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Abstracts of Selected Ph.D. Theses in the Area of Mobile Computing Awarded in 1998


Challenges to Reliable Data Transport over Heterogeneous Wireless Networks

Hari Balakrishnan

University of California at Berkeley, USA
August 1998

The full dissertation is available here
The Transmission Control Protocol (TCP) is the de facto standard for reliable data transmission in the Internet today. While TCP has been tuned to work well over traditional wired networks, its performance over wireless networks is much worse. This performance degradation results from several effects:

  • The preponderance of channel error-induced packet losses compared to congestion-induced losses.

  • Asymmetric effects and latency variation due to adverse interactions between media-access protocols and TCP.

  • Small TCP transmission windows due to low wireless channel bandwidths.

While TCP adapts well to network congestion, it does not adequately handle the vagaries of wireless media. In this thesis, we address these challenges in detail and design solutions to them. These solutions incorporate link-layer techniques as well as enhancements to TCP at the sender and receiver.

The adverse effects of wireless bit-errors on TCP performance arise primarily because TCP misinterprets wireless losses as being due to congestion. We present the design and implementation of a novel protocol, called the Berkeley Snoop Protocol, that exploits cross-layer protocol optimizations to improve performance. In addition, we design a mechanism called Explicit Loss Notification that can successfully distinguish between congestion and channel error-induced packet losses to substantially enhance end-to-end performance. These mechanisms have been demonstrated to provide performance improvements between 100% and 2000% across a range of bit-error rates in various cellular topologies in a Lucent WaveLAN-based wireless LAN, for data transfer to and from mobile hosts and when wireless transit links are present. Asymmetric networks pose a challenge to feedback-based protocols like TCP, because the characteristics of the reverse path used for acknowledgments (ACKs) have the potential to seriously affect forward performance. We classify asymmetry with respect to TCP performance into several categories, such as bandwidth asymmetry, latency asymmetry, loss-rate asymmetry, etc. We design several end-to-end and transport-aware link-layer solutions, such as ACK filtering, TCP sender adaptation and ACK reconstruction to obtain significantly better performance over such networks. These techniques lead to performance improvements for Internet access over packet radio networks such as Metricom's Ricochet network and for bandwidth-asymmetric networks such as Hybrid's wireless cable network.

Low channel bandwidths are common in many wireless networks as well as on many Internet paths today. This, and the short connections common in many Web transfers, lead to TCP windows being very small. This greatly affects its data-driven loss recovery mechanism and causes a large number of timeouts. In addition to implementing and evaluating the performance of TCP Selective Acknowledgments, we analyze data from packet-level traces of the Web server of the Atlanta Olympic Games to motivate enhancements to TCP's loss recovery mechanism. Our enhancements lead to an Enhanced Recovery scheme for TCP that greatly reduces the number of timeouts when windows are small.

In this research, we learn some general lessons about protocol design for heterogeneous networks. Specifically, we demonstrate the significant performance benefits of cross-layer protocol optimizations, of soft-state agents in the network infrastructure, and the advantages of data-driven loss recovery over timer-driven ones for better performance.

Today, wireless data networks are not widely deployed in the Internet despite the availability of a wide range of wireless technologies: one of the main reasons for this is their poor price-to-performance ratio for reliable data transfers. We believe (and hope!) that by significantly improving end-to-end TCP performance by up to an order of magnitude in some cases, our solutions will accelerate the widespread deployment and adoption of wireless data networks and make them an integral part of the Internet infrastructure of the future.


Information Dissemination Through Broadcast Delivery

Chi-Jiun Su

Electrical Engineering Department
Polytechnic University,Brooklyn, New York, USA
January 1998

Broadcast delivery is becoming a method of choice for the distribution of information to a large user population in many new applications. One of the major issues in broadcast delivery is scheduling of broadcast data such that the access delay of user's requests is minimized. The other complementary problem in push-based delivery is user's cache management in order to minimize the mismatch between the server's schedule and user's access pattern.

We formulate the problem of broadcast scheduling as a Markov Decision Process for both push-based and pull-based delivery and properties of the optimal scheduling policy are identified. We specify a class of near-optimal scheduling policies that incorporates some of the characteristics of the optimal policy and they turn out to yield good performance for both push- and pull-based delivery. The results are readily extended for a system with multiple broadcast channels and for information items of unequal lengths. We demonstrate by a numerical study that as the request generation rate increases, the achievable performance of the pull- and push-based systems becomes almost identical, and prove it for the case with equal access probabilities.

For push/pull hybrid delivery, we provide server's database partitioning, bandwidth allocation and scheduling algorithms and extensive numerical experiments are done to investigate their performance. The results are also extended for information items of unequal length.

