Tag Archives: Mark Weiser

Some Computer Science Issues in Ubiquitous Computing

by Mark Weiser
March 23, 1993

[to appear in CACM, July 1993]

Ubiquitous computing is the method of enhancing computer use by making many computers available throughout the physical environment, but making them effectively invisible to the user. Since we started this work at Xerox PARC in 1988, a number of researchers around the world have begun to work in the ubiquitous computing framework. This paper explains what is new and different about the computer science in ubiquitous computing. It starts with a brief overview of ubiquitous computing, and then elaborates through a series of examples drawn from various subdisciplines of computer science: hardware components (e.g. chips), network protocols, interaction substrates (e.g. software for screens and pens), applications, privacy, and computational methods. Ubiquitous computing offers a framework for new and exciting research across the spectrum of computer science.

A few places in the world have begun work on a possible next generation computing environment in which each person is continually interacting with hundreds of nearby wirelessly interconnected computers. The point is to achieve the most effective kind of technology, that which is essentially invisible to the user. To bring computers to this point while retaining their power will require radically new kinds of computers of all sizes and shapes to be available to each person. I call this future world “Ubiquitous Computing” (short form: “Ubicomp”) [Weiser 1991]. The research method for ubiquitous computing is standard experimental computer science: the construction of working prototypes of the necessary infrastructure in sufficient quantity to debug the viability of the systems in everyday use, using ourselves and a few colleagues as guinea pigs. This is an important step towards insuring that our infrastructure research is robust and scalable in the face of the details of the real world.

The idea of ubiquitous computing first arose from contemplating the place of today’s computer in actual activities of everyday life. In particular, anthropological studies of work life [Suchman 1985, Lave 1991] teach us that people primarily work in a world of shared situations and unexamined technological skills. However the computer today is isolated and isolating from the overall situation, and fails to get out of the way of the work. In other words, rather than being a tool through which we work, and so which disappears from our awareness, the computer too often remains the focus of attention. And this is true throughout the domain of personal computing as currently implemented and discussed for the future, whether one thinks of PC’s, palmtops, or dynabooks. The characterization of the future computer as the “intimate computer” [Kay 1991], or “rather like a human assistant” [Tesler 1991] makes this attention to the machine itself particularly apparent.

Getting the computer out of the way is not easy. This is not a graphical user interface (GUI) problem, but is a property of the whole context of usage of the machine and the affordances of its physical properties: the keyboard, the weight and desktop position of screens, and so on. The problem is not one of “interface”. For the same reason of context, this was not a multimedia problem, resulting from any particular deficiency in the ability to display certains kinds of realtime data or integrate them into applications. (Indeed, multimedia tries to grab attention, the opposite of the ubiquitous computing ideal of invisibility). The challenge is to create a new kind of relationship of people to computers, one in which the computer would have to take the lead in becoming vastly better at getting out of the way so people could just go about their lives.

In 1988, when I started PARC’s work on ubiquitous computing, virtual reality (VR) came the closest to enacting the principles we believed important. In its ultimate envisionment, VR causes the computer to become effectively invisible by taking over the human sensory and affector systems [Rheingold 91]. VR is extremely useful in scientific visualization and entertainment, and will be very significant for those niches. But as a tool for productively changing everyone’s relationship to computation, it has two crucial flaws: first, at the present time (1992), and probably for decades, it cannot produce a simulation of significant verisimilitude at reasonable cost (today, at any cost). This means that users will not be fooled and the computer will not be out of the way. Second, and most importantly, it has the goal of fooling the user — of leaving the everyday physical world behind. This is at odds with the goal of better integrating the computer into human activities, since humans are of and in the everyday world.

Ubiquitous computing is exploring quite different ground from Personal Digital Assistants, or the idea that computers should be autonomous agents that take on our goals. The difference can be characterized as follows. Suppose you want to lift a heavy object. You can call in your strong assistant to lift it for you, or you can be yourself made effortlessly, unconsciously, stronger and just lift it. There are times when both are good. Much of the past and current effort for better computers has been aimed at the former; ubiquitous computing aims at the latter.

The approach I took was to attempt the definition and construction of new computing artifacts for use in everyday life. I took my inspiration from the everyday objects found in offices and homes, in particular those objects whose purpose is to capture or convey information. The most ubiquitous current informational technology embodied in artifacts is the use of written symbols, primarily words, but including also pictographs, clocks, and other sorts of symbolic communication. Rather than attempting to reproduce these objects inside the virtual computer world, leading to another “desktop model” [Buxton 90], instead I wanted to put the new kind of computer also out in this world of concrete information conveyers. And because these written artifacts occur in many different sizes and shapes, with many different affordances, so I wanted the computer embodiments to be of many sizes and shapes, including tiny inexpensive ones that could bring computing to everyone.

The physical affordances in the world come in all sizes and shapes; for practical reasons our ubiquitous computing work begins with just three different sizes of devices: enough to give some scope, not enough to deter progress. The first size is the wall-sized interactive surface, analogous to the office whiteboard or the home magnet-covered refrigerator or bulletin board. The second size is the notepad, envisioned not as a personal computer but as analogous to scrap paper to be grabbed and used easily, with many in use by a person at once. The cluttered office desk or messy front hall table are real-life examples. Finally, the third size is the tiny computer, analogous to tiny individual notes or PostIts, and also like the tiny little displays of words found on book spines, lightswitches, and hallways. Again, I saw this not as a personal computer, but as a pervasive part of everyday life, with many active at all times. I called these three sizes of computers, respectively, boards, pads, and tabs, and adopted the slogan that, for each person in an office, there should be hundreds of tabs, tens of pads, and one or two boards. Specifications for some prototypes of these three sizes in use at PARC are shown in figure 1.

This then is phase I of ubiquitous computing: to construct, deploy, and learn from a computing environment consisting of tabs, pads, and boards. This is only phase I, because it is unlikely to achieve optimal invisibility. (Later phases are yet to be determined). But it is a start down the radical direction, for computer science, away from attention on the machine and back on the person and his or her life in the world of work, play, and home.

Hardware Prototypes

New hardware systems design for ubiquitous computing has been oriented towards experimental platforms for systems and applications of invisibility. New chips have been less important than combinations of existing components that create experimental opportunities. The first ubiquitous computing technology to be deployed was the Liveboard [Elrod 92], which is now a Xerox product. Two other important pieces of prototype hardware supporting our research at PARC are the Tab and the Pad.


The ParcTab is a tiny information doorway. For user interaction it has a pressure sensitive screen on top of the display, three buttons underneath the natural finger positions, and the ability to sense its position within a building. The display and touchpad it uses are standard commercial units.

The key hardware design problems in the pad are size and power consumption. With several dozens of these devices sitting around the office, in briefcases, in pockets, one cannot change their batteries every week. The PARC design uses the 8051 to control detailed interactions, and includes software that keeps power usage down. The major outboard components are a small analog/digital converter for the pressure sensitive screen, and analog sense circuitry for the IR receiver. Interestingly, although we have been approached by several chip manufacturers about our possible need for custom chips for the Tab, the Tab is not short of places to put chips. The display size leaves plenty of room, and the display thickness dominates total size. Off-the-shelf components are more than adequate for exploring this design space, even with our severe size, weight, and power constraints.

A key part of our design philosophy is to put devices in everyday use, not just demonstrate them. We can only use techniques suitable for quantity 100 replication, which excludes certain things that could make a huge difference, such as the integration of components onto the display surface itself. This technology, being explored at PARC, ISI, and TI, while very promising, is not yet ready for replication.

The Tab architecture is carefully balanced among display size, bandwidth, processing, and memory. For instance, the small display means that even the tiny processor is capable of four frame/sec video to it, and the IR bandwidth is capable of delivering this. The bandwidth is also such that the processor can actually time the pulse widths in software timing loops. Our current design has insufficient storage, and we are increasing the amount of non-volatile RAM in future tabs from 8k to 128k. The tab’s goal of postit-note-like casual use puts it into a design space generally unexplored in the commercial or research sector.


