In recent years, a wide variety of mobile computing devices has emerged, including portables, palmtops, and personal digit al assistants. Providing adequate network connectivity y for these devices will require a new generation of wireless LAN technology. In this paper we study media access protocols for a single channel wireless LAN being developed at Xerox Corporation’s Palo Alto Research Center. We start with the MACA media access protocol first proposed by Karn [9] and later refined by Biba [3] which uses an RTS-CTS-DATA packet exchange and binary exponential backoff. Using packet-level simulations, we examine various performance and design issues in such protocols, Our analysis leads to a new protocol, MACAW, which uses an RTS-CTS-DS-DATA-ACK message exchange and includes a significantly different backoff algorithm. 1

Mobile computing differs from desk-top computing because of the dynamic nature of system state: as users move, the sets of stationary and mobile objects they control and the types of information they wish to access change. Navigating a mobile environment can be aided by active maps that describe the location and characteristics of objects within some region as they change over time. We describe an active map service that keeps clients informed of changes in their environment. The primary issue driving our design is the question of scale: an active map service must be able to handle updates and queries over sufficiently large regions of space to satisfy clients' interests and must be able to handle peak loads that can occur when everyone in a region is moving around, for example, to attend a meeting. Our solution detects sets of clients that wish to receive the same active map information and then dynamically assigns multicast groups to them. To guarantee clients that they and the...

Computer applications traditionally expect a static execution environment. However, this precondition is generally not possible for mobile systems, where the world around an application is constantly changing. This thesis explores how to support and also exploit the dynamic configurations and social settings characteristic of mobile systems. More specifically, it advances the following goals: (1) enabling seamless interaction across devices; (2) creating physical spaces that are responsive to users; and (3) and building applications that are aware of the context of their use. Examples of these goals are: continuing in your office a program started at home; using a PDA to control someone else's windowing UI; automatically canceling phone forwarding upon return to your office; having an airport overheaddisplay highlight the flight information viewers are likely to be interested in; easily locating and using the nearest printer or fax machine; and automatically turning off a PDA's audible e-mail notification when in a meeting.

When a machine is connected to the Internet via a slow network, such as a 28.8 Kbps modem, the cumulative latency to communicate over the Internet to World Wide Web servers and then transfer documents over the slow network can be significant. We have built a system that optimistically transfers data that may be out of date, then sends either a subsequent confirmation that the data is current or a delta to change the older version to the current one. In addition, if both sides of the slow link already store the same older version, just the delta need be transferred to update it. Our mechanism is optimistic because it assumes that much of the time there will be sufficient idle time to transfer most or all of the older version before the newer version is available, and because it assumes that the changes between the two versions will be small relative to the actual document. Timings of retrievals of random URLs in the Internet support the former assumption, while experiments using a versi...

. This paper presents results from an eight-day network packet trace of MosquitoNet. MosquitoNet allows users of laptop computers to switch seamlessly between a metropolitan-area wireless network and a wired network (10 Mbit/s Ethernet) available in offices and on-campus residences. Results include the amount of user mobility between the wired and wireless networks, the amount of mobility within the wireless network, an examination of application end-to-end delays, and an examination of overall packet loss and reordering in the wireless network. We find that the average mobile host switches between the wired and wireless networks 14 times during the trace and moves within the wireless network five times. Round trip latencies in the wireless network are very high, with a minimum of 0.2 seconds. Even higher end-to-end delays, of up to hundreds of seconds, are due to packet loss and reordering. These delays cause users to change their usage patterns when connected to the wireless network....

Abstract: We propose a multiple-delivery transport service tailored for graphics and video transported over connections with wireless access. This service operates at the interface between the transport and application layers, balancing the subjective delay and image quality objectives of the application with the low reliability and limited bandwidth of the wireless link. While techniques like forward-error correction, interleaving and retransmission improve reliability over wireless links, they also increase latency substantially when bandwidth is limited. Certain forms of interactive multimedia datatypes can benefit from an initial delivery of a corrupt packet to lower the perceptual latency, as long as reliable delivery occurs eventually. Multiple delivery of successively refined versions of the received packet, terminating when a sufficiently reliable version arrives, exploits the redundancy inherently required to improve reliability without a traffic penalty. Modifications to acknowledgment-repeat-request (ARQ) methods to implement this transport service are proposed, which we term “leaky ARQ”. For the specific case of pixel-coded window-based text/graphics, we describe additional functions needed to more effectively support urgent delivery and asymptotic reliability. X server emulation suggests that users will accept a multi-second delay between a (possibly corrupt) packet and the ultimate reliablydelivered version. The relaxed delay for reliable delivery can be exploited for traffic capacity improvement using scheduling of retransmissions.

This paper describes the implementation and use of a platform designed to support collaborative multimedia applications in a mobile environment. The platform provides a programming interface compatible with emerging open systems standards and includes services for processing multimedia information. In addition, the platform provides feedback to applications and users on the state of their communications infrastructure - an important requirement in mobile environments. The services provided by the platform have been used to develop a collaborative multimedia application designed to support a specific class of mobile worker, i.e. field engineers. The design decisions taken in our implementation of both platform and application are evaluated, and our experiences are presented. Keywords: Mobile Computing, Open Distributed Processing, Co-operative Work, Adaptive Applications, Quality-of-Service. 1. Introduction This paper describes the results of on-going research at Lancaster aimed at p...

We propose a progressively reliable transport protocol for delivery of delay-sensitive multimedia over Internet connections with wireless access links. The protocol, termed "Leaky" ARQ, initially permits corrupt packets to be leaked to the receiving application and then uses retransmissions to progressively refine the quality of subsequent packet versions. A Web server would employ Leaky ARQ to quickly deliver a possibly corrupt first version of an image over a noisy bandlimited wireless link for immediate display by a Web browser. Later, Leaky ARQ's retransmissions would enable the browser to eventually display a cleaner image. Forwarding and displaying corrupt error-tolerant image data: (1) lowers the perceptual delay compared to fully reliable packet delivery, and (2) can be shown to produce images with lower distortion than aggressively compressed images when the delay budget only permits weak forward error correction. Leaky ARQ supports delaying of re-transmissions so that initial packet transmissions can be expedited, and cancelling of retransmissions associated with "out-of-date" data. Leaky ARQ can be parameterized to partially retransmit audio and video. We propose to implement Leaky ARQ by modifying Type-II Hybrid/"code combining" ARQ.

Recent growth in the number and quality of wireless network technologies has led to an increased interest in mobile computing. Furthermore, these technologies have now advanced sufficiently to allow "advanced applications" to be engineered. Applications such as these are characterised by complex patterns of distribution and interaction, support for collaboration and multimedia data, and are typically required to operate over heterogeneous networks and end-systems. Given these operating requirements, it is the author's contention that advanced applications must adapt their behaviour in response to changes in their environment in order to operate effectively. Such applications are termed adaptive applications. This thesis investigates the support required by advanced...

The availability of wireless network connections to laptop computers and PDA's has created interest in the issues surrounding mobile computing. However, enabling users to be genuinely mobile in their work requires more than a wireless connection. Distributed system services are needed to support the locating of people, equipment and software objects, and, especially for mobile multimedia applications, network transport protocols which can adapt to a wide range of networking conditions must be developed. This paper reviews some important mobility issues, looks at some of the systems requirements raised by user mobility, and describes some practical experiences with mobile applications at Olivetti Research Ltd. (ORL). Keywords Mobility, mobile multimedia, personalisation 1 INTRODUCTION User mobility is a more general concept than wireless terminal mobility or mobility enabled by portable computers. Instead of requiring the user to carry around a laptop or PDA it is in many instances mo...