Nomadic Computing and Communications
Nomadicity refers to the system support needed to provide a rich set of computing and communications capabilities and services, in a transparent and convenient form, to the nomad moving from place to place. This new paradigm is already manifesting itself as users travel to many different locations with laptops, personal digital assistants, cellular telephones, pagers, and so on. In this paper we discuss some of the open issues that must be addressed in the system support necessary for nomadicity. In addition, we present some additional considerations in the area of wireless communications, which forms one (and only one) component of nomadicity.
Currently, most users think of computers as associated with their desktop appliances or with a server located in a dungeon in some mysterious basement. However, many of those same users may be considered to be nomads, in that they own computers and communication devices that they carry about with them in their travels as they move between office, home, airplane, hotel, automobile, branch office, and so on. Moreover, even without portable computers or communications, there are many who travel to numerous locations in their business and personal lives and who require access to computers and communications when they arrive at their destinations. Indeed, even a move from a desk to a conference table in the same office constitutes a nomadic move since the computing platforms and communications capability may be considerably different at the two locations. The variety of portable computers is impressive, ranging from laptop computers, notebook computers, and personal digital assistants (or personal information managers) to "smart" credit card devices and wristwatch computers. In addition, the communication capability of these portable computers is advancing at a dramatic pacefrom high-speed modems to PCMCIA modems, e-mail receivers on a card, spread-spectrum hand-held radios, CDPD transceivers, portable GPS receivers, and gigabit satellite access, and so on.
The combination of portable computing with portable communications is changing the way we think about information processing (Weiser, 1991). We now recognize that access to computing and communications is necessary not only from "home base" but also while in transit and after reaching a destination.1
These ideas form the essence of a major shift to nomadicity (nomadic computing and communications), which we address in this paper. The focus is on the system support needed to provide a rich set of capabilities and services, in a transparent and convenient form, to the nomad moving from place to place.
NOTE: This work was supported by the Advanced Research Projects Agency, ARPA/CSTO, under Contract J-FBI-93-112, "Computer Aided Design of High Performance Network Wireless Networked Systems," and by the Advanced Research Projects Agency, ARPA/CSTO, under Contract DABT-63-C-0080, "Transparent Virtual Mobile Environment."
This paper contains material similar to that published by the author in a paper where the emphasis was on the research issues to be addressed in nomadic computing (Kleinrock, 1995).
We are interested in those capabilities that must be put in place to support nomadicity. The desirable characteristics for nomadicity include independence of location, motion, computing platform, communication device, and communication bandwidth, and widespread presence of access to remote files, systems, and services. The notion of independence does not refer here to the quality of service, but rather to the perception of a computing environment that automatically adjusts to the processing, communications, and access available at the moment. For example, the bandwidth for moving data between a user and a remote server could easily vary from a few bits per second (in a noisy wireless environment) to hundreds of megabits per second (in a hard-wired ATM environment); or the computing platform available to the user could vary from a low-powered personal digital assistant while traveling to a powerful supercomputer in a science laboratory. Indeed, today's systems treat radically changing connectivity or bandwidth/latency values as exceptions or failures; in the nomadic environment, these must be treated as the usual case. Moreover, the ability to accept partial or incomplete results is an option that must be made available because of the uncertainties of the informatics infrastructure.
The ability to automatically adjust all aspects of the user's computing, communication, and storage functionality in a transparent and integrated fashion is the essence of a nomadic environment.
Some of the key system parameters of concern include bandwidth, latency, reliability, error rate, delay, storage, processing power, interference, version control, file synchronization, access to services, interoperability, and user interface. These are the usual concerns for any computer-communication environment, but what makes them of special interest for us is that the values of these parameters change dramatically as the nomad moves from location to location. In addition, some totally new and primary concerns arise for the nomad such as weight, size, and battery life of the portable devices as well as unpredictability and wide variation in the communication devices and channels. The bottom line consideration in many nomadic applications is, of course, cost.
Many of the key parameters above focus on the lower levels of the architecture, and they have received the most attention from industry and product development to date. This is natural since hardware devices must focus on such issues. However, there is an enormous effort that must be focused on the middleware services if nomadicity is to be achieved. We identify a number of such services below, but we must recognize that they are in the early stages of identification and development.