The pad is really a family of notebook-sized devices. Our initial pad, the ScratchPad, plugged into a Sun SBus card and provided an X-window-system-compatible writing and display surface. This same design was used inside our first wall-sized displays, the liveboards, as well. Our later untethered pad devices, the XPad and MPad, continued the system design principles of X-compatibility, ease of construction, and flexibility in software and hardware expansion.

As I write, at the end of 1992, commercial portable pen devices have been on the market for two years, although most of the early companies have now gone out of business. Why should a pioneering research lab be building its own such device? Each year we ask ourselves the same question, and so far three things always drive us to continue to design our own pad hardware.

First, we need the right balance of features; this is the essence of systems design. The commercial devices all aim at particular niches, and so balance their design to that niche. For research we need a rather different balance, all the more so for ubiquitous computing. For instance, can the device communicate simultaneously along multiple channels? Does the O.S support multiprocessing? What about the potential for high-speed tethering? Is there a high-quality pen? Is there a high-speed expansion port sufficient for video in and out? Is sound in/out and ISDN available? Optional keyboard? Any one commercial device tends to satisfy some of these, ignore others, and choose a balance of the ones it does satisfy that optimize its niche, rather than ubiquitous computing-style scrap computing. The balance for us emphasizes communication, ram, multi-media, and expansion ports.

Second, apart from balance are the requirements for particular features. Key among these are a pen emphasis, connection to research environments like Unix, and communication emphasis. A high-speed (>64kbps) wireless capability is built into no commercial devices, nor do they generally have a sufficiently high speed port to which such a radio can be added. Commercial devices generally come with DOS or Penpoint, and while we have developed in both, they are not our favorite research vehicles because of lack of full access and customizability.

The third thing driving our own pad designs is ease of expansion and modification. We need full hardware specs, complete O.S. source code, and the ability to rip-out and replace both hardware and software components. Naturally these goals are opposed to best price in a niche market, which orients the documentation to the end user, and which keeps price down by integrated rather than modular design.

We have now gone through three generations of Pad designs. Six scratchpads were built, three XPads, and thirteen MPads, the latest. The MPad uses an FPGA for almost all random logic, giving extreme flexibility. For instance, changing the power control functions, and adding high-quality sound, were relatively simple FPGA changes. The Mpad has built-in both IR (tab compatible) and radio communication, and includes sufficient uncommitted space for adding new circuit boards later. It can be used with a tether that provides it with recharging and operating power and an ethernet connection. The operating system is a standalone version of the public-domain Portable Common Runtime developed at PARC [Weiser 89].


FIGURE 1 – some hardware prototypes in use at PARC


FIGURE 2 – Photographs of each of tabs, pads, boards (at end of paper).


The CS of Ubicomp

In order to construct and deploy tabs, pads, and boards at PARC, we found ourselves needing to readdress some of the well-worked areas of existing computer science. The fruitfulness of ubiquitous computing for new Computer Science problems clinched our belief in the ubiquitous computing framework.

In what follows I walk up the levels of organization of a computer system, from hardware to application. For each level I describe one or two examples of computer science work required by ubiquitous computing. Ubicomp is not yet a coherent body of work, but consists of a few scattered communities. The point of this paper is to help others understand some of the new research challenges in ubiquitous computing, and inspire them to work on them. This is more akin to a tutorial than a survey, and necessarily selective.

The areas I discuss below are: hardware components (e.g. chips), network protocols, interaction substrates (e.g. software for screens and pens), applications, privacy, and computational methods.

Issues of hardware components

In addition to the new systems of tabs, pads, and boards, ubiquitous computing needs some new kinds of devices. Examples of three new kinds of hardware devices are: very low power computing, low-power high-bits/cubic-meter communication, and pen devices.

Low Power

In general the need for high performance has dominated the need for low power consumption in processor design. However, recognizing the new requirements of ubiquitous computing, a number of people have begun work in using additional chip area to reduce power rather than to increase performance [Lyon 93]. One key approach is to reduce the clocking frequency of their chips by increasing pipelining or parallelism. Then, by running the chips at reduced voltage, the effect is a net reduction in power, because power falls off as the square of the voltage while only about twice the area is needed to run at half the clock speed.


Power = CL * Vdd2 * f

  • where CL is the gate capacitance, Vdd the supply voltage, and f the clocking frequency.


This method of reducing power leads to two new areas of chip design: circuits that will run at low power, and architectures that sacrifice area for power over performance. The second requires some additional comment, because one might suppose that one would simply design the fastest possible chip, and then run it at reduced clock and voltage. However, as Lyon illustrates, circuits in chips designed for high speed generally fail to work at low voltages. Furthermore, attention to special circuits may permit operation over a much wider range of voltage operation, or achieve power savings via other special techniques, such as adiabatic switching [Lyon 93].


A wireless network capable of accommodating hundreds of high speed devices for every person is well beyond the commercial wireless systems planned even ten years out [Rush 92], which are aimed at one low speed (64kbps or voice) device per person. Most wireless work uses a figure of merit of bits/sec x range, and seeks to increase this product. We believe that a better figure of merit is bits/sec/meter3. This figure of merit causes the optimization of total bandwidth throughout a three-dimensional space, leading to design points of very tiny cellular systems.

Because we felt the commercial world was ignoring the proper figure of merit, we initiated our own small radio program. In 1989 we built spread-spectrum transceivers at 900Mhz, but found them difficult to build and adjust, and prone to noise and multipath interference. In 1990 we built direct frequency-shift-keyed transceivers also at 900Mhz, using very low power to be license-free. While much simpler, these transceivers had unexpectedly and unpredictably long range, causing mutual interference and multipath problems. In 1991 we designed and built our current radios, which use the near-field of the electromagnetic spectrum. The near-field has an effective fall-off of r6 in power, instead of the more usual r2, where r is the distance from the transmitter. At the proper levels this band does not require an FCC license, permits reuse of the same frequency over and over again in a building, has virtually no multipath or blocking effects, and permits transceivers that use extremely low power and low parts count. We have deployed a number of near-field radios within PARC.


A third new hardware component is the pen for very large displays. We needed pens that would work over a large area (at least 60″x40″), not require a tether, and work with back projection. These requirements are generated from the particular needs of large displays in ubiquitous computing — casual use, no training, naturalness, multiple people at once. No existing pens or touchpads could come close to these requirements. Therefore members of the Electronics and Imaging lab at PARC devised a new infrared pen. A camera-like device behind the screen senses the pen position, and information about the pen state (e.g. buttons) is modulated along the IR beam. The pens need not touch the screen, but can operate from several feet away. Considerable DSP and analog design work underlies making these pens effective components of the ubiquitous computing system [Elrod 92].

Network Protocols

Ubicomp changes the emphasis in networking in at least four areas: wireless media access, wide-bandwidth range, real-time capabilities for multimedia over standard networks, and packet routing.

A “media access” protocol provides access to a physical medium. Common media access methods in wired domains are collision detection and token-passing. These do not work unchanged in a wireless domain because not every device is assured of being able to hear every other device (this is called the “hidden terminal” problem). Furthermore, earlier wireless work used assumptions of complete autonomy, or a statically configured network, while ubiquitous computing requires a cellular topology, with mobile devices frequently coming on and off line. We have adapted a media access protocol called MACA, first described by Phil Karn [Karn 90], with some of our own modifications for fairness and efficiency.

The key idea of MACA is for the two stations desiring to communicate to first do a short handshake of Request-To-Send-N-bytes followed by Clear-To-Send-N-bytes. This exchange allows all other stations to hear that there is going to be traffic, and for how long they should remain quiet. Collisions, which are detected by timeouts, occur only during the short RTS packet.

Adapting MACA for ubiquitous computing use required considerable attention to fairness and real-time requirements. MACA (like the original ethernet) requires stations whose packets collide to backoff a random time and try again. If all stations but one backoff, that one can dominate the bandwidth. By requiring all stations to adapt the backoff parameter of their neighbors we create a much fairer allocation of bandwidth.

Some applications need guaranteed bandwidth for voice or video. We added a new packet type, NCTS(n) (Not Clear To Send), to suppress all other transmissions for (n) bytes. This packet is sufficient for a basestation to do effective bandwidth allocation among its mobile units. The solution is robust, in the sense that if the basestation stops allocating bandwidth then the system reverts to normal contention.

