
among the corporations that have adopted
JANUARY–MARCH 2002
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it. Such examples show how business pres-
where not only the hardware but also the
Or, it could even be combined with an
sure is pushing the development of inte-
results of actions must be removed from the
existing device (such as a cell phone).
grated systems, which are based on ubi-
foreground. Similarly, robotics is an emerg-
Extending this model, a user’s personal
quitous computing principles.
ing field that mobilizes a computer and
server could also support interaction
The home is also a natural driver for ubi-
enables it to effect change at arbitrary loca-
through interfaces in the surrounding infra-
quitous computing system design. We are
tions in the real world.
structure. Users could access their personal
at an early evolutionary stage where some
data through a public kiosk or borrowed
homes have established a high-speed net-
Personal systems
laptop display (see Figure 2b). This
work connecting multiple PCs. Consumer
Personal systems give users access to
extended model is attractive because it
products are emerging that exploit this
computing independent of their physical
allows a favorable user experience (inter-
infrastructure—the home computer system
location at the cost of them having to carry
acting with personal data through a large
is slowly subsuming all the home commu-
some equipment. Discrete portable devices
display) without requiring users to carry
nication, entertainment, information deliv-
such as PDAs and cell phones are currently
the display. The personal server has recently
ery, and control systems.
the most useful personal systems. How-
become tractable owing to advances in
The automobile is a particularly out-
ever, these systems tend to be limited by
short-range wireless technologies (for
standing area of success for ubiquitous
their computational ability, integration
example, Bluetooth) and low-power pro-
computing. Modern cars have computer
with other devices (for example, your cell
cessing (for example, StrongARM), which
systems that integrate control of the engine,
phone communicating with your PDA),
can now support an acceptable user
transmission, climate, navigation, enter-
and interface capabilities (such as the dis-
experience.
tainment, and communication systems.
play’s size and quality). Today, computa-
Success in this domain is largely because
tional ability and integration are not hin-
Infrastructure systems
each car manufacturer has had complete
dered by a fundamental limit—advances
Unlike mobile systems, infrastructure
control over all aspects of its subsystems,
in processors and short-range wireless tech-
systems instrument a particular locale. So,
unlike the home and office domains. Also,
nology will eventually solve these prob-
many of the difficulties shift from issues of
power for computation is not a limitation
lems—but interface capabilities are.
size, weight, and performance to those of
because it is a small fraction of what is
Some systems, typically termed wearable
deployment, management, and processing.
needed to accelerate the car.
computers, rely on hardware such as heads-
For example, imagine thousands of minia-
up displays and one-handed keyboards to
ture temperature sensors deployed around
Where we’re headed
provide the interface to the computer. This
a room—How did they get there? How is
Ubiquitous computing focuses on getting
model is attractive because it provides a
the data collected? How are faulty com-
computing “beyond the desktop.” This
fully functional computing experience
ponents identified and replaced?15 Tasks
immediately presents a set of research chal-
wherever the user might be. However, these
that are tractable when the human/com-
lenges associated with removing the cus-
interfaces can be overly intrusive, requiring
puter ratio is close to one suddenly become
tomary stationary display-and-keyboard
a great deal of the user’s attention; this mit-
difficult when the number of computing
model, because for most people the PC is
igates their widespread acceptance. Cur-
devices increases.
still their focus. Most current ubiquitous
rently, these devices are typically fairly
Several hardware projects, such as the
computing research projects fall into two
bulky belt-worn devices, but they will
Berkeley motes,16 have started to explore
categories: personal systems, which include
shrink as technology progresses, thereby
this space by creating a fairly small wireless
mobile and wearable systems, and infra-
lending themselves to better industrial
sensor platform. This lets researchers
structure systems, which are associated
design and integration.
actively explore the networking protocols
with a particular physical locale. For both
Personal servers14 (see Figure 2a) can en-
necessary to organize large sets of nodes.
categories, novel interaction modalities,
hance ubiquitous access to data. They form
Currently about the size of three stacked
such as speech or pen processing, become a
the computation and storage center of a
US quarter-dollar coins, these devices will
necessary component because they don’t
person’s digital experience. Devices such
keep shrinking until they reach the size of
require bulky displays or input devices.
