Rose P. Tsang

Multipath Routing in Mobile Ad Hoc Networks: Issues and Challenges

Mobile ad hoc networks (MANETs) consist of a collection of wireless mobile nodes which dynamically exchange data among themselves without the reliance on a fixed base station or a wired backbone network. MANET nodes are typically distinguished by their limited power, processing, and memory resources as well as high degree of mobility. In such networks, the wireless mobile nodes may dynamically enter the network as well as leave the network. Due to the limited transmission range of wireless network nodes, multiple hops are usually needed for a node to exchange information with any other node in the network. Thus routing is a crucial issue to the design of a MANET. In this paper, we specifically examine the issues of multipath routing in MANETs. Multipath routing allows the establishment of multiple paths between a single source and single destination node. It is typically proposed in order to increase the reliability of data transmission (i.e., fault tolerance) or to provide load balancing. Load balancing is of especial importance in MANETs because of the limited bandwidth between the nodes. We also discuss the application of multipath routing to support application constraints such as reliability, load-balancing, energy-conservation, and Quality-of-Service (QoS).

Efficient selective frame discard algorithms for stored video delivery across resource constrained networks

Video delivery from a server to a client across a network is an important component of many multimedia applications. While delivering a video stream across a resource constrained network, loss of frames may be unavoidable. Under such circumstances, it is desirable to find a server transmission schedule that can efficiently utilize the network resources while maximizing the perceived quality-of-service (QoS) at the client. To address this issue, we introduce the notion of selective frame discard at the server and formulate the optimal selective frame discard problem using a QoS based cost function. Given network bandwidth and client buffer constraints, we develop an O(N log N) algorithm to find the minimum number of frames that must be discarded in order to meet these constraints. The correctness of the algorithm is also formally established. Since the computational complexity of the optimal algorithm for solving the optimal selective frame discard problem is prohibitively high in general, we also develop several efficient heuristic algorithms for selective frame discard. These algorithms are evaluated using JPEG video traces.

Virtual path layout design on ATM networks

The paper examines the efficient layout of virtual paths (VPs) in an ATM network. The ATM network consists of ATM switches and their attached network end users, which may be gateways, routers, and hosts. The physical topology, the offered traffic, and call setup matrices of the network end users are assumed to be given. The problem is formulated as a flow-based optimization problem. A heuristic approach is presented which (i) establishes VPs according to physical network-specific and application-specific constraints and a cost function, (ii) provides multipaths between each source destination user pair to minimize the cell blocking probability and to increase network resilience, and (iii) uses a novel VP combining process which is guaranteed to always satisfy the switching constraints. Simulation results are presented for the proposed VP planning policy. Guidelines for the design of robust VP layouts and the efficient establishment of VCs are also presented.<<ETX>>

Quality of service routing without global information exchange

We propose a novel localized QoS routing approach, under which no global QoS state information exchange among network nodes is needed. This approach has many advantages over the conventional global QoS routing approach based on information exchange among routers. We develop two localized QoS routing schemes and demonstrate through simulations that the proposed localized QoS routing approach is indeed a viable alternative to the global QoS routing approach.

Universal Service in a Ubiquitous Digital Network

Before there was the digital divide there was the analog divide– and universal service was the attempt to close that analogdivide. Universal service is becoming ever more complex in terms ofregulatory design as it becomes the digital divide. In order to evaluatethe promise of the next generation Internet with respect to the digitaldivide this work looks backwards as well as forwards in time. Byevaluating why previous universal service mechanisms failed andsucceeded this work identifies specific characteristics ofcommunications systems – in particular in billing and managinguncertainty – and argues that these characteristics underliesuccess or failure in terms of technological ubiquity. Developing a setof characteristics of services rather than a set of services is afundamental break with the tradition of universal service. In fact, theimplications of our proposal is that basic characteristics in theoffering of the service rather than the absolute price are critical toclose the digital divide: certainty of total charge, ability to avoiddeposits or disconnection via best effort service, and payer-basedcontrol of all charges. While all of these principles sound obvious infact none of these hold in the telephony network. Universal service hasevolved from common carriage (serve all with no discrimination) to aright to basic services (100% penetration). Universal service isnow discussed as the digital divide, as the access to information asopposed to services becomes increasingly critical. However, we arediscussing in this paper access to the bits and the network rather thanaccess to the information (or intellectual property) once connected. Theprovision of universal service is seen as a technical problem only in thatthe technology costs money – universal service debates have longbeen the domain of economists. Yet the design of protocols has been thedomain of engineers, the building of systems the corporate domain, andthe discussion of equity the interest of ethicists. The design ofprotocols can define the parameters of the corporate decision-makers,the variables of the economist, and the questions for the ethicist. Thedesign decisions made at the fundamental levels can make communicationsequity more or less likely. In this work I focus on the design ofprotocols for the next generation Internet, protocols which willfundamentally change the best-effort nature of Internet services.Building on the economic and ethnographic work of others I argue thatthe effects of protocols adoption on universal service can be predictedto some degree. By examination of past and current technologies Iexamine a set of technical mechanisms to determine how such mechanismsmight harm or enhance universal service. I define each mechanism (e.g.denial of entry) and offer observations about each particularmechanisms implicit pricing assumptions. I close with a discussion ofinterest to ethicists and regulators on evaluating communicationsprotocols with respect to universal access. Protocols for developingmultiple qualities of service for packet-switched networks have focusedon economic efficiency (e.g. Mackie-Mason, 1995; Choi, Stahl &Winston, 1997; Shapiro & Varian, 1998), billing to encouragewidespread adoption of network innovations (e.g. Xie & Sirbu, 1985)and billing in a manner consistent with the underlying network (e.g.Clark, 1996). Here we examine a set of protocols which include varyingquality of service mechanisms with respect to the compatibility of theprotocols with universal access.