When a number of mobile units share a single basestation, that basestation may be a bottleneck for communication. For fairness, a basestation with N > 1 nonempty output queues needs to contend for bandwidth as though it were N stations. We therefore make the basestation contend just enough more aggressively that it is N times more likely to win a contention for media access.

Two other areas of networking research at PARC with ubiquitous computing implications are gigabit networks and real-time protocols. Gigabit-per-second speeds are important because of the increasing number of medium speed devices anticipated by ubiquitous computing, and the growing importance of real-time (multimedia) data. One hundred 256kbps portables per office implies a gigabit per group of forty offices, with all of PARC needing an aggregate of some five gigabits/sec. This has led us to do research into local-area ATM switches, in association with other gigabit networking projects [Lyles 92].

Real-time protocols are a new area of focus in packet-switched networks. Although real-time delivery has always been important in telephony, a few hundred milliseconds never mattered in typical packet-switched applications like telnet and file transfer. With the ubiquitous use of packet-switching, even for telephony using ATM, the need for real-time capable protocols has become urgent if the packet networks are going to support multi-media applications. Again in association with other members of the research community, PARC is exploring new protocols for enabling multimedia on the packet-switched internet [Clark 92].

The internet routing protocol, IP, has been in use for over ten years. However, neither it nor its OSI equivalent, CLNP, provides sufficient infrastructure for highly mobile devices. Both interpret fields in the network names of devices in order to route packets to the device. For instance, the “13” in IP name is interpreted to mean net 13, and network routers anywhere in the world are expected to know how to get a packet to net 13, and all devices whose name starts with 13 are expected to be on that network. This assumption fails as soon as a user of a net 13 mobile device takes her device on a visit to net 36 (Stanford). Changing the device name dynamically depending on location is no solution: higher level protocols like TCP assume that underlying names won’t change during the life of a connection, and a name change must be accompanied by informing the entire network of the change so that existing services can find the device.

A number of solutions have been proposed to this problem, among them Virtual IP from Sony [Teraoka 91], and Mobile IP from Columbia University [Ioannidis 93]. These solutions permit existing IP networks to interoperate transparently with roaming hosts. The key idea of all approaches is to add a second layer of IP address, the “real” address indicating location, to the existing fixed device address. Special routing nodes that forward packets to the right real address, and keep track of where this address is, are required for all approaches. The internet community has a working group looking at standards for this area (contact [email protected] for more information).

Interaction Substrates

Ubicomp has led us into looking at new substrates for interaction. I mention four here that span the space from virtual keyboards to protocols for window systems.

Pads have a tiny interaction area — too small for a keyboard, too small even for standard handprinting recognition. Handprinting has the further problem that it requires looking at what is written. Improvements in voice recognition are no panacea, because when other people are present voice will often be inappropriate. As one possible solution, we developed a method of touch-printing that uses only a tiny area and does not require looking. As drawbacks, our method requires a new printing alphabet to be memorized, and reaches only half the speed of a fast typist [Goldberg 93].

Liveboards have a huge interaction area, 400 times that of the tab. Using conventional pulldown or popup menus might require walking across the room to the appropriate button, a serious problem. We have developed methods of location-independent interaction by which even complex interactions can be popped up at any location. [Kurtenbach 93].

The X window system, although designed for network use, makes it difficult for windows to move once instantiated at a given X server. This is because the server retains considerable state about individual windows, and does not provide convenient ways to move that state. For instance, context and window IDs are determined solely by the server, and cannot be transferred to a new server, so that applications that depend upon knowing their value (almost all) will break if a window changes servers. However, in the ubiquitous computing world a user may be moving frequently from device to device, and wanting to bring windows along.

Christian Jacobi at PARC has implemented a new X toolkit that facilitates window migration. Applications need not be aware that they have moved from one screen to another; or if they like, they can be so informed with an upcall. We have written a number of applications on top of this toolkit, all of which can be “whistled up” over the network to follow the user from screen to screen. The author, for instance, frequently keeps a single program development and editing environment open for days at a time, migrating its windows back and forth from home to work and back each day.

A final window system problem is bandwidth. The bandwidth available to devices in ubiquitous computing can vary from kilobits/sec to gigabits/sec, and with window migration a single application may have to dynamically adjust to bandwidth over time. The X window system protocol was primarily developed for ethernet speeds, and most of the applications written in it were similarly tested at 10Mbps. To solve the problem of efficient X window use at lower bandwidth, the X consortium is sponsoring a “Low Bandwidth X” (LBX) working group to investigate new methods of lowering bandwidth. [Fulton 93].


Applications are of course the whole point of ubiquitous computing. Two examples of applications are locating people and shared drawing.

Ubicomp permits the location of people and objects in an environment. This was first pioneered by work at Olivetti Research Labs in Cambridge, England, in their Active Badge system [Want 92]. In ubiquitous computing we continue to extend this work, using it for video annotation, and updating dynamic maps. For instance, the picture below (figure 3) shows a portion of CSL early one morning, and the individual faces are the locations of people. This map is updated every few seconds, permitting quick locating of people, as well as quickly noticing a meeting one might want to go to (or where one can find a fresh pot of coffee).


Figure 3. Display of CSL activity from personal locators.


PARC, EuroPARC, and the Olivetti Research Center have built several different kinds of location servers. Generally these have two parts: a central database of information about location that can be quickly queried and dumped, and a group of servers that collect information about location and update the database. Information about location can be deduced from logins, or collected directly from an active badge system. The location database may be organized to dynamically notify clients, or simply to facilitate frequent polling.

Some example uses of location information are: automatic phone forwarding, locating an individual for a meeting, and watching general activity in a building to feel in touch with its cycles of activity (important for telecommuting).

PARC has investigated a number of shared meeting tools over the past decade, starting with the CoLab work [Stefik 87], and continuing with videodraw and commune [Tang 91]. Two new tools were developed for investigating problems in ubiquitous computing. The first is Tivoli [Pedersen 93], the second Slate, each based upon different implementation paradigms. First their similarities: they both emphasize pen-based drawing on a surface, they both accept scanned input and can print the results, they both can have several users at once operating independently on different or the same pages, they both support multiple pages. Tivoli has a sophisticated notion of a stroke as spline, and has a number of features making use of processing the contents and relationships among strokes. Tivoli also uses gestures as input control to select, move, and change the properties of objects on the screen. When multiple people use Tivoli each must be running a separate copy, and connect to the others. On the other hand, Slate is completely pixel based, simply drawing ink on the screen. Slate manages all the shared windows for all participants, as long as they are running an X window server, so its aggregate resource use can be much lower than Tivoli, and it is easier to setup with large numbers of participants. In practice we have used slate from a Sun to support shared drawing with users on Macs and PCs. Both Slate and Tivoli have received regular use at PARC.

Shared drawing tools are a topic at many places. For instance, Bellcore has a toolkit for building shared tools [Hill 93], and Jacobsen at LBL uses multicast packets to reduce bandwidth during shared tool use. There are some commercial products [Chatterjee 92], but these are usually not multi-page and so not really suitable for creating documents or interacting over the course of a whole meeting. The optimal shared drawing tool has not been built. For its user interface, there remain issues such as multiple cursors or one, gestures or not, and using an ink or a character recognition model of pen input. For its substrate, is it better to have a single application with multiple windows, or many applications independently connected? Is packet-multicast a good substrate to use? What would it take to support shared drawing among 50 people, 5,000 people? The answers are likely both technological and social.

Three new kinds of applications of ubiquitous computing are beginning to be explored at PARC. One is to take advantage of true invisibility, literally hiding machines in the walls. An example is the Responsive Environment project led by Scott Elrod. This aims to make a building’s heat, light, and power more responsive to individually customized needs, saving energy and making a more comfortable environment.

A second new approach is to use so-called “virtual communities” via the technology of MUDs. A MUD, or “Multi-User Dungeon,” is a program that accepts network connections from multiple simultaneous users and provides access to a shared database of “rooms”, “exits”, and other objects. MUDs have existed for about ten years, being used almost exclusively for recreational purposes. However, the simple technology of MUDs should also be useful in other, non-recreational applications, providing a casual environment integrating virtual and real worlds [Curtis 92].