as a cell phone or PDA-like appliance com-
smart dust17 and can no longer be seen or
Outside these two categories, the inter-
municate directly with this central server,
directly manipulated. The Berkeley Pico
action of computers with the physical
providing a common representation of a
Radio project is pursing a single-chip sys-
world, without direct human involvement,
user’s data. These devices, therefore, merely
tem that incorporates both processing and
is rapidly becoming more important as the
represent an interface into this central
radio frequency subsystems.18
total amount of deployable computation
repository. With this model, the personal
Although the basic hardware of embed-
increases. A system’s ability to proactively
server could be located out of easy reach—
ded devices can be relatively simple, con-
monitor and react to the real world is instru-
for example, in a user’s shoe, handbag, or
siderable hardware challenges remain.
mental for truly ubiquitous computing,
belt clip—without causing inconvenience.
Power is a primary concern. Even though
40
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http://computer.org/pervasive
(a)
(b)
Personal server
Figure 2. Personal servers: (a) Intel Research’s prototype; (b) using resident display devices such as a wall-mounted display, or a
community tablet computer, to view personal data stored on a user’s personal server.
each node might have a battery lifetime of
has no place in systems with either very
will greatly increase computers’ impact on
several months, the mean time to battery
small displays or no display at all. Speech
our lives by removing people as a major lim-
failure for the entire system can be short if
and vision interfaces have done well in
iting factor from the processing stream. A
it contains many devices. Furthermore, the
infrastructure-based systems that have both
few automatic systems have already signif-
environmental impact of these systems is
significant computation resources and a
icantly affected our lives: thermostats, air-
not well understood—dropping a thou-
static environment.19 But they have trou-
plane autopilots, automated factories, and
sand sensing devices out of an airplane on
ble in mobile systems that are computa-
antilock brakes, for example. Such systems
a disaster zone is relatively easy, but how
tionally impoverished or need to operate in
still require human intervention: thermostat
do you reclaim the devices?
dynamic environments that might be, for
maintenance, airplane landings, factory con-
The fundamental problem with shrink-
example, very loud or dark. Complex
struction, and deciding when to apply the
ing hardware devices is that they quickly
touch-based interfaces20 that deal with a
brakes. The challenge is to make these sys-
become too small and numerous for peo-
significant amount of data input or output
tems proactive, where they can anticipate
ple to relate to them: they are literally out
are problematic. They tend to be task spe-
and react to physical world conditions (for
of sight, so they will quickly become out of
cific, with interface hardware crafted for a
example, deciding to apply the brakes),
mind. One direct byproduct of these mul-
specific application, so they have not gained
instead of just reacting to them (for exam-
tidevice systems is that the individual net-
wide acceptance.
ple, deploying an air bag when you crash).21
work nodes will not be named in any
The most exciting advances in ubiqui-
The salient distinction between these two
human-understandable way—there will
tous interfaces will likely be new display
models is that one is human-centric, which
simply be too many of them to keep track
technologies that enable rich visual output
requires close involvement to effect correct
of. System security rapidly becomes a big
without a bulky flat-screen display. E-Ink
operation, while the other is human-super-
concern: how do you know if a node should
(electronic ink) Corp., for example, uses a
vised, requiring minimum involvement but
be listening in on a wireless conversation if
system of microcapsules to create flexible
still achieving an intelligent, useful result.
you can’t even keep track of which nodes
display surfaces, which would be consider-
A proactive system must closely and reli-
you have in the system?
ably more convenient than a traditional
ably integrate sensors and actuators with
rigid LCD panel. Such technology, in con-
the physical world. This task is closely
Proactive interaction
junction with an abstracted computation
related to building the infrastructure-based
Both personal and infrastructure-based
model such as the personal server, brings us
systems we described earlier. However,
systems ultimately require some kind of
one step closer to a world where we can
proactive systems will require greater
user interface to let humans interact with
access personally relevant information
sophistication in the components deployed
them. Nondesktop interface modalities,
quickly and conveniently, without relying
in the environment, both to enable the
such as pen, speech, vision, and touch, are
on bulky, fragile display systems. Interface
capability to affect the physical world and
attractive in ubiquitous computing systems
hardware technology poses different diffi-
to quickly, robustly, and accurately pro-
because they require less of a user’s atten-
culties for mobile systems than for desktop
cess real-world data. For example, for a
tion than a traditional desktop interface.
computers. For example, speech interfaces
building-scale temperature-monitoring
However, the mainstream use of these tech-
for mobile devices are inherently problem-
application, slowly reporting distributed
niques depends largely on their hardware
atic because they often operate in noisy
temperatures to a central server would be
requirements and interface capabilities.
environments, requiring noise cancellation
sufficient. However, an earthquake re-
For example, pen computing has largely
or directional-microphone techniques.
sponse system would need to actively
been successful on PDA-class devices with
Directly integrating computing with the
dampen vibrations at many nodes through-
a well-defined and accessible display. But it
real world, without human intervention,
out a building.
