Edward Chan

Cache invalidation scheme for mobile computing systems with real-time data

In this paper, we propose a cache invalidation scheme called Invalidation by Absolute Validity Interval (IAVI) for mobile computing systems. In IAVI, we define an absolute validity interval (AVI), for each data item based on its dynamic property such as the update interval. A mobile client can verify the validity of a cached item by comparing the last update time and its AVI. A cached item is invalidated if the current time is greater than the last update time plus its AVI. With this self-invalidation mechanism, the IAVI scheme uses the invalidation report to inform the mobile clients about changes in AVIs rather than the update event of the data items. As a result, the size of the invalidation report can be reduced significantly. Through extensive simulation experiments, we have found that the performance of the IVAI scheme is significantly better than other methods such as bit sequence and timestamp.

An efficient method for generating location updates for processing of location-dependent continuous queries

Recent advances in mobile computing and mobile communication technology have led to the emergence of many innovative mobile computing applications. Some of them require providing support to location-dependent continuous queries (LDCQs) on moving objects. The result of a location-dependent query depends on the current locations of the moving objects. When the query is specified as continuous, the requesting client can get continuously changing results. In order to provide correct and timely results to requesting clients, the locations of moving objects have to be closely monitored. In this paper, we propose an adaptive monitoring method (AMM) for managing the locations of moving objects to maintain the correctness of the results of query evaluation without significantly increasing the wireless bandwidth requirements. Extensive simulation experiments have been conducted to investigate the performance of the proposed method as compared to plain dead-reckoning (PDR).

Coding-Aware Multi-path Routing in Multi-Hop Wireless Networks

To overcome the inherent lossy property of wireless links and increase network throughput, many multi-path routing protocols have been proposed to improve the reliability and latency of packet delivery in wireless networks. Multi-path routing protocols, however, do not take advantage of existing coding opportunities to maximize network throughput. In this paper, we propose a novel coding-aware multi-path routing protocol (CAMP), which forwards packets over multiple paths dynamically based on path reliability and coding opportunity. CAMP employs a route discovery mechanism which returns to the source multiple paths along with ETX (Expected Transmission Count) of all links on each path. Using a novel forwarding mechanism, CAMP splits the traffic among multiple paths and actively creates instead of passively waiting for coding opportunity by switching its path to maximize the switching gain. Experimental results demonstrate that CAMP can achieve much higher throughput than comparable schemes for delivering packets in wireless networks.

Energy efficient multipath routing in large scale sensor networks with multiple sink nodes

Due to the battery resource constraint, it is a critical issue to save energy in wireless sensor networks, particularly in large sensor networks. One possible solution is to deploy multiple sink nodes simultaneously. In this paper, we propose a protocol called MRMS (Multipath Routing in large scale sensor networks with Multiple Sink nodes) which incorporates multiple sink nodes, a new path cost metric for improving path selection, dynamic cluster maintenance and path switching to improve energy efficiency. MRMS is shown to increase the lifetime of sensor nodes substantially compared to other algorithms based on a series of simulation experiments.

Energy-Efficient Cache Replacement Policies for Cooperative Caching in Mobile Ad Hoc Network

Data caching on mobile clients is widely seen as an effective solution to improve system performance. In particular, cooperative caching, based on the idea of sharing and coordination of cache data among multiple users, can be particularly effective for information access in mobile ad hoc networks where mobile clients moving frequently and network topology changing dynamically. Most existing cache strategies perform replacement independently, and they seldom consider coordinated replacement and energy saving issues in the context of a mobile ad hoc network. This paper analysed the impact of energy on designing a cache replacement policy and formulate the energy-efficient coordinated cache replacement problem (ECORP) as a 0-1 knapsack problem. A heuristic algorithm called ECORP-greedy and an optimal solution called ECORP-OPT are presented to solve the problem. Simulations show that the proposed policies can significantly reduce energy consumption and access latency when compared to other replacement policies.

Towards Location-aware Topology in both Unstructured and Structured P2P Systems

A self-organizing peer-to-peer system is built upon an application level overlay, whose topology is independent of underlying physical network. A well-routed message path in such systems may result in a long delay and excessive traffic due to the mismatch between logical and physical networks. In order to solve this problem, we present a family of Peer-exchange Routing Optimization Protocols (PROP) to reconstruct the overlay. It includes two policies: PROP- G for generic condition and PROP-0 for optimized one. Both theoretical analysis and simulation experiments show that these two protocols greatly reduce the average latency of the overlay and achieve a location-aware topology with low overhead. Their overall performance can be further improved if combined with other recent approaches. Specifically, PROP-G can be easily applied to both structured and unstructured systems without the loss of their primary characteristics, such as efficient routing and anonymity. PROP- O, on the other hand, is more efficient, especially in a heterogeneous environment where nodes have different processing capabilities.