We identify the optimal user's cache management strategy for push-based delivery, that minimize the mean response time of user's requests. Computational issues are also discussed and cache update policies with limited look-ahead are given as implementable approximation to the optimal policy. We present some interesting special cases for which limited look-ahead policies are optimal. It is also shown that the same formulation can be used when the objective is to minimize the number of deadline misses.

Finally, we consider the problem of joint broadcast scheduling and cache management for push-based delivery and propose an approach which yields up to 40% performance improvement over traditional non-joint schemes when prefetching is done at user's side. A two-level scheduling algorithm is given which remedies some pathological cases and performs slightly better than one-level schemes.


A Proxy Based Filtering Mechanism For the Mobile Environment

Bruce Zenel

Columbia University, NY, USA
February 1998

Host mobility complicates the standard networking model in unexpected ways. It increases network heterogeneity, causing difficulty within applications that expect a high or constant amount of network bandwidth. Mobility increases cost when a user switches from a relatively inexpensive high bandwidth network to a low bandwidth pay-by-call or pay-by-byte network. Mobility may also reduce security when a user moves from a "safe" network onto a foreign one.

This situation can be improved by introducing a proxy between the mobile client and its peer. The purpose of this intermediary is to process data moving between the client and peer. The type of processing performed depends on the desired result: the proxy can hide network heterogeneity via data filtering, reduce cost by using compression, and increase security through the use of secure protocols between the client and proxy. This dissertation presents a general purpose proxy system and how it has been applied to the mobile environment. The overall architecture is described, examples of its use given, and a study on its feasibility and performance is presented.


On the Broadcast and Clustering Problems in Peer-to-Peer Networks

Stefano Basagni

Università di Milano, Italy
May 1998

The full dissertation is available here
In this thesis, we study two problems in the field of wireless mobile computing, namely, the broadcast and the clustering problems for peer-to-peer networks, i.e., wireless networks in which all nodes can move (these networks are also often termed ad hoc networks, or multi-hop networks).

The first problem we are interested in is about algorithms for implementing broadcast primitives in peer-to-peer networks. Informally, broadcast is the task initiated by a single node, called the source, whose goal is to send a single message m to all the nodes in the network. This problem has been widely investigated, and several algorithms have been proposed for peer-to-peer networks. Most of them use probabilistic techniques to deliver m to all the nodes (randomized algorithms), or assume that the topology of the network is known to each node (centralized algorithms). Both these assumptions give efficient broadcast protocols, but lead to algorithms that are not suitable when time-dependent applications are run over the network, or when the topology of the network is uncertain, as in a mobile environment. Thus, we have investigated solutions to the broadcast problem that overcome these limitations. The protocols we present here have the following characteristics. They are deterministic, so that an a priori known bound on the maximum delay of message transmission can be easily determined. They are delivery-guaranteed, in that the correct delivery of a message is always guaranteed within a bounded amount of time, and they are completely distributed, i.e., we assume that the nodes in the network do not have an a priori knowledge of the entire network topology and not even of their neighbors. The bounds induced by the algorithms are shown to be optimal for networks with certain topologies, and they partially bridge the gap between a previously known logarithmic lower bound for broadcast protocols with the stated required properties and a trivial linear upper bound.

The other problem that we have investigated is the clustering problem for peer-to-peer networks. Clustering a network means to partition its nodes in clusters so that certain desirable properties are guaranteed. Examples of issues addresses by clustering include the need to control the spatial reuse of the channel (e.g., in terms of frequency and/or time), and the need to reduce routing and control information overhead. Recent research in the field shows that an effective solution to this problem can be obtained through the organization of nodes in clusters. In this dissertation we characterize the clustering problem as a maximum weight independent set (MWIS) problem. Once assigned a weight to each node of the network, we look for clusterheads so that the set of clusterheads is an independent set with the maximum weight. The weights are introduced to reflect how a node is to be preferred for the role of clusterhead. We show that a simple variation of the greedy algorithm for the MWIS problem leads to a clustering algorithm with provable properties on the performance and to some characteristics of optimality. We finally give a completely distributed implementation of the centralized clustering algorithm and, by using the techniques developed for the broadcast problem, we obtain efficient (deterministic) bounds on the distributed clustering problem for peer-to-peer networks.


Mobile Data Access

Brian Noble

Carnegie Mellon University, Pittsburgh, PA
May 1998

The full dissertation is available here
Mobile devices and wireless networks are becoming more powerful and affordable, leading to the growing importance of mobile data access. Unfortunately, mobile environments are inherently turbulent; the resources available to mobile clients change dramatically and unpredictably over time.

This dissertation puts forth the claim that clients must adapt their behavior to such turbulence by trading quality of fetched data for performance in fetching it. Such adaptation is best provided by application-aware adaptation --- a collaboration between the operating system and its applications. In this collaboration, the system is responsible for providing the mechanisms for adaptation, while applications are free to set adaptive policies.