A third new approach is the use of collaboration to specify information filtering. Described in the December 1992 issue of Communcations of the ACM, this work by Doug Terry extends previous notions of information filters by permitting filters to reference other filters, or to depend upon the values of multiple messages. For instance, one can select all messages that have been replied to by Smith (these messages do not even mention Smith, of course), or all messages that three other people found interesting. Implementing this required inventing the idea of a “continuous query”, which can effectively sample a changing database at all points in time. Called “Tapestry”, this system provides new ways for people to invisibly collaborate.

Privacy of Location

Cellular systems inherently need to know the location of devices and their use in order to properly route information. For instance, the traveling pattern of a frequent cellular phone user can be deduced from the roaming data of cellular service providers. This problem could be much worse in ubiquitous computing with its more extensive use of cellular wireless. So a key problem with ubiquitous computing is preserving privacy of location. One solution, a central database of location information, means that the privacy controls can be centralized and so perhaps done well — on the other hand one break-in there reveals all, and centrality is unlikely to scale worldwide. A second source of insecurity is the transmission of the location information to a central site. This site is the obvious place to try to snoop packets, or even to use traffic analysis on source addresses.

Our initial designs were all central, initially with unrestricted access, gradually moving towards controls by individual users on who can access information about them. Our preferred design avoids a central repository, but instead stores information about each person at that person’s PC or workstation. Programs that need to know a person’s location must query the PC, and run whatever gauntlet of security the user has chosen to install there. EuroPARC uses a system of this sort.

Accumulating information about individuals over long periods is both one of the more useful things to do, and also most quickly raises hackles. A key problem for location is how to provide occasional location information for clients that need it while somehow preventing the reliable accumulation of long-term trends about an individual. So far at PARC we have experimented only with short-term accumulation of information to produce automatic daily diaries of activity [Newman 90].

It is important to realize that there can never be a purely technological solution to privacy, that social issues must be considered in their own right. In the computer science lab we are trying to construct systems that are privacy enabled, that can give power to the individual. But only society can cause the right system to be used. To help prevent future oppressive employers or governments from taking this power away, we are also encouraging the wide dissimenation of information about location systems and their potential for harm. We have cooperated with a number of articles in the San Jose Mercury News, the Washington Post, and the New York Times on this topic. The result, we hope, is technological enablement combined with an informed populace that cannot be tricked in the name of technology.

Computational Methods

An example of a new problem in theoretical computer science emerging from ubiquitous computing is optimal cache sharing. This problem originally arose in discussions of optimal disk cache design for portable computer architectures. Bandwidth to the portable machine may be quite low, while its processing power is relatively high, introducing as a possible design point the compression of pages in a ram cache, rather than writing them all the way back over a slow link. The question arises of the optimal strategy for partitioning memory between compressed and uncompressed pages.

This problem can be generalized as follows [Bern 93]:

The Cache Sharing Problem. A problem instance is given by a sequence of page requests. Pages are of two types, U and C (for uncompressed and compressed), and each page is either IN or OUT. A request is served by changing the requested page to IN if it is currently OUT. Initially all pages are OUT. The cost to change a type-U (type-C) page from OUT to IN is CU (respectively, CC). When a requested page is OUT, we say that the algorithm missed. Removing a page from memory is free.

Lower Bound Theorem: No deterministic, on-line algorithm for cache sharing can be c-competitive for

  • c < MAX (1+CU/(CU+CC), 1+CC/(CU+CC))

This lower bound for c ranges from 1.5 to 2, and no on-line algorithm can approach closer to the optimum than this factor. Bern et al also construct an algorithm that achieves this factor, therefore providing an upper bound as well. They further propose a set of more general symbolic programming tools for solving competitive algorithms of this sort.

Concluding remarks

As we start to put tabs, pads, and boards into use, phase I of ubiquitous computing should enter its most productive period. With this substrate in place we can make much more progress both in evaluating our technologies and in choosing our next steps. A key part of this evaluation is using the analyses of psychologists, anthropologists, application writers, artists, marketers, and customers. We believe they will find some things right; we know they will find some things wrong. Thus we will begin again the cycle of cross-disciplinary fertilization and learning. Ubicomp seems likely to provide a framework for interesting and productive work for many more years or decades, but we have much to learn about the details.

Acknowledgements: This work was funded by Xerox PARC. Portions of this work were sponsored by DARPA. Ubiquitous computing is only a small part of the work going on at PARC; we are grateful for PARC’s rich, cooperative, and fertile environment in support of the document company.Bern 93. Bern, M., Greene, D., Raghunathan. On-line algorithms for cache sharing. 25th ACM Symposium on Theory of Computing, San Diego, 1993.

Buxton 90. Buxton, W. (1990). Smoke and Mirrors. Byte,15(7), July 1990. 205-210.

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Clark 92. Clark, David D., Shenker, Scott, Zhang, Lixia. Supporting real-time applications in an integrated services packet network:architecture and mechanism. SIGCOMM ’92 Conference Proceedings. Communicatins architectures and protcosl. August 17-20, 1992. Baltimore, Maryland. Computer Communication Review. Vol. 22, no. 4, October 1992.published by Accosication for Comptuing Machinery, New York, NY. pp. 14-26

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This is an archive of Mark Weiser’s ubiquitous computing website (ubiq.com) which disappeared from the internet in 2018 some time after Mark Weiser passed away. We wanted to preserve Mark Weiser’s knowledge about ubiquitous computing and are permanently hosting a selection of important pages from ubiq.com.

Ubiquitous computers: The Computer for the 21st Century

by Mark Weiser

Scientific American Ubicomp Paper after Sci Am editing

September, 1991

The most profound technologies are those that disappear. They weave themselves into the fabric of everyday life until they are indistinguishable from it.

Consider writing, perhaps the first information technology: The ability to capture a symbolic representation of spoken language for long-term storage freed information from the limits of individual memory. Today this technology is ubiquitous in industrialized countries. Not only do books, magazines and newspapers convey written information, but so do street signs, billboards, shop signs and even graffiti. Candy wrappers are covered in writing. The constant background presence of these products of “literacy technology” does not require active attention, but the information to be conveyed is ready for use at a glance. It is difficult to imagine modern life otherwise.

Silicon-based information technology, in contrast, is far from having become part of the environment. More than 50 million personal computers have been sold, and nonetheless the computer remains largely in a world of its own. It is approachable only through complex jargon that has nothing to do with the tasks for which which people actually use computers. The state of the art is perhaps analogous to the period when scribes had to know as much about making ink or baking clay as they did about writing.

The arcane aura that surrounds personal computers is not just a “user interface” problem. My colleagues and I at PARC think that the idea of a “personal” computer itself is misplaced, and that the vision of laptop machines, dynabooks and “knowledge navigators” is only a transitional step toward achieving the real potential of information technology. Such machines cannot truly make computing an integral, invisible part of the way people live their lives. Therefore we are trying to conceive a new way of thinking about computers in the world, one that takes into account the natural human environment and allows the computers themselves to vanish into the background.

Such a disappearance is a fundamental consequence not of technology, but of human psychology. Whenever people learn something sufficiently well, they cease to be aware of it. When you look at a street sign, for example, you absorb its information without consciously performing the act of reading.. Computer scientist, economist, and Nobelist Herb Simon calls this phenomenon “compiling”; philosopher Michael Polanyi calls it the “tacit dimension”; psychologist TK Gibson calls it “visual invariants”; philosophers Georg Gadamer and Martin Heidegger call it “the horizon” and the “ready-to-hand”, John Seely Brown at PARC calls it the “periphery”. All say, in essence, that only when things disappear in this way are we freed to use them without thinking and so to focus beyond them on new goals.

The idea of integrating computers seamlessly into the world at large runs counter to a number of present-day trends. “Ubiquitous computing” in this context does not just mean computers that can be carried to the beach, jungle or airport. Even the most powerful notebook computer, with access to a worldwide information network, still focuses attention on a single box. By analogy to writing, carrying a super-laptop is like owning just one very important book. Customizing this book, even writing millions of other books, does not begin to capture the real power of literacy.