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location problem mentioned in the previ-
ous paragraph.
Hard problems & physical
limitations
Three hard problems have been the core
challenge of ubiquitous computing hard-
ware and will likely continue to be far into
the future: size and weight, energy, and the
user interface. These problems transcend
individual points on the technology curve,
partly because they are somewhat contra-
dictory—as we’ll see, a solution in one
space greatly confounds that in another.
Ongoing solutions to these problems,
therefore, will have to come from “outside
the box” and remove the fundamental
roadblocks.
Size and weight
Size and weight significantly impede
Figure 3. Sony’s Aibo robot dog.
ubiquitous computing because they con-
tinually remind the user of the hardware’s
presence. This limitation manifests itself
From a personal perspective, proactive
namely, Sony’s Aibo (see Figure 3)—have
both in mobile systems when the user must
computing pushes us toward systems that
reached the market. Abstractly, robots are
carry the device and during the setup and
can monitor and affect our bodies directly.
a novel type of disappearing hardware: they
configuration of infrastructure-based sys-
For example, automatically handling dia-
allow computation to directly affect the real
tems involving many pieces of equipment.
betes requires the ability to both monitor
world without heavily instrumenting the
Appropriately, the two main contributors
blood glucose levels and administer
environment.
to a device’s size and weight stem from the
insulin—tasks requiring specialized hard-
Software issues aside, significant hard-
other two fundamental problems: batteries
ware. A system such as this would still
ware challenges exist, particularly for the
and the user interface.
require significant advances in software
sensing technologies that will enable proac-
The Itsy pocket computer (see Figure
reliability to be feasible.
tive robotics. Vision is the canonical tech-
4)22 exemplifies this situation. It is just 70
nique for a robot to determine where it is,
percent larger than its 2.2 watt-hour bat-
Robotics
what objects are around it, and where they
tery and 320- × 200-pixel display. In addi-
Now more popular than ever, robotics
are. The generalized vision problem re-
tion, these two components represent 60
presents an interesting confluence between
quired to deal with dynamically changing
percent of the total weight without the case
mobile and proactive systems: allowing a
physical locations is quite difficult. One
but 43 percent with the case.
computer system to affect the real world
solution is to instrument a particular envi-
Overall, device sizes have been shrink-
without a priori instrumentation of the
ronment with sensors, beacons, or both to
ing at an incredible rate owing to system-
environment. Most robotic systems either
aid a robot in a particular locale—the
level integration. But devices are reaching
perform a very specific task, such as factory
equivalent of GPS for buildings. A high-
the point where they can’t get much smaller
automation, or are remotely controlled by
accuracy indoor-positioning system would
and still be usable or provide additional
a person. In general, autonomous robots
better allow an autonomous robot to func-
benefit. Moreover, such reductions might
have difficulties dealing with dynamic phys-
tion within a building’s confines. Addi-
increase, rather than decrease, the cogni-
ical situations—primarily with determin-
tionally, tagging interesting objects in the
tive load. For example, many new cars
ing where they are, as well as with identi-
environment would help a robot identify
include a key fob for locking the doors and
fying objects in the environment. Similar to
and locate them. By exploiting infrastruc-
arming the alarm. If the fob were the size
handheld devices, the hardware for con-
ture-based computing, as we discussed ear-
of a brick, few people would use it because
structing robots is shrinking rapidly, mak-
lier, robotics can make significant strides
it would be too large. Alternatively, if it
ing them cheaper and more capable.
toward being truly proactive and auto-
were the size of a penny, few people would
Recently, the first real consumer robots—
nomous without solving the generalized
use it owing to difficulty manipulating the
42
PERVASIVE computing
http://computer.org/pervasive
Figure 4. The Itsy pocket computer
exemplifies the trend that the size and
weight of energy sources and I/O devices
is dominating the size and weight of
mobile computers.
buttons. In this case, the most suitable size
and weight are inextricably tied to the
device’s intended function.
