Adaptive range control using directional antennas in mobile ad hoc networks
This paper presents ARC (Adaptive Range Control), a communication range control mechanism using directional antennas to be implemented across multiple layers. ARC uses directional reception for range control rather than directional transmission such that extended communication links do not increase interference to other ongoing communications. It adaptively controls the communication range by estimating dynamically changing local network density based on the transmission activities around each network node. The experimental results using simulation with detailed physical layer, IEEE 802.11 DCF MAC, and AODV protocol models have shown the successful adaptation of communication range with ARC for varied network densities and traffic loads. ARC improves the packet delivery ratio by a factor of 9 at the maximum for sparse networks while it maintains the increased network capacity for dense networks.

title: Exploiting medium access diversity in rate adaptive wireless LANs
abstract: Recent years have seen the growing popularity of multi-rate wireless network devices (e.g., 802.11a cards) that can exploit variations in channel conditions and improve overall network throughput. Concurrently, rate adaptation schemes have been developed that selectively increase data transmissions on a link when it offers good channel quality. In this paper, we propose a Medium Access Diversity (MAD) scheme that leverages the benefits of rate adaptation schemes by aggressively exploiting multiuser diversity. The basic mechanism of MAD is to obtain instantaneous channel condition information from multiple receivers and selectively transmit data to a receiver that improves the overall throughput of the network, while maintaining temporal fairness among multiple data flows. We identify and address the challenges in the design and implementation of MAD's three phases: channel probing, data transmission, and receiver scheduling. We also use analytical models to examine the tradeoff between network performance improvement and overhead of channel probing, and derive an asymptotic performance bound for the receiver scheduling algorithms used by MAD. Results from the analysis and the extensive simulations demonstrate that, on average, MAD can improve the overall throughput of IEEE 802.11 wireless LANs by 50% as compared with the best existing rate adaptation scheme.

title: An alternative time management mechanism for distributed simulations
abstract: Over the past few years, there has been a keen interest in the management of time in distributed simulation environments. Previous emphasis in time management (TM) services has been based on time stamp ordering, which is both computation and bandwidth intensive. This article discusses an alternative approach to time management based on causal ordering. Traditional causal ordering protocols incur a large amount of communication overhead, which is generally of the order of N2 for a distributed system of N processes. A new causal ordering protocol proposed by the authors, the Modified Schiper-Eggli-Sandoz (MSES) protocol, is presented in this article. This new protocol minimizes the control information overhead of causal ordering by using the direct dependency tracking technique. The MSES protocol works well in both unicast and multicast environments, without relying on information about the underlying network topology and communication pattern among the processes of the distributed system. The MSES protocol has been successfully implemented as a middleware on top of DMSO RTI. Experiments have been conducted to benchmark the performance of the new time management mechanism with respect to the existing TM mechanisms available in DMSO RTI. The simulation scenarios of the experiments vary with different degrees of inter-federate dependency and federate event granularities. The ordering limitations of the causality based TM mechanism are addressed in this article and the trade-off of the degree of event ordering and execution speed of simulations is discussed.

title: Improving scalability of wireless network simulation with bounded inaccuracies
abstract: Discrete event network simulators have emerged as popular tools for verification and performance evaluation of wireless networks. Nevertheless, the desire to model such networks at high fidelity implies high computational costs, limiting most researchers the ability to simulate networks with thousands of nodes. Previous attempts to optimize simulation of large-scale wireless networks have not appropriately modeled accumulation of weak interference, thereby suffering inaccuracies that may be further magnified in the evaluation of upper-layer protocols. This article presents a comprehensive analysis on the effects of common optimization techniques for large-scale wireless network simulation on the overall network performance. Based on the analysis, it formulates distance limit derivation and mobility update reduction that introduce bounded inaccuracy to the radio propagation simulation. It further proposes a novel technique, Lazy Event Scheduling with Corrective Retrospection, that reduces simulation events twenty-five fold without introducing any inaccuracy at all. The experimental results show that these optimizations can substantially improve the runtime performance of an already efficient wireless network simulator, by a factor of up to 55 for wireless networks with 3200 nodes without compromising the simulation's accuracy.

title: Optimizing parallel execution of detailed wireless network simulation
abstract: With parallel and discrete event simulation (PDES) techniques, the runtime performance of detailed wireless network simulation can be improved significantly without compromising fidelity of the simulation results. However, modelling characteristics of wireless communications such as signal propagation and interference may severely hinder the potential speedup yielded by PDES. This paper proposes various optimization techniques to address three major concerns in achieving efficient parallel execution of wireless network simulation: i.e., (1) reducing communication and computation overhead of simulating signal propagation across multiple logical processes; (2) reducing synchronization overhead among logical processes; (3) minimizing event scheduling overhead within individual logical processes. These techniques have been implemented in a parallel version of GloMoSim and QualNet. The experimental results with mobile ad hoc networking scenarios demonstrate that the proposed optimization techniques can improve the performance of parallel wireless network simulation by up to an order of magnitude.