Furthermore, although ubiquitous computers may employ sound and video in addition to text and graphics, that does not make them “multimedia computers.” Today’s multimedia machine makes the computer screen into a demanding focus of attention rather than allowing it to fade into the background.

Perhaps most diametrically opposed to our vision is the notion of “virtual reality,” which attempts to make a world inside the computer. Users don special goggles that project an artificial scene on their eyes; they wear gloves or even body suits that sense their motions and gestures so that they can move about and manipulate virtual objects. Although it may have its purpose in allowing people to explore realms otherwise inaccessible — the insides of cells, the surfaces of distant planets, the information web of complex databases — virtual reality is only a map, not a territory. It excludes desks, offices, other people not wearing goggles and body suits, weather, grass, trees, walks, chance encounters and in general the infinite richness of the universe. Virtual reality focuses an enormous apparatus on simulating the world rather than on invisibly enhancing the world that already exists.

Indeed, the opposition between the notion of virtual reality and ubiquitous, invisible computing is so strong that some of us use the term “embodied virtuality” to refer to the process of drawing computers out of their electronic shells. The “virtuality” of computer-readable data — all the different ways in which it can be altered, processed and analyzed — is brought into the physical world.

How do technologies disappear into the background? The vanishing of electric motors may serve as an instructive example: At the turn of the century, a typical workshop or factory contained a single engine that drove dozens or hundreds of different machines through a system of shafts and pulleys. Cheap, small, efficient electric motors made it possible first to give each machine or tool its own source of motive force, then to put many motors into a single machine.

A glance through the shop manual of a typical automobile, for example, reveals twenty-two motors and twenty-five more solenoids. They start the engine, clean the windshield, lock and unlock the doors, and so on. By paying careful attention it might be possible to know whenever one activated a motor, but there would be no point to it.

Most of the computers that participate in embodied virtuality will be invisible in fact as well as in metaphor. Already computers in light switches, thermostats, stereos and ovens help to activate the world. These machines and more will be interconnected in a ubiquitous network. As computer scientists, however, my colleagues and I have focused on devices that transmit and display information more directly. We have found two issues of crucial importance: location and scale. Little is more basic to human perception than physical juxtaposition, and so ubiquitous computers must know where they are. (Today’s computers, in contrast, have no idea of their location and surroundings.) If a computer merely knows what room it is in, it can adapt its behavior in significant ways without requiring even a hint of artificial intelligence.

Ubiquitous computers will also come in different sizes, each suited to a particular task. My colleagues and I have built what we call tabs, pads and boards: inch-scale machines that approximate active Post-It notes, foot-scale ones that behave something like a sheet of paper (or a book or a magazine), and yard-scale displays that are the equivalent of a blackboard or bulletin board.

How many tabs, pads, and board-sized writing and display surfaces are there in a typical room? Look around you: at the inch scale include wall notes, titles on book spines, labels on controls, thermostats and clocks, as well as small pieces of paper. Depending upon the room you may see more than a hundred tabs, ten or twenty pads, and one or two boards. This leads to our goals for initially deploying the hardware of embodied virtuality: hundreds of computers per room.

Hundreds of computers in a room could seem intimidating at first, just as hundreds of volts coursing through wires in the walls did at one time. But like the wires in the walls, these hundreds of computers will come to be invisible to common awareness. People will simply use them unconsciously to accomplish everyday tasks.

Tabs are the smallest components of embodied virtuality. Because they are interconnected, tabs will expand on the usefulness of existing inch-scale computers such as the pocket calculator and the pocket organizer. Tabs will also take on functions that no computer performs today. For example, Olivetti Cambridge Research Labs pioneered active badges, and now computer scientists at PARC and other research laboratories around the world are working with these clip-on computers roughly the size of an employee ID card. These badges can identify themselves to receivers placed throughout a building, thus making it possible to keep track of the people or objects to which they are attached.

In our experimental embodied virtuality, doors open only to the right badge wearer, rooms greet people by name, telephone calls can be automatically forwarded to wherever the recipient may be, receptionists actually know where people are, computer terminals retrieve the preferences of whoever is sitting at them, and appointment diaries write themselves. No revolution in artificial intelligence is needed–just the proper imbedding of computers into the everday world. The automatic diary shows how such a simple thing as knowing where people are can yield complex dividends: meetings, for example, consist of several people spending time in the same room, and the subject of a meeting is most likely the files called up on that room’s display screen while the people are there.

My colleague Roy Want has designed a tab incorporating a small display that can serve simultaneously as an active badge, calendar and diary. It will also act as an extension to computer screens: instead of shrinking a program window down to a small icon on the screen, for example, a user will be able to shrink the window onto a tab display. This will leave the screen free for information and also let people arrange their computer-based projects in the area around their terminals, much as they now arrange paper-based projects in piles on desks and tables. Carrying a project to a different office for discussion is a simple as gathering up its tabs; the associated programs and files can be called up on any terminal.

The next step up in size is the pad, something of a cross between a sheet of paper and current laptop and palmtop computers. Bob Krivacic at PARC has built a prototype pad that uses two microprocessors, a workstation-sized display, a multi-button stylus, and a radio network that can potentially handle hundreds of devices per person per room.

Pads differ from conventional portable computers in one crucial way. Whereas portable computers go everywhere with their owners, the pad that must be carried from place to place is a failure. Pads are intended to be “scrap computers” (analogous to scrap paper) that can be grabbed and used anywhere; they have no individualized identity or importance.

One way to think of pads is as an antidote to windows. Windows were invented at PARC and popularized by Apple in the Macintosh as a way of fitting several different activities onto the small space of a computer screen at the same time. In twenty years computer screens have not grown much larger. Computer window systems are often said to be based on the desktop metaphor–but who would ever use a desk whose surface area is only 9″ by 11″?

Pads, in contrast, use a real desk. Spread many electronic pads around on the desk, just as you spread out papers. Have many tasks in front of you and use the pads as reminders. Go beyond the desk to drawers, shelves, coffee tables. Spread the many parts of the many tasks of the day out in front of you to fit both the task and the reach of your arms and eyes, rather than to fit the limitations of CRT glass-blowing. Someday pads may even be as small and light as actual paper, but meanwhile they can fulfill many more of paper’s functions than can computer screens.

Yard-size displays (boards) serve a number of purposes: in the home, video screens and bulletin boards; in the office, bulletin boards, whiteboards or flip charts. A board might also serve as an electronic bookcase from which one might download texts to a pad or tab. For the time being, however, the ability to pull out a book and place it comfortably on one’s lap remains one of the many attractions of paper. Similar objections apply to using a board as a desktop; people will have to get used to using pads and tabs on a desk as an adjunct to computer screens before taking embodied virtuality even further.

Boards built by Richard Bruce and Scott Elrod at PARC currently measure about 40 by 60 inches and display 1024×768 black-and-white pixels. To manipulate the display, users pick up a piece of wireless electronic “chalk” that can work either in contact with the surface or from a distance. Some researchers, using themselves and their coleagues as guinea pigs, can hold electronically mediated meetings or engage in other forms of collaboration around a liveboard. Others use the boards as testbeds for improved display hardware, new “chalk” and interactive software.

For both obvious and subtle reasons, the software that animates a large, shared display and its electronic chalk is not the same as that for a workstation. Switching back and forth between chalk and keyboard may involve walking several steps, and so the act is qualitatively different from using a keyboard and mouse. In addition, body size is an issue — not everyone can reach the top of the board, so a Macintosh-style menu bar may not be a good idea.

We have built enough liveboards to permit casual use: they have been placed in ordinary conference rooms and open areas, and no one need sign up or give advance notice before using them. By building and using these boards, researchers start to experience and so understand a world in which computer interaction casually enhances every room. Liveboards can usefully be shared across rooms as well as within them. In experiments instigated by Paul Dourish of EuroPARC and Sara Bly and Frank Halasz of PARC, groups at widely separated sites gathered around boards — each displaying the same image — and jointly composed pictures and drawings. They have even shared two boards across the Atlantic.