Energy
The challenge for any mobile system is
to reduce user involvement in managing
even generating, energy. For example, MIT
and cannot autonomously signal their pres-
its power consumption. Energy is a nec-
researchers have exploited the human body
ence. The ability to transmit energy, when
essary resource for virtually all comput-
as an energy source by constructing sneak-
combined with robotics, gives us the capa-
ing systems, but any reference to it
ers that use flexible piezoelectric structures
bility for mobile computation that can
detracts from a positive user experience.
to generate energy (see Figure 5).26,27 Sim-
recharge itself.
The degree to which an energy source dis-
ilarly, solar radiation, thermal gradients,
tracts the user depends on the intended
mechanical vibration, and even gravita-
User interfaces
application, the hardware implementing
tional fields all represent potential power
Rendering user interfaces invisible is fun-
the application, and the energy source’s
sources for a mobile device.15
damentally difficult owing to the tradeoff
characteristics.
Additionally, storage technologies under
between size and weight and usability.
Solutions to this problem fall into two
development promise much greater energy
Reducing the size and weight will make the
approaches. The first is to reduce power con-
densities than those of conventional bat-
device less visible but might decrease its
sumption. Part of this approach consists of
teries. In the near term, one of the more
usability. The degree to which these two
designing energy-aware software that can
promising technologies is fuel cells, partic-
components matter depends on the specific
identify the hardware states that provide a
ularly direct methanol fuel cells.28 Pure
properties of the interface and of the appli-
given service level and select those that are
methanol fuel offers an energy density
cation for which it is being used.
most energy efficient. For instance, in sys-
roughly 40 times that of a Lithium-ion
For example, to open or lock a door, a
tems with a microprocessor whose energy
polymer battery, but 70 to 90 percent of
user might use a remote control contain-
consumption is greater at high speeds, the
this chemical energy is lost in conversion to
ing one button that unlocks and opens the
software can select the lowest speed possi-
electricity. In the longer term, technologies
door when pressed once and locks it when
ble that still achieves the required task’s per-
such as MIT’s MEMS (microelectro-
pressed twice in quick succession. Because
formance.23 The control software can also
mechanical systems)-based microturbine
the single button serves multiple purposes,
modify the quality of service it seeks to
and associated micro electric generator
users will likely find it more demanding
deliver.24 For instance, to save energy, the
might provide highly compact energy
than the original interfaces (the door knob
software could reduce the frame rate, or size,
sources with significantly longer lifetimes.
and door lock). Alternatively, the remote
of an MPEG movie, incrementally resulting
An alternative to acquiring energy is to
control could have two buttons, one for
in a corresponding loss of fidelity. Or, the
transmit energy to a mobile device, reduc-
opening and one for locking and unlocking
software could forward a voice utterance to
ing its need for an autonomous power
the door. At the expense of increased size
a remote system for recognition rather than
source. This technique is difficult to do
and weight, this solution substitutes a spa-
expending local energy on the task.24 Soft-
safely over a long range, but it is applica-
tial differentiation (two single-function
ware systems are just beginning to address
ble to the field of passive electronic tagging.
buttons) for a temporal one (a multiple-
these issues concerning energy awareness.
For example, Radio Frequency Identifica-
function button). These two solutions rep-
The second approach is to find alterna-
tion29 tags are inductively powered by the
resent the chief user-interface tradeoff that
tive and improved energy sources.
tag reader, typically up to a maximum of
makes good user interface design for small
Although battery energy densities are pro-
one meter, employing load modulation to
ubiquitous devices fundamentally hard.
jected to increase approximately 10 per-
transmit their data back to the interroga-
Today’s most popular user interfaces
cent annually for the next three years,25 the
tor. Such passive tags have unlimited life-
include buttons, keyboards, mice, point-
energy storage costs of batteries will likely
times, are smaller, and cost less; however,
ers, LCD panels, touch screens, micro-
remain significant. This will lead us to
unlike battery-powered (active) tags, they
phones, and speakers. These elements are
explore other technologies for storing, and
can communicate only over a short range
designed for high-rate information flow,
JANUARY–MARCH 2002
PERVASIVE computing
43
R E A C H I N G F O R W E I S E R ’ S V I S I O N
each suited to a specific application class.
