GUEST
EDITORS’NOTE
Jssues in Mobile Cumpufinq Systems he vision of m o b i l e computing systems is at once compelling and elusive: the goal of “anytime, anywhere access to information, communication, and service” is easy t o articulate, but PARVIZ difficult technical issues M U R R A Y S. M A Z E R separate contemporary systems from those to which we aspire. The physical precursors for mobile computing are improving at a rapid pace: mobile devices and their components are becoming smaller, faster, and denser, and communications capabilities are becoming more diverse and pervasive. However, software systems (including applications, middleware, and operating systems) lag in exploiting these improvements. This Special Issue is dedicated to exploring some of the important problems facing computing systems researchers as we move toward the exciting world of always-accessible information systems. One may view this brave new world of computing as a combination of the following three elements: Stationary computers and network devices, which we will call the “infrastructure” Mobile computers, of varying physical properties (such as weight, power, and size) and computational capabilities, which will likely be “resource-poor“relative to the infrastructure Wireline and wireless communications channels of varying bandwidth, cost, reliability, and service properties The overarching feature of tomorrow’s systems will be adaptability, to computational capabilities, communication properties, and physical and virtual context. This new design requirement will pervade all aspects of system software, from hardware devices through the operating system and middleware to applications, including user interfaces. For example, as a communication channel‘s quality of service changes, how far up will knowledge of that change propagate, and how will those components adapt? An application that can no longer assume high-bandwidth communications will need to adapt in order t o access information without long delays o r high cost; it might adapt the quality of information retrieved (for example, degrading the fidelity of images), adapt the interaction mode (disabling long latency functions), or adapt the distribution of work (delegating some function to a middleware gateway). High-level questions to be addressed include: How will we overcome the inadequacies of classical software structures? How will we incorporate support for new requirements, such as locating mobile devices and exploiting asymmetric communication? How will we adapt cur0
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rent applications to the requirements of mobile computing, and how will we develop new applications? Some researchers t a k e a n evolutionary approach by extending contemporary architectures a n d services; for KERMAN~ HENRYCHANG example, they extend client/server systems by adapting the cIient or server or by adding intermediaries. Others are adopting new paradigms of information services, such as ubiquitous computing. Both approaches are represented within this issue, in a selection intendeid to stimulate the reader to investigate further, not to cover the spectrum of active research and development efforts addressing these problems. The first paper, by Bagrodia, Chu, Kleinrock, and Popek, presents a high-level view of the broad range of issues involved. They focus the discussion on how one might extend today’s system architectures t o support nomadicig, so that systems could provide ongoing, transparent, integrated access to computing and communications while t h e user moves from place t o place. T h e authors propose the goal of “transparent virtual networking,” which should permit users and programs t o be as effective as possible in environments of uncertain connectivity without changes to the manner in which they operate. While we are far from achieving that goal today, the paper discusses an architecture which is intended to produce systems that can adjust to varying processing, commmunications, and access properties. The authors touch upon resultant issues such as mobile file systems, message queuing, intelligent databases for resource discovery, and large-scale performance modeling. In contrast to the preceding approach, which is primarily evolutionary, researchers at the Xerox Palo Alto Research Center are pursuing a revolutionary vision which they term “Ubiquitous Computing.” The second paper, by Want et al., reports on one of Xerox’s experimental systems, the ParcTab. The philosophy of Ubiquitous Computing aims to enrich our computing experience by embedding into the environment many computing devices of different types, both personal and anonymous, fixed and mobile, large and small, and by using the physical and virtual context to provide more flexible personal access to information. This p a p e r describes in detail t h e motivation, implementation, and evaluation of the ParcTab system. Mobile users often access d a t a t h a t “normally” reside in stationary network-based data sources; increasingly, mobile systems cache those data. A critical issue involves the degree of consistency maintained between the data on a mobile device and their counterparts on the sta-
IEEE Personal Communications December 1995
G U E S T EDITORS' NOTE tionary systems.The paper by Honeyman and Huston examines this problem in the context of mobile file systems. In particular, the authors examine trade-offs between the latency of file system access and degrees of file consistency under variations in network. availability. They discuss how a file service may adaptively relax cache consistency guarantees to achieve acceptable performance in the face of changing communication proplxties. The next paper, by Acharya, Franklin, and Zdonik, explores the provocative concept of broadcast disks as a way to provide data to mobile clients in a context of asymmetric communication. A common assumption for current and future mobile systems is that the communication channel from the infrastructure to mobile devices has much more capacity than the reverse channel, resulting in asymmetric communication. In the broadcast disk approach, a server continuously and repeatedly broadcasts