There are a number of reasons why nomadicity is of interest. For example, nomadicity is clearly a newly emerging technology that already surrounds the user. Indeed, this author judges it to be a paradigm shift in the way computing will be done in the future. Information technology trends are moving in this direction. Nomadic computing and communications is a multidisciplinary and multi-institutional effort. It has a huge potential for improved capability and convenience for the user. At the same time, it presents at last as huge a problem in interoperability at many levels. The contributions from any investigation of nomadicity will be mainly at the middleware level. The products that are beginning to roll out have a short-term focus; however, there is an enormous level of interest among vendors (from the computer manufacturers, the networking manufacturers, the carriers, and so on) for long-range development and product planning, much of which is now under way. Whatever work is accomplished now will certainly be of immediate practical use.
There are fundamental new research problems that arise in the development of a nomadic architecture and system. Let us consider a sampling of such problems, which we break out into systems issues and wireless networking issues.
One key problem is to develop a full system architecture and set of protocols for nomadicity. These should provide for a transparent view of the user's dynamically changing computing and communications environment. The protocols must satisfy the following kinds of requirements:
In addition, the following components can help in meeting these requirements:
A second research problem is to develop a reference model for nomadicity that will allow for a consistent discussion of its attributes, features, and structure. This should be done in a way that characterizes the view of the system as seen by the user, and the view of the user as seen by the system. The dimensions of this reference model might include the following:
A third research problem is to develop mathematical models of the nomadic environment. These models will allow one to study the performance of the system under various workloads and system configurations as well as to develop design procedures.
As mentioned above, the area of nomadic computing and communications is multidisciplinary. Following is a list of the disciplines that contribute to this area (in top-down order):
The reason that the last three items in this list are included is that we intend that the nomadic environment include the concept of an intelligent room. Such a room has embedded in its walls, furniture, floor, and other aspects all manner of sensors (to detect who and what is in the room), actuators, communicators, logic, cameras, etc. Indeed, one would hope to be able to speak to the room and say, for example, "I need some books on the subject of spread spectrum radios," and perhaps three books would reply. The replies would also offer to present the table of contents of each book, as well, perhaps, as the full text and graphics. Moreover, the books would identify where they are in the room, and, if such were the case, might add that one of the books is three doors down the hall in a colleague's office!
There are numerous other systems issues of interest that we have not addressed here. One of the primary issues is that of security, which involves privacy as well as authentication. Such matters are especially difficult in a nomadic environment, because the nomad often finds that the computing and communication devices are outside the careful security walls of his or her home organization. This basic lack of physical security exacerbates the problem of achieving nomadicity.
We have only touched on some of the systems issues relevant to nomadicity. Let us now discuss some of the wireless networking issues of nomadicity.
Wireless Networking Issues
It is clear that a great many issues regarding nomadicity arise whether or not there is access to wireless communications. However, with such access, a number of interesting considerations arise.
Access to wireless communications provides two capabilities to the nomad: It allows for communication from various (fixed) locations without being connected directly into the wireline network, and it allows the nomad to communicate while traveling. Although the bandwidth offered by wireless communication media varies over as enormous a range as does the wireline network bandwidth, the nature of the error rate, fading behavior, interference level, and mobility issues for wireless are considerably different, so that the algorithms and protocols require some new and different forms from those of wireline networks (Katz, 1994). For example, the network algorithms to support wireless access are far more complex than for the wireline case; some of these are identified below. Whereas the location of a user or a device is a concern for wireline networks as described above, the details of tracking a user moving in a wireless environment add to the complexity and require rules for handover, roaming, and so on.
The cellular radio networks so prevalent today have an architecture that assumes the existence of a cell base station for each cell of the array; the base station controls the activity of its cell. The design considerations of such cellular networks are reasonably well understood and are being addressed by an entire industry (Padgett et al., 1995). We discuss these no further here.3
There is, however, another wireless networking architecture of interest that assumes no base stations (Jain et al., 1995; Short et al., 1995). Such wireless networks are useful for applications that require "instant" infrastracture, among others. For example, disaster relief, emergency operations, special military operations, and clandestine operations are all cases where no base station infrastructure can be assumed. In the case of no base stations, maintaining communications is considerably more difficult. For example, it may be that the destination for a given reception is not within range of the transmitter, and some form of relaying is therefore required; this is known as "multihop" communications. Moreover, since there are no fixed-location base stations, then the connectivity of the network is subject to considerable change as devices move around and/or as the medium change its characteristics. A number of new considerations arise in these situations, and new kinds of network algorithms are needed to deal with them.
