2
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.
Systems 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:
•
Interoperation among many kinds of infrastructures
(e.g., wireline and wireless);
•
Ability to deal with unpredictability of user
behavior, network capability, and computing platform;
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•
Ability to provide for graceful degradation;
•
Ability to scale with respect to heterogeneity,
address space, quality of service (QOS), bandwidth, geographical
dimensions, number of users, and so on;
•
Integrated access to services;
•
Ad hoc access to services;
•
Maximum independence between the network and the
applications from both the user's viewpoint and the development
viewpoint;
•
Ability to match the nature of what is transmitted
to the network bandwidth availability (i.e., compression,
approximation, partial information, and so on);
•
Cooperation among system elements such as sensors,
actuators, devices, network, operating system, file system,
middleware, services, applications, and so on; and
•
Ability to locate users, devices, services, and
the like.
In addition, the following components can help in meeting these
requirements:
•
An integrated software framework that presents a
common virtual network layer;
•
Appropriate replication services at various
levels;
•
File synchronization;
•
Predictive caching;
•
Consistency services;
•
Adaptive database management;
•
Location services (to find people and devices via
tracking, forwarding, searching, etc.)—Mobile IP (Perkins,
1995) is an example of an emerging standard here;
•
Discovery of resources; and
•
Discovery of profile.
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:
•
System state consistency (i.e., Is the system
consistent at the level of e-mail, files, database, applications,
and so on?);
•
Functionality (this could include the bandwidth of
communications, the nature of the communication infrastructure, and
the quality of service provided); and
•
Locality, or awareness (i.e., How aware is the
user of the local environment and its resources, and how aware is
the environment of the users and their profiles?).
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):
•
Advanced applications, such as multimedia or
visualization;
•
Database systems;
•
File systems;
•
Operating systems;
•
Network systems;
•
Wireless communications;
•
Low-power, low-cost radio technology;
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•
Micro-electro-mechanical systems (MEMS) sensor
technology;
•
MEMS actuator technology; and
•
Nanotechnology.
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:
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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:
•
Can you reach your destination?: Power
Control
•
What access method should you use?: Network
Access Control
•
Which channel (or code) should you use?:
Channel Assignment Control
•
Will you interfere with another transmission?:
Power and Medium Access Control
•
When do you allow a new "call" into the system?:
Admission Control
•
For different multiplexed streams, can you achieve
the required QOS (e.g., bandwidth, loss, delay, delay jitter,
higher-order statistics, etc.)?: Multimedia Control
•
What packet size should you use?: System
Design
•
How are errors to be handled?: Error
Control
•
How do you handle congestion?: Congestion
Control
•
How do you adapt to failures?: Degradation
Control
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:
•
Is there a path to your destination?: Path
Control
•
Does giant stepping help (Takagi and Kleinrock,
1984)?: Power Control
•
What routing procedure should you use?: Routing
Control
•
When should you reroute existing calls?:
Reconfiguration Control
•
How do you assign bandwidth and QOS along the
path?: Admission Control and Channel Assignment
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:
•
Do you track, forward, or search for your
destination?: Location Control
•
What network reconfiguration strategy should you
use?: Adaptive Topology Control
•
How should you use reconfigurable and adaptive
base stations?: Adaptive Base Station Control
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
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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 data—say, the 60-GHz
range—would 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.
Conclusion
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.
References
Jain, R., J. Short, L. Kleinrock, S.
Nazareth, and J. Villasenor. 1995. "PC-notebook Based Mobile
Networking: Algorithms, Architectures and Implementations," ICC
'95, pp. 771–777, June.
Katz, R.H. 1994. "Adaptation and Mobility
in Wireless Information Systems," IEEE Personal Communications
Magazine 1(1):6–17.
Kleinrock, L. 1995. "Nomadic
Computing—An Opportunity," Computer Communications Review,
ACM SIGCOMM 25(1):36–40.
Nomadic Working Team (NWT). 1995.
"Nomadicity in the NII," Cross-Industry Working Team, Corporation
for National Research Initiatives, Reston, Va.
Padgett, J.E., C.G. Gunther, and T.
Hattori. 1995. "Overview of Wireless Personal Communications,"
IEEE Communications Magazine 33(1):28–41.
Perkins, Charles. 1995. "IP Mobility
Support," an Internet draft produced for the Internet Engineering
Task Force; see
http://www.ietf.cnri.reston.va.us/ids.by.wg/mobil.
Short, J., R. Bagrodia, and L. Kleinrock.
1995. "Mobile Wireless Network System Simulation," Proceedings
of ACM Mobile Computing & Networking Conference (Mobicom
'95), pp. 195–205, November.
Takagi, H., and L. Kleinrock. 1984.
"Optimal Transmission Ranges for Randomly Distributed Packet Radio
Terminals," IEEE Transactions on Communications, Vol.
COM-32, No. 3, pp. 246–257, March.
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94–104.
Notes
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.
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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.
Representative terms from entire chapter:
base stations
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