data across a high-capacity channel to a client community, providing a "disk" from which clients can rei rieve data, thereby relieving the mobile device from some of its storage requirements. The authors discuss how to create an arbitrarily fine-grained memory hierarchy from this mechanism and explore interactions with client-$de caching. In most settings, the communication bandwidth between the mobile device and the infrastructure will be low in both directions. An important technique for reducing the amount of traffic across this narrow channel is to use a process that sits in the infrastructure and acts on behalf of the mobile client. By knowing the characteristics of the mobile device, this "network-based agent" can perform a number of functions for its client, such as service negotiation and data prefetchirig, filtering, and compression. The final paper in this Special Issue, by Ramjee, La Porta, and Veeraragahvan, explores this approach in the context of mobile terminals within a PCS environment. In particular, a network-based agent process can reduce control signaling traffic for a mobile terminal by carrying out complex service negotiations and then communicating with the mobile terminal using simple messages. The authors examine the performance of seberal schemes for migrating the agent process. As these papers demonstrate, mobile computing is hard. It pushes the adaptability of software systems to the extreme, with numerous interesting challenges arising in all layers of system and application components. At the core of many of the issues are the nature and capabilities of the communication channel. Developers of communication systems can greatly impact the development and deployment of future mobile-capable environments by creating more adaptive communication systems with higher bandwidth, larger space coverage, and better interoperabil-
IEEE Personal Communications December 1995
ity. Users and their applications will always make more demands on the system than the system can deliver. We wish to thank Hamid Ahmadi, the Editor-inChief of IEEE Personal Communications, for the privilege of serving as guest editors for this issue. We also thank the authors for contributing superb papers and the reviewers for helping to make the papers even better. We encourage others working on mobile computing systems to share new results and viewpoints through the pages of this publication.
Biography MURRAY S . MAZERreceived the Ph.D. in computer science from the University of Toronto, where he was Junior Fellow at Massey College and Trinity College. He received undergraduate training at the University of Saskatchewan and the Hebrew University of Jerusalem. He is currently principal research fellow at the OSF Research Institute, a computer systems think tank in Cambridge, Massachusetts. The OSF RI is funded by governments and corporations worldwide t o develop innovative technology for World Wide Web infrastructure and applications, and for scalable, fault-tolerant, secure, microkernel-based operating systems. Dr. Mazer currently leads a project on support for ongoing adaptive mobile access t o Web-based information resources under conditions of variable or intermittent connectivity. He also consults on projects investigating Web support for collaborative activities, DCE-based secure Web technology, and automated browsing assistants for easier location, management, and sharing of Internet-based information. Previously, he worked at Digital Equipment Corporation, where his research addressed issues in distributed systems, including data replication, location awareness, and disconnected operation. In addition, he has served as consultant t o the legal community on technical issues in software patents. He can be contacted at
[email protected]. Parviz Kermani [SM] received the Ph.D. degree in computer science at UCLA in 1977, and since then he has been with IBM at the T. 1. Watson Research Center. While at UCLA he participated in the ARPA network project and did research in the design and evaluation of switching and flow control techniques in computer communication networks under the supervision of Professor Leonard Kleinrock. His pioneering work on a new switching technique, Cut-Through switching, was later used in many innovative switching and networking architectures. Since joining IBM, he has been involved in a number of diverse research projects spanning from theoretical work t o product development. He has made a number of contributions t o different parts of IBM communication products. Dr. Kermani has many publications in diverse fields related t o data communication networks and has chaired technical sessions in a number of international conferences. He has also received a number of other awards and recognitions for his innovative work for which IBM holds patents. Dr. Kermani has served on the editorial board of /E€€ Computer Networks and ISDNSystems for a number of years. He is also an adjunct faculty member at the graduate center of Polytechnic University in New York, where he teaches graduate courses in computer communication networks. His current interests are in the area of mobile computing and personal system communications. He is currently manager of the mobility software group at the T. J. Watson Reserach Center. He can be reached at kermani@watson. ibm.com. HENRY CHANGis a research staff member in the mobility software group at the IBM T. J. Watson Research Center. His area of activity includes mobile file systems, mobile object application frameworks and internet mobility services. Before joining the mobility group, he adapted the MACH microkernel with parallel scheduling and memory extensions for the IBM RP3 64-way multiprocessor. He received his Ph.D. in computer sciences from the University of Wisconsin a t Madison i n 1987, where he b u i l t a distributed microkernel, Charlotte, on a cluster of VAXes, with a thesis on the performance study of distributed migratable processes. He received the B.S. in electrical engineering at the National Taiwan University in 1979. He is a member of the ACM and the Buddhism Association of the United States.
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