The dissertation next describes the design and implementation of Odyssey, a platform for mobile data access. This discussion focuses on the mechanisms provided by the system, the architecture comprising those mechanisms, and the application programming interface from which applications construct adaptive policies. The dissertation then presents three applications that make use of these mechanisms: a video player, a web browser, and a speech recognition system. These applications adapt their behavior to changes in available network bandwidth.

There are three questions to ask of this prototype and its applications. First, how agile can the prototype be in the face of changing network bandwidth? Second, does adaptation to substantial changes in bandwidth provide benefit to individual applications? Third, is the collaboration between the system and applications necessary when several applications are run concurrently?

These questions cannot be answered simply by subjecting the prototype to a real wireless network. Such networks provide neither stable nor repeatable performance, and hence are not suitable for comparative evaluations. Instead, the prototype is evaluated using trace modulation. This technique allows one to capture the performance characteristics of a wireless network over a short period of time, and reliably recreate that performance in an otherwise live system. Evaluating the system under modulation shows that Odyssey has good agility with respect to changes in network bandwidth, that individual applications can benefit from adaptive strategies, and that the system's involvement in adaptation is crucial for concurrent applications.


Energy-conserving Protocols for Wireless Data Networks

Jason Keith Redi

Boston University, MA, USA
March 1998

One of the most important factors which will influence the future success of ubiquitous mobile computing is the convenience and usefulness of the mobile devices. Major defining factors of the utility of these nodes will be the lifetime, size and weight of the power source. As active battery research does not predict a substantial increase in the energy capacity of today's batteries, it is important to minimize the power required by the mobile device. Among existing protocols for wireless packet reception, classical access and automatic repeat request (ARQ) protocols are either not energy conserving, or lead to unacceptable delays. In this research, we consider methods to substantially reduce the amount of power necessary for reception of packets at the mobile devices.

We first assume an error-free channel between the base station and the mobile node. We introduce three protocols which conserve energy by reducing the amount of time that the mobile receiver is in the ``ON'' state, while meeting the application delay constraints. We analyze the protocols and consider their performance under a variety of traffic loads. We next address a few of their individual drawbacks through enhancements such as dynamic adaptation of the node wakeup schedules. Since one of the defining features of the wireless environment is the error-prone channel, we follow with protocols designed to conserve power in the multi-path fading environment. As uplink transmissions can typically require twice as much energy as reception, it is particularly important to reduce the number of acknowledgments sent by the mobile node's automatic repeat request (ARQ) protocol. We propose four energy-conserving alternatives to the classical protocols and show through analysis and simulation that the proposed protocols allow a substantial reduction in the energy used for acknowledgments in the uplink, without unreasonably increasing the average packet delay, or substantially reducing the throughput capacity. Finally, we adapt these protocols into a dynamic, asymmetric version where the complexity of the protocol is off-loaded to the base station in order to more easily support multiple traffic types, reduce the requirements of the mobile node, and more efficiently handle changing channel conditions in the presence of multipath fading.


On Optimizing Performance in Mobile Packet Radio Networks

Madhavi W. Subbarao

Department of Electrical and Computer Engineering
The Johns Hopkins University, Baltimore, MD, USA
March 1998

The full dissertation is available here
Mobile packet radio networks are a class of mobile wireless systems. They consist of a collection of movable terminals distributed over a wide geographical area and the radio links which connect them. In the design of such a network, there are many physical layer, data link layer, and network layer components that must be determined, such as the signaling scheme, error-correction coding, transmission range, routing strategy, and nodal transmission probability. The best choice for these different design parameters is not straightforward since their effects on network performance are interdependent. Two important concerns for efficient network performance are to find the optimum transmission range and optimum code rate to be used by each node. The optimum combination is a tradeoff between the distance covered by a transmission and the amount of data conveyed by the transmission. To capture this tradeoff, we introduce a new performance measure, information efficiency, which can be viewed as a measure of spectral efficiency for wireless networks. We analyze two types of spread-spectrum networks using the slotted ALOHA channel access protocol, a frequency-hopping network and a direct-sequence network. By maximizing the information efficiency and local throughput, we find the jointly optimum code rate, transmission range, and slotted ALOHA transmission probability for each node in the network. We show that using trellis-coded modulation as the coding scheme leads to the highest information efficiency in a direct-sequence packet radio network. Finally, we focus on the benefits of conserving power in these systems and develop a power efficient routing algorithm for packet radio networks.