Liveboards can also be used as bulletin boards. There is already too much data for people to read and comprehend all of it, and so Marvin Theimer and David Nichols at PARC have built a prototype system that attunes its public information to the people reading it. Their “scoreboard” requires little or no interaction from the user other than to look and to wear an active badge.

Prototype tabs, pads and boards are just the beginning of ubiquitous computing. The real power of the concept comes not from any one of these devices; it emerges from the interaction of all of them. The hundreds of processors and displays are not a “user interface” like a mouse and windows, just a pleasant and effective “place” to get things done.

What will be most pleasant and effective is that tabs can animate objects previously inert. They can beep to help locate mislaid papers, books or other items. File drawers can open and show the desired folder — no searching. Tabs in library catalogs can make active maps to any book and guide searchers to it, even if it is off the shelf and on a table from the last reader.

In presentations, the size of text on overhead slides, the volume of the amplified voice, even the amount of ambient light, can be determined not by accident or guess but by the desires of the listeners in the room at that moment. Software tools for instant votes and consensus checking are already in specialized use in electronic meeting rooms of large corporations; tabs can make them widespread.

The technology required for ubiquitous computing comes in three parts: cheap, low-power computers that include equally convenient displays, a network that ties them all together, and software systems implementing ubiquitous applications. Current trends suggest that the first requirement will easily be met. Flat-panel displays containing 640×480 black-and-white pixels are now common. This is the standard size for PC’s and is also about right for television. As long as laptop, palmtop and notebook computers continue to grow in popularity, display prices will fall, and resolution and quality will rise. By the end of the decade, a 1000×800-pixel high-contrast display will be a fraction of a centimeter thick and weigh perhaps 100 grams. A small battery will provide several days of continuous use.

Larger displays are a somewhat different issue. If an interactive computer screen is to match a whiteboard in usefulness, it must be viewable from arm’s length as well as from across a room. For close viewing the density of picture elements should be no worse than on a standard computer screen, about 80 per inch. Maintaining a density of 80 pixels per inch over an area several feet on a side implies displaying tens of millions of pixels. The biggest computer screen made today has only about one fourth this capacity. Such large displays will probably be expensive, but they should certainly be available.

Central-processing unit speeds, meanwhile, reached a million instructions per second in 1986 and continue to double each year. Some industry observers believe that this exponential growth in raw chip speed may begin to level off about 1994, but that other measures of performance, including power consumption and auxiliary functions, will still improve. The 100-gram flat-panel display, then, might be driven by a single microprocessor chip that executes a billion operations per second and contains 16 megabytes of onboard memory along with sound, video and network interfaces. Such a processor would draw, on average, a few percent of the power required by the display.

Auxiliary storage devices will augment the memory capacity. Conservative extrapolation of current technology suggests that match-book size removable hard disks (or the equivalent nonvolatile memory chips) will store about 60 megabytes each. Larger disks containing several gigabytes of information will be standard, and terabyte storage — roughly the capacity of the Library of Congress — will be common. Such enormous stores will not necessarily be filled to capacity with usable information. Abundant space will, however, allow radically different strategies of information management. A terabyte of space makes deleting old files virtually unnecessary, for example.

Although processors and displays should be capable of offering ubiquitous computing by the end of the decade, trends in software and network technology are more problematic. Software systems today barely take any advantage of the computer network. Trends in “distributed computing” are to make networks appear like disks, memory, or other non-networked devices, rather than to exploit the unique capabilities of physical dispersion. The challenges show up in the design of operating systems and window systems.

Today’s operating sytems, like DOS and Unix, assume a relatively fixed configuration of hardware and software at their core. This makes sense for both mainframes and personal computers, because hardware or operating system software cannot reasonably be added without shutting down the machine. But in an embodied virtuality, local devices come and go, and depend upon the room and the people in it. New software for new devices may be needed at any time, and you’ll never be able to shut off everything in the room at once. Experimental “micro-kernel” operating systems, such as those developed by Rick Rashid at Carnegie-Mellon University and Andy Tanenbaum at Vrije University in Amsterdam, offer one solution. Future operating systems based around tiny kernels of functionality may automatically shrink and grow to fit the dynamically changing needs of ubiquitous computing.

Today’s window systems, like Windows 3.0 and the X Window System, assume a fixed base computer on which information will be displayed. Although they can handle multiple screens, they do not do well with applications that start out in one place (screen, computer, or room) and then move to another. For higher performance they assume a fixed screen and input mode and use the local computer to store information about the application–if any of these change, the window system stops working for that application. Even window systems like X that were designed for use over networks have this problem–X still assumes that an application once started stays put. The solutions to this problem are in their infancy. Systems for shared windows, such as those from Brown University and Hewlett-Packard Corporation, help with windows, but have problems of performance, and do not work for all applications. There are no systems that do well with the diversity of inputs to be found in an embodied virtuality. A more general solution will require changing the kinds of protocols by which application programs and windows interact.

The network connecting these computers has its own challenges. On the one hand, data transmission rates for both wired and wireless networks are increasing rapidly. Access to gigabit-per-second wired nets is already possible, although expensive, and will become progressively cheaper. (Gigabit networks will seldom devote all of their bandwidth to a single data stream; instead, they will allow enormous numbers of lower-speed transmissions to proceed simultaneously.) Small wireless networks, based on digital cellular telephone principles, currently offer data rates between two and 10 megabits per second over a range of a few hundred meters. Low-power wireless networks transmitting 250,000 bits per second to each station will eventually be available commercially.

On the other hand, the transparent linking of wired and wireless networks is an unsolved problem. Although some stop-gap methods have been developed, engineers must develop new communication protocols that explicitly recognize the concept of machines that move in physical space. Furthermore the number of channels envisioned in most wireless network schemes is still very small, and the range large (50-100 meters), so that the total number of mobile devices is severely limited. The ability of such a system to support hundreds of machines in every room is out of the question. Single-room networks based on infrared or newer electromagnetic technologies have enough channel capacity for ubiquitous computers, but they can only work indoors.

Present technologies would require a mobile device to have three different network connections: tiny range wireless, long range wireless, and very high speed wired. A single kind of network connection that can somehow serve all three functions has yet to be invented.

Neither an explication of the principles of ubiquitous computing nor a list of the technologies involved really gives a sense of what it would be like to live in a world full of invisible widgets. To extrapolate from today’s rudimentary fragments of embodied virtuality resembles an attempt to predict the publication of Finnegan’s Wake after just having invented writing on clay tablets. Nevertheless the effort is probably worthwhile:

Sal awakens: she smells coffee. A few minutes ago her alarm clock, alerted by her restless rolling before waking, had quietly asked “coffee?”, and she had mumbled “yes.” “Yes” and “no” are the only words it knows.

Sal looks out her windows at her neighborhood. Sunlight and a fence are visible through one, but through others she sees electronic trails that have been kept for her of neighbors coming and going during the early morning. Privacy conventions and practical data rates prevent displaying video footage, but time markers and electronic tracks on the neighborhood map let Sal feel cozy in her street.

Glancing at the windows to her kids’ rooms she can see that they got up 15 and 20 minutes ago and are already in the kitchen. Noticing that she is up, they start making more noise.

At breakfast Sal reads the news. She still prefers the paper form, as do most people. She spots an interesting quote from a columnist in the business section. She wipes her pen over the newspaper’s name, date, section, and page number and then circles the quote. The pen sends a message to the paper, which transmits the quote to her office.

Electronic mail arrives from the company that made her garage door opener. She lost the instruction manual, and asked them for help. They have sent her a new manual, and also something unexpected — a way to find the old one. According to the note, she can press a code into the opener and the missing manual will find itself. In the garage, she tracks a beeping noise to where the oil-stained manual had fallen behind some boxes. Sure enough, there is the tiny tab the manufacturer had affixed in the cover to try to avoid E-mail requests like her own.

On the way to work Sal glances in the foreview mirror to check the traffic. She spots a slowdown ahead, and also notices on a side street the telltale green in the foreview of a food shop, and a new one at that. She decides to take the next exit and get a cup of coffee while avoiding the jam.