where the computer and the real world are
devices that use the baseline minimum
For example, a keyboard and display seem
tightly integrated. Nonetheless, direct
power for the job or occupy the smallest
ideal for writing a book, but a microphone
neural interfaces embody tremendous risks,
possible volume. Processors will continue
and speaker would probably be better for
not the least of which is loss of human
to shrink while increasing in capability and
communicating with another person. For
autonomy. Clearly, this area requires much
capacity. However, new applications will
other applications, such as alerting a user
more research, but if we can overcome the
demand ever-greater processing capabilities,
that he or she has received mail, these user
challenges, such interfaces would go a long
not the least of which will be those of com-
interfaces are unnecessarily complex.
way toward reaching Weiser’s vision.
munication and security. For example,
Other, more unobtrusive, mechanisms such
implementing public key encryption with a
as the ambientROOM,30 explore how to
Future challenges
typical microcontroller instruction set leads
communicate low-latency, low-importance
We will soon be able to include com-
to large code size, high power requirements,
information via interfaces that require lit-
puter hardware into virtually every man-
and slow performance.33 The challenge will
tle direct attention. For example, variances
ufactured product, and provide a wireless
be to include appropriate primitives that
in a projected image’s intricacy could indi-
infrastructure to let these devices commu-
make these operations more efficient in all
cate if there is unread mail, thereby mak-
nicate directly or indirectly. But what will
these dimensions while permitting the evo-
ing information available to the user unob-
they communicate about and in what pro-
lution of security algorithms.
trusively through a directed glance.
tocol? How will a user or user’s applica-
As we crowd more and more frequencies
Although these interfaces are distinctly
tion know what devices to use for what
with wireless communications, we will need
separate from our bodies, the natural direc-
purpose? How will a user know when
to make our radio systems adaptive. Devices
tion for disappearing interfaces is for the
invisible devices are present, functioning,
will need to adjust their bandwidth require-
two to blend together. Several researchers
and not compromising privacy? What
ments on the basis of what other devices are
are exploring the possibility of interpret-
actions can a user take if the situation is
in the radio neighborhood. Such a necessity
highlights the critical problem of system evo-
A key element of ubiquitous computing
lution. How will the development and
deployment of new devices affect the devices
applications is knowing the precise
we already have deployed? Flexibility will
be needed, not only in terms of software
spatial–temporal relationships between
updates to adjust how a device is used but
also in terms of wireless communication.
people and objects.
Software radios are promising in this sec-
ond dimension; they let a device change how
ing information from our neurons to let us
unacceptable? These questions suggest that
it uses the spectrum to be compatible with
control computers and other machines by
hardware, software, user interaction, and
its neighbors.34
just thinking about doing so.31 Early work
applications all have unresolved issues that
involving a monkey with neural implants
we must address before ubiquitous com-
Disappearing software
has demonstrated the ability to gather suf-
puting will truly reach Weiser’s goal of
To make hardware disappear, we also
ficient information to let a robot mimic the
improving, rather than further complicat-
need to make software disappear. Today’s
monkey’s arm movement.31 Similarly,
ing, our lives.
software is too monolithic and stovepiped,
neural implants can feed information into
and is written with many assumptions
the brain, removing the need for humans to
Unresolved hardware issues
about hardware and software resources.
gather information through their senses.
We have already discussed some of the
The ubiquitous computing environment
This approach’s potential is suggested by
challenges for our hardware platforms.
that we must create will be much more
recent research employing cochlear
Specifically, we will need to continue to
dynamic. Devices, objects, and people will
implants to help those with hearing loss to
manage our devices’ power requirements.
be constantly moving around, creating an
communicate better and become more
Making devices ubiquitous can’t be coupled
ever-changing set of resources—different
aware of their surroundings.32 For exam-
with the need to change batteries, or even
user input/output interfaces, displays, and
ple, such neural implants could bring infor-
recharge them, when thousands of devices
windowing systems—that will be available
mation (such as “you’ve got mail”) to the
are involved. The solutions presented ear-
to our applications. Also, some devices
user’s attention by fooling the brain into
lier only partially solve the problems; some
might lose their ability to communicate
thinking a subtle but noticeable image
are highly experimental and might not be
with others owing to interference or envi-
has been projected onto the retina.
practical for unforeseen reasons.
ronmental conditions. How will we con-
These interfaces form the personal-inter-
Microprocessors can also continue evolv-
struct applications to operate in such envi-
action version of proactive computing,
ing. The need will persist for minimal
ronments?