Translucent Cache Management for Mobile Computing

Maria R. Ebling

Carnegie Mellon University, USA
March 1998

The full dissertation is available here
Recently, distributed files systems have aggressively exploited caching to provide high availability to their clients. Such systems allow users to access and modify cached data even when clients become disconnected (e.g. a laptop removed from its docking station) or weakly connected (e.g. a laptop connected via a modem) to servers. In doing so, these file systems violate one of the key properties of caching: transparency.

When disconnected or weakly connected, these systems cannot always maintain the illusion of connectivity. For instance, the system may not be able to service a cache miss or may only be able to do so with a lengthy delay. When the illusion of connectivity breaks down, the usability of the system is undermined. Early designers of these systems either did not recognize or failed to appreciate the extent to which users would be affected. After extensive experience with the Coda file system, however, we have witnessed the arduous learning curve experienced by new Coda users. We have also observed occasional, but serious, confusion caused by modal behaviors introduced to cope with the range of network connectivity encountered by mobile clients.

This dissertation describes the design, implementation, and evaluation of the CodaConsole, a graphical interface that makes caching translucent to users of such systems. To make caching translucent, the interface exposes critical aspects of caching to support availability, while hiding noncritical details to preserve usability. For example, the system informs users about the availability of their tasks, but does not bother them with minutia of cache consistency. The interface also offers opportunities for users to control where network resources are spent.

To evaluate the CodaConsole, I performed a usability test. This study showed that users were able to learn how to use the interface in about one hour. They were able to operate the interface with little difficulty. They were also able to understand problems indicated by the interface and then determine what actions might resolve those problems. Perhaps the most surprising result, however, was the fact that novice Coda users performed almost as well as expert Coda users. This interface successfully makes caching, in the presence of disconnected or weakly connected operation, translucent.


Context-Aware and Location Systems

Giles John Nelson

University of Cambridge, United Kingdom
January 1998

The full dissertation is available here
Computing systems are becoming increasingly mobile, with users interacting with many networked devices, both fixed and portable, over the course of a day. This has lead to the emergence of a new paradigm of computing, the context-aware system. A user's context can be described as their relationship to computing devices, colleagues and the surrounding physical environment. Applications using such a system are made aware of changes in the physical and computing environment, and adjust themselves accordingly. For example, with knowledge of the locations of people and equipment, a suitable nearby display can be determined to deliver a message to a mobile user. Context-aware systems are currently ad-hoc in nature, typically developed around a particular technology and oriented towards supporting a particular type of application. Current location systems are similarly ad-hoc with no notion of abstraction above a particular locating technology. Furthermore, little analysis has been conducted into the application requirements of a location system. This general lack of systems design makes current approaches difficult to extend and generalise.

This thesis proposes a systems architecture for the support of context-aware applications. It is asserted that such an architecture requires three main areas of capability. Firstly, a context-aware system needs to monitor the physical environment and make this information available in a uniform way. Secondly, location information requires higher-level management together with a representation of static parts of the physical domain. Further, support is required to enable complex, asynchronous monitoring of spatial areas. Thirdly, application support is required to access distributed sensor and location services, so facilitating the simple specification of context-aware scenarios.


Roam: A Scalable Replication System for Mobile and Distributed Computing

David Howard Ratner

UCLA, Los Angeles, CA, USA
January 1998

The full dissertation is available here
Mobile computing is rapidly becoming a way of life. Users carry their lap- tops, PDAs, and other portable devices with them almost constantly, whether their mobility takes them across town or across the world. Recent hardware innovations and improvements in chip technology have made mobile computing truly feasible.

Unfortunately, unlike the hardware industry, much of today's system software is not ``mobile-ready.'' Such is the case with the replication service. Nomadic users require replication to store copies of critical data on their mobile machines, since disconnected or poorly connected machines must rely primarily on local resources. However, the existing replication services are designed for stationary environments, and do not provide mobile users with the capabilities they require. Replication in mobile environments requires fundamentally different solutions than those previously proposed, because nomadicity presents a fundamentally new and different computing paradigm. Mobile users require several key features from the replication system: the ability for direct synchronization between any two replicas, the capability for widespread scaling and large numbers of replicas, and detailed control over what files reside on their local (mobile) replica.

Today's replication systems do not and cannot provide these features. Therefore, mobile users must adapt their behavior to match the service provided to them, complicating their ability to work while mobile and effectively hindering mobility by costing the user additional time, money, and system-management effort simply to operate in a nomadic mode.

Here we present Roam, a replication system designed to satisfy the requirements of the mobile user. Roam is based on the Ward Model, a replication architecture specifically aimed at mobile environments. Together with the Ward Model and new, distributed algorithms such as improved garbage collection techniques and version vector management, Roam provides a scalable replication solution for the mobile user. We describe not only the design and motivation of such a system, but its implementation and performance as well.

The ACM Special Interest Group on Mobility of Systems, Users, Data and Computing