title: Performance Evaluation of Directional Adaptive Range Control in Mobile Ad Hoc Networks
abstract: This paper presents DARC (Directional Adaptive Range Control), a range control mechanism using directional antennas to be implemented across multiple layers. DARC uses directional reception for range control rather than directional transmission in order to achieve both range extension and high spatial reuse. It adaptively controls the communication range by estimating dynamically changing local network density based on the transmission activities around each network node. The experimental results using simulation with detailed physical layer, IEEE 802.11 DCF MAC, and AODV protocol models have shown the successful adaptation of communication range with DARC for varied network densities and traffic loads. DARC improves the packet delivery ratio by a factor of 9 at the maximum for sparse networks while it maintains the increased network capacity for dense networks. Further, as each node adaptively changes the communication range, the network delivers up to 20% more packets with DARC compared to any fixed range configurations.

title: Further analysis of the Internet key exchange protocol
abstract: Secure communication over the Internet becomes an essential requirement for any value-added Internet application. The use of cryptography for secure communication brings out the requirement of cryptographic key management. This paper examines some security issues on the Internet Key Exchange (IKE) protocol specified in RFC 2409. The investigation is intended for clarifying some specification ambiguities in RFC 2409 and facilitating a correct implementation of the IKE protocol.

title: Undeniable billing in mobile communication
abstract: In the current mobile communication networks, users have to trtistnetwork operators to make correct. charges over the cds they made. There islack of evidence to resolve possible disputti+ over thenurnberofcdls and the duration of each cd. Such a concern may grow when users roam among different network operators. This paper proposes an efficient solution to undeniable b]~ng when a mobile user roams into foreign networks. This is achieved by the combination of digitd signature and hash-chaining techniques. Mobile users need to submit a digitd signature when requesting a cd and release chained h~<h vahtes during the session so that the cd and its duration are undeniable. Our mechanism is tight-weighted regarding the storage requirement and comput ation overhead~ on a mobile user, and is apphcable for mobile phone users equipped with a smart card. Ke~ords: undeniable bi~ng, mobile communication se curity, non-repudiation, cryptographic protocol

title: Analysis and Improvement of Micali's Fair Contract Signing Protocol
abstract: In PODC 2003, Micali presented a fair electronic exchange protocol for contract signing with an invisible trusted party [17]. The protocol was filed as a US patent No 5666420 in 1997 . In the protocol, two mutually distrusted parties exchange their commitments to a contract in a fair way such that either each of them can obtain the other’s commitment, or neither of them does. The protocol is optimistic in the sense that the trusted party need not be involved in the protocol unless a dispute occurs. In this paper, we show that Micali’s protocol cannot achieve the claimed fairness. In resolving a dispute, the trusted party may face a dilemma situation that no matter what it does, one of the exchanging parties can succeed in cheating. In other words, there is always a party who can get the other’s commitment without the other party obtaining his. We further propose a revised version of contract signing protocol that preserves fairness while remaining optimistic.

title: An improved ellipse propagation model for location estimation in facilitating ubiquitous computing
abstract: Positioning is a crucial technology for ubiquitous computing. A directional propagation model - the ellipse propagation model (EPM) is proposed by our research group for locating a mobile station (MS) within a radio cellular network with an accuracy that can enable a number of location based services to realize ubiquitous computing. By using a geometric algorithm, the location of the mobile station can be estimated. However, since one parameter in our geometric algorithm is fixed, errors may be induced as the surrounding environment changes. In view of this, we would like to propose a new algorithm - the iterative algorithm to provide the positioning based on EPM. With the technical support of two local mobile phone operators, we have conducted a series of experiments using real data and experiment results showed that the proposed iterative algorithm outperforms the geometric algorithm by a good margin of 18% in terms of average error.

title: Intermediary non-repudiation protocols
abstract: In commercial transactions, an intermediary might be involved to help transacting parties to conduct their business. Nevertheless, the intermediary may not be fully trusted. We introduce the concept of intermediary (or agent) in a non-repudiation protocol, define the aims of intermediary non-repudiation protocols, and analyze their security requirements. We present a simple scenario with only one recipient, followed by a more complicated framework where multiple recipients are involved and collusion between them is possible.