To elaborate on some of the issues of concern if there are no base stations, we take three possible scenarios:
1. Static topology with one-hop communications. In this case, there is no motion among the system elements, and all transmitters can reach their destinations without any relays. The issues of concern, along with the needed network algorithms (shown in bold print), are as follows:
2. Static topology with multihop communications. Here the topology is static again, but transmitters may not be able to reach their destinations in one hop, and so multihop relay communications is necessary in some cases. The issues of concern, along with the needed network algorithms (shown in bold print), include all of the above plus the following:
3. Dynamic topology with multihop. In this case, the devices (radios, users, etc.) are allowed to move, which cause the network connectivity to change dynamically. The issues of concern, along with the needed network algorithms (shown in bold print), include all of the above plus the following:
These lists of considerations are not complete but are only illustrative of the many interesting research problems that present themselves in this environment. The net result of these considerations is that the typical 7-layer OSI model for networking must be modified to account for these new considerations. For example, we must ask what kind of network operating system (NOS) should be developed, along with other network functions (Short et al., 1995); what mobility modules must be introduced to support these new services; and so on.
This section addresses mainly the network algorithm issues and does not focus on the many other issues involved with radio design, hardware design, tools for CAD, system drivers, and so on. What is important is that the network algorithms must be supported by the underlying radio (e.g., to provide signal-to-interference ratios, ability to do power control, change codes in CDMA environments, and the like). These obviously have an impact on the functionality, structure, and convenience of the appliance that the user must carry around, as well as on its cost.
If we ask what are the great applications of wireless technology that affect the fabric of our society, then education applications stand out among the most significant. In this application, a wireless infrastructure could serve to provide connectivity in a cost-effective fashion to rural areas that are difficult to serve otherwise; it could
serve within a school to provide flexible sharing of devices as they move from location to location. For long-distance wireless access, it seems that direct broadcast satellite (DBS) technology would be great benefit, but it should also provide a decent up-channel as well. For in-building wireless access, the availability of unlicensed spectrum for datasay, the 60-GHz rangewould serve a number of education applications nicely.
One might ask what role government could play in helping to bring about some of the advantages just described. The allocation of spectrum is one of the major ways in which government can assist. Currently, most spectrum is assigned for long periods of time to specific types of services; it seems that a more liberal view on the kinds of uses for radio bandwidth would encourage innovative applications, services, and efficient sharing of this bandwidth. Any action (such as spectrum allocation and use) that encourages the introduction of innovative services is to be encouraged by whatever means government has available.
This paper presents nomadicity as a new paradigm in the use of computer and communications technology and outlines a number of challenging problems. It is clear that our existing physical and logical infrastructure must be extended to support nomadicity in the many ways described here. The implication is that we must account for nomadicity at this early stage in the development and deployment of the NII; failure to do so will seriously inhibit the growth of nomadic computing and communications. In addition to those issues we raise here, there are far more we have not yet identified. Those will arise only as we probe the frontiers of nomadic computing and communications.
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Padgett, J.E., C.G. Gunther, and T. Hattori. 1995. "Overview of Wireless Personal Communications," IEEE Communications Magazine 33(1):28–41.
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1. Moreover, one may have more than a single "home base"; in fact, there may be no well-defined "home base" at all.
2. Some of the ideas presented in this section were developed with two groups with which the author has collaborated in work on nomadic computing and communications. One of these is the Nomadic Working Team (NWT) of the Cross Industrial Working Team (XIWT); the author is the chairman of the NWT, and this working team recently published a white paper on nomadic computing (NWT, 1995). The second group is a set of his colleagues at the UCLA Computer Science Department who are working on an ARPA-supported effort known as TRAVLER, of which he is principal investigator.
3. Wireless LANs come in a variety of forms. Some of them are centrally controlled and therefore have some of the same control issues as cellular systems with base stations; others have distributed control, in which case they behave more like the no-base-station systems we discuss in this section.