Once Sal arrives at work, the foreview helps her to quickly find a parking spot. As she walks into the building the machines in her office prepare to log her in, but don’t complete the sequence until she actually enters her office. On her way, she stops by the offices of four or five colleagues to exchange greetings and news.

Sal glances out her windows: a grey day in silicon valley, 75 percent humidity and 40 percent chance of afternoon showers; meanwhile, it has been a quiet morning at the East Coast office. Usually the activity indicator shows at least one spontaneous urgent meeting by now. She chooses not to shift the window on the home office back three hours — too much chance of being caught by surprise. But she knows others who do, usually people who never get a call from the East but just want to feel involved.

The telltale by the door that Sal programmed her first day on the job is blinking: fresh coffee. She heads for the coffee machine.

Coming back to her office, Sal picks up a tab and “waves” it to her friend Joe in the design group, with whom she is sharing a virtual office for a few weeks. They have a joint assignment on her latest project. Virtual office sharing can take many forms–in this case the two have given each other access to their location detectors and to each other’s screen contents and location. Sal chooses to keep miniature versions of all Joe’s tabs and pads in view and 3-dimensionally correct in a little suite of tabs in the back corner of her desk. She can’t see what anything says, but she feels more in touch with his work when noticing the displays change out of the corner of her eye, and she can easily enlarge anything if necessary.

A blank tab on Sal’s desk beeps, and displays the word “Joe” on it. She picks it up and gestures with it towards her liveboard. Joe wants to discuss a document with her, and now it shows up on the wall as she hears Joe’s voice:

“I’ve been wrestling with this third paragraph all morning and it still has the wrong tone. Would you mind reading it?”

“No problem.”

Sitting back and reading the paragraph, Sal wants to point to a word. She gestures again with the “Joe” tab onto a nearby pad, and then uses the stylus to circle the word she wants:

“I think it’s this term ‘ubiquitous’. Its just not in common enough use, and makes the whole thing sound a little formal. Can we rephrase the sentence to get rid of it?”

“I’ll try that. Say, by the way Sal, did you ever hear from Mary Hausdorf?”

“No. Who’s that?”

“You remember, she was at the meeting last week. She told me she was going to get in touch with you.”

Sal doesn’t remember Mary, but she does vaguely remember the meeting. She quickly starts a search for meetings in the past two weeks with more than 6 people not previously in meetings with her, and finds the one. The attendees’ names pop up, and she sees Mary. As is common in meetings, Mary made some biographical information about herself available to the other attendees, and Sal sees some common background. She’ll just send Mary a note and see what’s up. Sal is glad Mary did not make the biography available only during the time of the meeting, as many people do…

In addition to showing some of the ways that computers can find their way invisibly into people’s lives, this speculation points up some of the social issues that embodied virtuality will engender. Perhaps key among them is privacy: hundreds of computers in every room, all capable of sensing people near them and linked by high-speed networks, have the potential to make totalitarianism up to now seem like sheerest anarchy. Just as a workstation on a local-area network can be programmed to intercept messages meant for others, a single rogue tab in a room could potentially record everything that happened there.

Even today, although active badges and self-writing appointment diaries offer all kinds of convenience, in the wrong hands their information could be stifling. Not only corporate superiors or underlings, but overzealous government officials and even marketing firms could make unpleasant use of the same information that makes invisible computers so convenient.

Fortunately, cryptographic techniques already exist to secure messages from one ubiquitous computer to another and to safeguard private information stored in networked systems. If designed into systems from the outset, these techniques can ensure that private data does not become public. A well-implemented version of ubiquitous computing could even afford better privacy protection than exists today. For example, schemes based on “digital pseudonyms” could eliminate the need to give out items of personal information that are routinely entrusted to the wires today, such as credit card number, social security number and address.

Jim Morris of Carnegie-Mellon University has proposed an appealing general method for approaching these issues: build computer systems to have the same privacy safeguards as the real world, but no more, so that ethical conventions will apply regardless of setting. In the physical world, for example, burglars can break through a locked door, but they leave evidence in doing so. Computers built according to Morris’s rule would not attempt to be utterly proof against cracker, but they would be impossible to enter without leaving the digital equivalent of fingerprints.

By pushing computers into the background, embodied virtuality will make individuals more aware of the people on the other ends of their computer links. This development carries the potential to reverse the unhealthy centripetal forces that conventional personal computers have introduced into life and the workplace. Even today, people holed up in windowless offices before glowing computer screens may not see their fellows for the better part of each day. And in virtual reality, the outside world and all its inhabitant effectively ceases to exist. Ubiquitous computers, in contrast, reside in the human world and pose no barrier to personal interactions. If anything, the transparent connections that they offer between different locations and times may tend to bring communities closer together.

My colleagues and I at PARC believe that what we call ubiquitous computing will gradually emerge as the dominant mode of computer access over the next twenty years. Like the personal computer, ubiquitous computing will enable nothing fundamentally new, but by making everything faster and easier to do, with less strain and mental gymnastics, it will transform what is apparently possible. Desktop publishing, for example, is fundamentally not different from computer typesetting, which dates back to the mid 1960’s at least. But ease of use makes an enormous difference.

When almost every object either contains a computer or can have a tab attached to it, obtaining information will be trivial: “Who made that dress? Are there any more in the store? What was the name of the designer of that suit I liked last week?” The computing environment knows the suit you looked at for a long time last week because it knows both of your locations, and, it can retroactively find the designer’s name even if it did not interest you at the time.

Sociologically, ubiquitous computing may mean the decline of the computer addict. In the 1910’s and 1920’s many people “hacked” on crystal sets to take advantage of the new high tech world of radio. Now crystal-and-cat’s whisker receivers are rare, because radios are ubiquitous. In addition, embodied virtuality will bring computers to the presidents of industries and countries for nearly the first time. Computer access will penetrate all groups in society.

Most important, ubiquitous computers will help overcome the problem of information overload. There is more information available at our fingertips during a walk in the woods than in any computer system, yet people find a walk among trees relaxing and computers frustrating. Machines that fit the human environment, instead of forcing humans to enter theirs, will make using a computer as refreshing as taking a walk in the woods.

This is an archive of Mark Weiser’s ubiquitous computing website (ubiq.com) which disappeared from the internet in 2018 some time after Mark Weiser passed away. We wanted to preserve Mark Weiser’s knowledge about ubiquitous computing and are permanently hosting a selection of important pages from ubiq.com.

Designing Calm Technology

by Mark Weiser and John Seely Brown

Xerox PARC
December 21, 1995


Bits flowing through the wires of a computer network are ordinarily invisible. But a radically new tool shows those bits through motion, sound, and even touch. It communicates both light and heavy network traffic. Its output is so beautifully integrated with human information processing that one does not even need to be looking at it or near it to take advantage of its peripheral clues. It takes no space on your existing computer screen, and in fact does not use or contain a computer at all. It uses no software, only a few dollars in hardware, and can be shared by many people at the same time. It is called the “Dangling String”.

Created by artist Natalie Jeremijenko, the “Dangling String” is an 8 foot piece of plastic spaghetti that hangs from a small electric motor mounted in the ceiling. The motor is electrically connected to a nearby Ethernet cable, so that each bit of information that goes past causes a tiny twitch of the motor. A very busy network causes a madly whirling string with a characteristic noise; a quiet network causes only a small twitch every few seconds. Placed in an unused corner of a hallway, the long string is visible and audible from many offices without being obtrusive. It is fun and useful. The Dangling String meets a key challenge in technology design for the next decade: how to create calm technology. 

We have struggled for some time to understand the design of calm technology, and our thoughts are still incomplete and perhaps even a bit confused. Nonetheless, we believe that calm technology may be the most important design problem of the twenty-first century, and it is time to begin the dialogue.

The Periphery

Designs that encalm and inform meet two human needs not usually met together. Information technology is more often the enemy of calm. Pagers, cellphones, newservices, the World-Wide-Web, email, TV, and radio bombard us frenetically. Can we really look to technology itself for a solution?