44
PERVASIVE computing
http://computer.org/pervasive
Figure 5. Energy-scavenging shoes using
piezoelectric material, developed at MIT’s
Media Laboratory. With these shoes,
normal walking motion can generate
sufficient energy to broadcast an ID
every three to five steps.27 Photo
courtesy of the MIT Media Laboratory.
To do this, we must even further decou-
ple our applications into small pieces of
code, spread across ubiquitous hardware,
that can come together as needed and
expect to have connections constantly
enabled and disabled. We must create data
interchange formats where the data not
software handle multiple users, but also
structure,40,41 or are based on proximity
only is self-describing but also can find its
the hardware must be able to accurately
(for example, electronic tags). We desper-
way from the device that created it to its
identify and differentiate between the mul-
ately need tags that can be located within
destination as autonomously and securely
tiple users. Addressing these issues using
a few millimeters, are cheap to create (for
as possible.35 We must develop mecha-
what we’ve learned from the desktop
example, by printing), and are completely
nisms for devices to advertise their capa-
metaphor seems less promising because
passive. Ideally, we would also want the
bilities so that applications, whether in the
that interface has not been developed or
ability to detect that two tagged objects are
infrastructure or on portable devices, can
optimized for casual use by multiple users.
physically touching rather than just in close
become aware of and select from the
With computing capability in every
proximity.42,43 Coming up with the tech-
panorama of available resources.
object, users will want to take advantage
nologies that provide these capabilities,
Discovery services36,37 (for example, Uni-
of the devices they encounter throughout
even if initially imperfectly, is another key
versal Plug and Play) to some extent already
their day without worrying about owner-
challenge. For more on this topic, see
enable such cyber foraging.38 These first-
ship or security at every step. Consider how
“Connecting the Physical World with Per-
generation systems require highly capable
we use paper and pencil: we can easily jot
vasive Networks,” in this issue.
devices that can download code and enjoy
down notes and later identify the author
stable and relatively high-bandwidth con-
based solely on the handwriting. The note-
Applications
nections. This approach will not scale to
taking process directly incorporates this
Currently, most applications are based
the myriad sensing and processing devices
process; there is no explicit authentication
on ownership of relatively large, multi-
that will surround us. They will employ
mechanism. Similarly, physical control
purpose hardware devices with only the
minimal computational elements and exist
guarantees privacy: we put the paper in our
most limited interaction with the physical
in small communication cells with highly
pocket and can hide it from others’ eyes.
world in which people live. We need to dis-
variable communication and processing
What metaphors will we have for the elec-
cover and enable those most compelling
properties. This scenario requires that mul-
tronic paper that can communicate with
applications that will let us deploy the first
tiple devices replace a single device. Adap-
other devices? Without some kind of phys-
truly ubiquitous systems. Yet, as we have
tation at this level is another challenge we
ical icon, how do we seamlessly control
already discussed, the most difficult part
must tackle to achieve long-lived systems
sharing content with other people?
of this will be to give these systems an evo-
that can evolve gracefully.
A key element of ubiquitous computing
lutionary path, in contrast to today’s
applications is knowing the precise spatial–
approach of completely reengineering all
Interaction design
temporal relationships between people and
the component devices. Software engi-
Although new technologies are creating
objects. Such knowledge succinctly helps
neering will take center stage in this effort.
new ways of interacting with our compu-
specify intent, an integral component of
Clearly, we need a new set of abstractions
tations, we could end up trading the prob-
user interfaces. But the resolution of loca-
to make writing such applications possi-
lems of one user interface for those of
tion systems needs to improve dramati-
ble—abstractions that support software
another. For example, wireless technolo-
cally.39 Current commercial indoor loca-
deployment in separate modules on thou-
gies permit a device we are carrying to
tion systems are either too coarse,
sands of devices, cyber foraging, and hard-
interact with a public display. But what
operating primarily at the room level7 (for
ware sharing. This is a different world of
happens when multiple potential users are
example, the Versus Information System),
software development than we are accus-
surrounding a display? Not only must the
or require prohibitively expensive infra-
tomed to.
JANUARY–MARCH 2002
PERVASIVE computing
45
R E A C H I N G F O R W E I S E R ’ S V I S I O N
Many hardware components on the hard problems associated with ubiquitous Trends through 2004, MPUs, MCUs, DSPs
necessary to build ubiqui-
computing. And a special thank you to Mark Weiser,
and Cores, Gartner Group, Stamford,
in memory, for his visionary ideas in the early 1990s.