Approaches for broadcasting temporal data in mobile computing systems

Abstract Rapid advances in mobile communication technology have spawned many new mobile applications. A key element in many of these systems is the need to distribute real-time information from a database server to mobile clients. While data broadcast has been shown to be an efficient data dissemination technique, many issues such as selection of broadcast data and caching strategies at mobile clients are still active research areas. In this paper, we consider an important characteristic of many mobile computing systems which has often been ignored in the design of broadcast algorithms: the fact that many data items are associated with temporal constraints on their validity. We introduce the notion of absolute validity interval (AVI) to capture the temporal constraints of the data items, formulate a temporal data model and examine both static and dynamic approaches to select data items based on their access frequencies as well as their AVI. The reason for considering the AVI of the data items in broadcast selection is to increase the client cache hit probability so that the access delay for a data item will be much reduced. Based on the results from extensive simulation experiments, it is concluded the AVI-based approaches, by improving cache hit probability, can significantly improve the mean response time and reduce the number of deadline missing requests.

Hypergraph-based data link layer scheduling for reliable packet delivery in wireless sensing and control networks with end-to-end delay constraints

Abstract Many mission-critical and safety-critical applications in networked wireless sensing and control systems have stringent reliability requirements and timing constraints on end-to-end ( E2E ) packet delivery. Late arrivals of packets could severely degrade overall system performance and cause serious problems in system operation. In this paper, we study the data link layer scheduling problem to maximize the reliability of E2E packet delivery in TDMA-based wireless sensing and control networks ( WSCNs ) subject to specified delay constraints. We propose to organize the physical network nodes into logical hypernodes and form a hypergraph for improved scheduling flexibility. Based on the hypergraph, we introduce two data link layer scheduling schemes to maximize the E2E reliability in packet delivery without violating any delay constraints of the packets. The first scheme, named dedicated scheduling , decides how many time slots (TSs) for each hypernode along the path to the destination should be allocated to transmit a packet, and the packets are only transmitted in their scheduled TSs. The second scheme, named shared scheduling , allows the packets to share their scheduled TSs, and thus further improve the E2E reliability. We apply these two schemes in both single-path routing and any-path routing in WSCNs. Asymptotic analysis of the proposed schemes is provided and extensive simulation experiments are performed to illustrate their effectiveness in improving the E2E reliability of packet delivery under different network settings.

Energy Efficient Residual Energy Monitoring in Wireless Sensor Networks

A crucial issue in the management of sensor networks is the continuous monitoring of residual energy level of the sensors in the network. With the large number of sensors in a typical network, the energy monitoring process can itself be very energy intensive. In this article, we propose a hierarchical approach to construct a continuous energy map of a sensor network. Our method consists of a topology discovery and clustering phase, followed by an aggregation phase when energy information collected is abstracted and merged into energy contours in which nodes with similar energy level are grouped into the same region. The topology of the monitoring tree is restructured periodically to distribute energy cost among all nodes fairly, which helps to reduce the impact of the monitoring scheme on the lifetime of the sensor network. Simulation results indicate that our method is able to generate accurate energy maps with much lower energy cost compared with traditional monitoring approaches.

Cache Invalidation Strategies for Mobile Ad Hoc Networks

Cache invalidation is an important issue in mobile caching and has received much attention by researchers. However most existing studies on cache invalidation strategies are limited to cellular wireless networks in which the mobile terminals can communicate with the base stations directly. In this paper, we address the seldom studied issue of cache invalidation in mobile ad hoc networks (MANETs), where the use of multi-hop communications poses serious challenges. We propose three cache invalidation strategies for MANETs. The three methods, namely pull on demand (POD), modified amnesic terminals (MAT) and pull-based amnesic terminals (PAT) are adapted from Barbara and Imielinskis classic cache invalidation schemes to handle the special requirements of MANETs. Both analytical results as well as simulation experiments are used to examine the performance of these three schemes, and it is shown that PAT provides the best overall performance.