Robust message routing for mobile (wireless) ad hoc networks.

This report describes the results of research targeting improvements in the robustness of message transport in wireless ad hoc networks. The first section of the report provides an analysis of throughput and latency in the wireless medium access control (MAC) layer and relates the analysis to the commonly used 802.11 protocol. The second section describes enhancements made to several existing models of wireless MAC and ad hoc routing protocols; the models were used in support of the work described in the following section. The third section of the report presents a lightweight transport layer protocol that is superior to TCP for use in wireless networks. In addition, it introduces techniques that improve the performance of any ad hoc source routing protocol. The fourth section presents a novel, highly scalable ad hoc routing protocol that is based on geographic principles but requires no localization hardware.

Detection of nuclear material at border crossings using motion correlation

This paper considers the problem that arises when a vehicle carrying nuclear material is detected approaching a border crossing. As quickly as possible, and with automation we wish to identify which vehicle among all those in the area is likely to be carrying the source. We show that if the border crossing area has technology for tracking the position of vehicles, we can correlate observed movements with observed changes in levels of detected radiation - for as the vehicle carrying the material gets closer to the detector, the stronger will be the detected radiation. We use a simulation model that captures the stop-and-go dynamics of a border crossing area to evaluate our ideas, and find a highly successful technique that tracks which vehicles move just when detected radiation changes, coupled with fitting radiation intensity/distance observations to an inverse-square law. This method almost always isolates the sought vehicle just as soon as the minimum number of data observations is obtained

Scheduling algorithms for a high-speed switch supporting real-time periodic traffic sources

The successful operation of mission critical systems requires a sophisticated control network which provides for the real-time delivery of data from a very large number of diverse sources such as sensors, audio/video surveillance, computational sources, etc., as well as remote monitoring and diagnosis sources. It is expected that many of the traffic sources will be in the form of periodic sources with real-time requirements. In this study, we propose fast scheduling algorithms for multiplexing periodic source flows with real-time delivery requirements. We assume a single switch environment with periodic source flows being supported over a constant bit rate (CBR) type circuit. We undertake a systematic study beginning from the simplest scenario where all traffic flows consist of data frames of the same size and with the same real-time delay requirement. In this case, we prove that the FCFS algorithm is optimal. We conclude with the most difficult case of examining CBR flows of variable bandwidth requirements and variable delay requirements. Algorithms and analysis are presented for all the cases. The simulation results show the substantial performance gains provided by the proposed algorithms.

The effect of various ATM switch architectures on VBR video performance

One of the most important components of an Asynchronous Transfer Mode (ATM) net- work is the switch. Switch design is not a part of the ATM standards so vendors use a wide variety of techniques to build their switches. In this paper, we present experimental results of switching and multiplexing real-time Variable Bit Rate (VBR) digital video traffic (JPEG, MPEG-1, and MPEG-2) through two different ATM switch architectures. Real-time VBR traffic, such as digital video, is particularly interesting due to its high demands in terms of bandwidth, real-time delivery and processing requirements. Our ex- periments show that the ‘fastest’ switches, i.e., lowest latencies, do not necessarily perform better when transmitting VBR video. The impact of the high speed network components’ characteristics, such as switch fabric architecture, buffering strategies, and higher layer transport protocols (i.e., UDP, TCP/IP), are illustrated through the experimental results.