Conn., 22 Jan. 2001.
tous computing systems
are now available. Key
14. R. Want and B. Schilit, “Guest Editors’
improvements since Weiser’s original vision
Introduction: Expanding the Horizons of
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the AUTHORS
31. A. Regalado, “Miguel Nicolelis: Brain-
Machine Interfaces,” Technology Rev.,
Roy Want is a principal engineer at Intel Research. His interests include ubiquitous
Jan./Feb. 2001, pp. 98–100.
computing wireless protocols, hardware design, embedded systems, distributed
systems, and automatic identification and microelectromechanical systems. While at
32. M. Nicolelis, “Actions from Thoughts,”
Olivetti Research, he developed the Active Badge, a system for automatically locating
Nature, vol. 409, no. 6818, 18 Jan. 2001,
people in a building. As part of Xerox PARC’s Ubiquitous Computing program, he led
pp. 403–407.
the ParcTab project, one of the first context-aware computer systems. At PARC, Want
also managed the Embedded Systems group. He received his BA and PhD in computer
33. A. Perrig, “SPINS: Security Protocols for
science from Churchill College, Cambridge University. Contact him at Intel Corp., 2200
Sensor Networks,” Proc. 7th Ann. ACM
Mission College Blvd., Santa Clara, CA 95052; roy.want@intel.com; www.ubicomp.com/want.
Int’l Conf. Mobile Computing and Net-
working (MobiCom 2001), ACM Press,
Trevor Pering is a research scientist at Intel Research. His research interests include
New York, 2001, pp. 189–199.
many aspects of mobile and ubiquitous computing, including usage models, power
management, novel form factors, and software infrastructure. He received his PhD
34. V. Bose, D. Wetherall, and J. Guttag, “Next
in electrical engineering from the University of California, Berkeley, with a focus on
Century Challenges: RadioActive Net-
operating system power management. He is a member of the ACM. Contact him at
works,” Proc. ACM Mobile Computing and
trevor.pering@intel.com.
Networking (Mobicom), ACM Press, New
York, 1999, pp. 242–248.
35. M. Esler et al., “Next Century Challenges:
Gaetano Borriello is a faculty member of the University of Washington’s Department
Data-Centric Networking for Invisible Com-
of Computer Science and Engineering. He is on a two-year leave to establish a new
puting—The Portolano Project at the Uni-
Intel research center adjacent to the University of Washington campus. His research
versity of Washington,” Proc. MobiCom
interests are in the design, development, and deployment of computing systems with
1999, ACM Press, New York, 1999, pp.
particular emphasis on mobile and ubiquitous devices and their application. His most
256–262.
recent research accomplishments include the development of the Chinook design
system for heterogeneous distributed embedded processors. He received his BS
36. W. Adjie-Winoto et al., “The Design and
in electrical engineering from the Polytechnic Institute of New York, his MS in elec-
Implementation of an Intentional Naming
trical engineering from Stanford University, and his PhD in computer science from the University of
System,” Operating Systems Rev., vol. 34,
California, Berkeley. He received an NSF Presidential Young Investigator Award in 1998 and a University
no. 5, Dec. 1999, pp. 186–201.
of Washington Distinguished Teaching Award in 1995. Contact him at the Dept. of Computer Science &
Eng., Univ. of Washington, Box 352350, Seattle, WA 98195-2350; gaetano@cs.washington.edu.
37. J. Waldo, Jini Architecture Overview, tech.
report, Sun Microsystems, Palo Alto, Calif.,
Keith I. Farkas is a senior member of the research staff of Compaq Computer’s
Jan. 1999.
Western Research Lab. His research interests include microprocessor design and
software and hardware techniques for managing and optimizing computer system
38. M. Satyanarayanan, “Pervasive Computing:
energy consumption. He received his PhD from the University of Toronto. He is a
Vision and Challenges,” IEEE Personal
member of the IEEE and ACM. Contact him at keith.farkas@compaq.com.
Comm., vol. 8, no. 4, Aug. 2001, pp. 10–17.
39. J. Hightower and G. Borriello, “Location
Systems for Ubiquitous Computing,” Com-
JANUARY–MARCH 2002
PERVASIVE computing
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