But some technology does lead to true calm and comfort. There is no less technology involved in a comfortable pair of shoes, in a fine writing pen, or in delivering the New York Times on a Sunday morning, than in a home PC. Why is one often enraging, the others frequently encalming? We believe the difference is in how they engage our attention. Calm technology engages both the center and the periphery of our attention, and in fact moves back and forth between the two.

We use “periphery” to name what we are attuned to without attending to explicitly. Ordinarily when driving our attention is centered on the road, the radio, our passenger, but not the noise of the engine. But an unusual noise is noticed immediately, showing that we were attuned to the noise in the periphery, and could come quickly to attend to it.

It should be clear that what we mean by the periphery is anything but on the fringe or unimportant. What is in the periphery at one moment may in the next moment come to be at the center of our attention and so be crucial. The same physical form may even have elements in both the center and periphery. The ink that communicates the central words of a text also, though choice of font and layout, peripherally clues us into the genre of the text. 

A calm technology will move easily from the periphery of our attention, to the center, and back. This is fundamentally encalming, for two reasons.

First, by placing things in the periphery we are able to attune to many more things than we could if everything had to be at the center. Things in the periphery are attuned to by the large portion of our brains devoted to peripheral (sensory) processing. Thus the periphery is informing without overburdening.

Second, by recentering something formerly in the periphery we take control of it. Peripherally we may become aware that something is not quite right, as when awkward sentences leave a reader tired and discomforted without knowing why. By moving sentence construction from periphery to center we are empowered to act, either by finding better literature or accepting the source of the unease and continuing. Without centering the periphery might be a source of frantic following of fashion; with centering the periphery is a fundamental enabler of calm through increased awareness and power.

Not all technology need be calm. A calm videogame would get little use; the point is to be excited. But too much design focuses on the object itself and its surface features without regard for context. We must learn to design for the periphery so that we can most fully command technology without being dominated by it. 

Our notion of technology in the periphery is related to the notion of affordances, due to Gibson by popularized by Norman. An affordance is a relationship between an object in the world and the intentions, perceptions, and capabilities of a person. The side of a door that only pushes out affords this action by offering a flat pushplate. The idea of affordance, powerful as it is, tends to describe the surface of a design. For us the term “affordance” does not reach far enough into the periphery where a design must be attuned to but not attended to.

Three signs of calm technology

Technologies encalm as they empower our periphery. This happens in two ways. First, as already mentioned, a calming technology may be one that easily moves from center to periphery and back. Second, a technology may enhance our peripheral reach by bringing more details into the periphery. An example is a video conference that, by comparison to a telephone conference, enables us to attune to nuances of body posture and facial expression that would otherwise be inaccessible. This is encalming when the enhanced peripheral reach increases our knowledge and so our ability to act without increasing information overload.

The result of calm technology is to put us at home, in a familiar place. When our periphery is functioning well we are tuned into what is happening around us, and so also to what is going to happen, and what has just happened. We are connected effortlessly to a myriad of familiar details. This connection to the world around we called “locatedness”, and it is the fundamental gift that the periphery gives us.

Examples of calm technology

To deepen the dialogue we now examine a few designs in terms of their motion between center and periphery, peripheral reach, and locatedness. Below we consider inner office windows, Internet Multicast, and once again the Dangling String.

inner office windows

We do not know who invented the concept of glass windows from offices out to hallways. But these inner windows are a beautifully simple design that enhances peripheral reach and locatedness. 

The hallway window extends our periphery by creating a two-way channel for clues about the environment. Whether it is motion of other people down the hall (its time for a lunch; the big meeting is starting), or noticing the same person peeking in for the third time while you are on the phone (they really want to see me; I forgot an appointment), the window connects the person inside to the nearby world.

Inner windows also connect with those who are outside the office. A light shining out into the hall means someone is working late; someone picking up their office means this might be a good time for a casual chat. These small clues become part of the periphery of a calm and comfortable workplace.

Office windows illustrate a fundamental property of motion between center and periphery. Contrast them with an open office plan in which desks are separated only by low or no partitions. Open offices force too much to the center. For example, a person hanging out near an open cubicle demands attention by social conventions of privacy and politeness. There is less opportunity for the subtle clue of peeking through a window without eavesdropping on a conversation. The individual, not the environment, must be in charge of moving things from center to periphery and back. 

The inner office window is a metaphor for what is most exciting about the Internet, namely the ability to locate and be located by people passing by on the information highway.

Internet Multicast

A technology called Internet Multicast may become the next World Wide Web (WWW) phenomenon. Sometimes called the MBone (for Multicast backBONE), multicasting was invented by a then graduate student at Stanford University, Steve Deering.

Whereas the World Wide Web (WWW) connects only two computers at a time, and then only for the few moments that information is being downloaded, the MBone continuously connects many computers at the same time. To use the familiar highway metaphor, for any one person the WWW only lets one car on the road at a time, and it must travel straight to its destination with no stops or side trips. By contrast, the MBone opens up streams of traffic between multiple people and so enables the flow of activities that constitute a neighborhood. Where the WWW ventures timidly to one location at a time before scurrying back home again, the MBone sustains ongoing relationships between machines, places, and people.

Multicast is fundamentally about increasing peripheral reach, derived from its ability to cheaply support multiple multimedia (video, audio, etc.) connections all day long. Continuous video from another place is no longer television, and no longer video-conferencing, but more like a window of awareness. A continuous video stream brings new details into the periphery: the room is cleaned up, something important may be about to happen; everyone got in late today on the east coast, must be a big snowstorm or traffic tie-up. 

Multicast shares with videoconferencing and television an increased opportunity to attune to additional details. Compared to a telephone or fax, the broader channel of full multimedia better projects the person through the wire. The presence is enhanced by the responsiveness that full two-way (or multiway) interaction brings. 

Like the inner windows, Multicast enables control of the periphery to remain with the individual, not the environment. A properly designed real-time Multicast tool will offer, but not demand. The MBone provides the necessary partial separation for moving between center and periphery that a high bandwidth world alone does not. Less is more, when less bandwidth provides more calmness. 

Multicast at the moment is not an easy technology to use, and only a few applications have been developed by some very smart people. This could also be said of the digital computer in 1945, and of the Internet in 1975. Multicast in our periphery will utterly change our world in twenty years.

Dangling String

Let’s return to the dangling string. At first it creates a new center of attention just by being unique. But this center soon becomes peripheral as the gentle waving of the string moves easily to the background. That the string can be both seen and heard helps by increasing the clues for peripheral attunement.

The dangling string increases our peripheral reach to the formerly inaccessible network traffic. While screen displays of traffic are common, their symbols require interpretation and attention, and do not peripheralize well. The string, in part because it is actually in the physical world, has a better impedance match with our brain’s peripheral nerve centers.

In Conclusion

It seems contradictory to say, in the face of frequent complaints about information overload, that more information could be encalming. It seems almost nonsensical to say that the way to become attuned to more information is to attend to it less. It is these apparently bizarre features that may account for why so few designs properly take into account center and periphery to achieve an increased sense of locatedness. But such designs are crucial. Once we are located in a world, the door is opened to social interactions among shared things in that world. As we learn to design calm technology, we will enrich not only our space of artifacts, but also our opportunities for being with other people. Thus may design of calm technology come to play a central role in a more humanly empowered twenty-first century.


Gibson, J. The Ecological Approach to Visual Perception. New York: Houghton Mifflin, 1979.

Norman, D.A. The Psychology of Everyday Things. New York: Basic Books, 1988.

MBone. http://www.best.com/~prince/techinfo/mbone.html 

Brown, J.S. and Duguid, P. Keeping It Simple: Investigating Resources in the Periphery. To appear in Solving the Software Puzzle. Ed. T. Winograd, Stanford University. Spring 1996. 

Weiser, M. The Computer for the Twenty-First Century. Scientific American. September 1991.

Brown, J.S. http://www.startribune.com/digage/seelybro.htm 

Weiser, M. http://www.ubiq.com/weiser

This is an archive of Mark Weiser’s ubiquitous computing website (ubiq.com) which disappeared from the internet in 2018 some time after Mark Weiser passed away. We wanted to preserve Mark Weiser’s knowledge about ubiquitous computing and are permanently hosting a selection of important pages from ubiq.com.