Xiaoxin Wu

Defending against wormhole attacks in mobile ad hoc networks

In ad hoc networks, malicious nodes can deploy wormhole attacks to fabricate a false scenario on the proximity relationship among mobile nodes. A classification of the attacks according to the format of the wormholes is proposed. This forms a basis to identify the detection capability of various approaches. An analysis shows that earlier approaches focus on the prevention of wormholes among neighbors that trust each other. As a more generic approach, we present an end-to-end scheme that can detect wormholes on a multi-hop route. Only the trust between the source and the destination is assumed. The mechanism uses geographic information to detect anomalies in neighbor relations and node movements. To reduce the computation and storage overhead, we present a scheme called cell-based open tunnel avoidance (COTA) to manage the information. COTA requires a constant space for every node on the path and the computation overhead increases linearly to the number of detection packets. We prove that the savings do not deteriorate the detection capability. Various schemes to control communication overhead are studied. The simulation and experiments on real devices show that the proposed mechanism can be combined with existent routing protocols to defend against wormhole attacks. Copyright

AO2P: ad hoc on-demand position-based private routing protocol

Privacy is needed in ad hoc networks. An ad hoc on-demand position-based private routing algorithm, called AO2P, is proposed for communication anonymity. Only the position of the destination is exposed in the network for route discovery. To discover routes with the limited routing information, a receiver contention scheme is designed for determining the next hop. Pseudo identifiers are used for data packet delivery after a route is established. Real identities (IDs) for the source nodes, the destination nodes, and the forwarding nodes in the end-to-end connections are kept private. Anonymity for a destination relies on the difficulty of matching a geographic position to a real node ID. This can be enforced by the use of secure position service systems. Node mobility enhances destination anonymity by making the match of a node ID with a position momentary. To further improve destination privacy, R-AO2P is proposed. In this protocol, the position of a reference point, instead of the position of the destination, is used for route discovery. Analytical models are developed for evaluating the delay in route discovery and the probability of route discovery failure. A simulator based on ns-2 is developed for evaluating network throughput. Analysis and simulation results show that, while AO2P preserves communication privacy in ad hoc networks, its routing performance is comparable with other position-based routing algorithms.

Integrating heterogeneous wireless technologies: a cellular aided mobile Ad Hoc network (CAMA)

A mobile ad hoc network is a collection of wireless terminals that can be deployed rapidly. Its deficiencies include limited wireless bandwidth efficiency, low throughput, large delays, and weak security. Integrating it with a well-established cellular network can improve communication and security in ad hoc networks, as well as enrich the cellular services. This research proposes a cellular-aided mobile ad hoc network (CAMA) architecture, in which a CAMA agent in the cellular network manages the control information, while the data is delivered through the mobile terminals (MTs). The routing and security information is exchanged between MTs and the agent through cellular radio channels. A position-based routing protocol, the multi-selection greedy positioning routing (MSGPR) protocol, is proposed. At times due to the complicated radio environment, the position information is not precise. Even in these cases, the MT can still find its reachable neighbors (the association) by exchanging “hello” messages. This association is used in complement with the position information to make more accurate routing decisions. Simulation results show that the delivery ratio in the ad hoc network is greatly improved with very low cellular overhead. The security issues in the proposed architecture and the corresponding solutions are addressed. The experimental study shows that CAMA is much less vulnerable than a pure ad hoc network.

Hierarchical architectures in the third-generation cellular network

Third-generation wireless communication faces the challenges of rapidly increasing mobile user demand against limited radio bandwidth. Splitting cells into smaller cells can reduce the frequency reuse distance to improve network capacity within a certain area. Other than increasing the cost of the fixed infrastructure, cell splitting also causes the problem of increasing handoff rate and event the handoff failure rate when high-speed users roam in the network. To solve this problem, larger cells are overlaid on these smaller cells, and different classes of users (usually classified by speed) are initially assigned to the proper types of cells (i.e., proper tiers). We call this kind of cellular network a hierarchical cellular network. In this study, we review the different design techniques in the hierarchical architecture and some analytical tools to study the performance of these designs.

MADF: Mobile-assisted data forwarding for wireless data networks

In a cellular network, if there are too many data users in a cell, data may suffer long delay, and systems quality-of-service (QoS) will degrade. Some traditional schemes such as dynamic channel-allocation scheme (DCA) will assign more channels to hot (or overloaded) cells through a central control system (CC) and the throughput increase will be upper bounded by the number of new channels assigned to the cell. In mobile-assisted data forwarding (MADF), we add an ad-hoc overlay to the fixed cellular infrastructure and special channels-called forwarding channels-are used to connect mobile units in a hot cell and its surrounding cold cells without going through the hot cells base station. Thus, mobile units in a hot cell can forward data to other cold cells to achieve load balancing. Most of the forwarding-channel management work in MADF is done by mobile units themselves in order to relieve the load from the CC. The traffic increase in a certain cell will not be upper bounded by the number of forwarding channels. It can be more if the users in hot cell are significantly far away from one another and these users can use the same forwarding channels to forward data to different cold neighboring cells without interference. We find that, in a system using MADF, under a certain delay requirement, the throughput in a certain cell or for the whole network can be greatly improved.

A crossing-tier location update/paging scheme in hierarchical cellular networks

Location update/paging strategies have been widely studied in the traditional single-tier cellular networks. We propose and evaluate a novel crossing-tier location update/paging scheme that can be used in a hierarchical macrocell/microcell cellular network. Location update is proceeded only in the macrocell tier, where a location area (LA) is made up by larger macrocells. A mobile user will stay in such a LA for longer time. Therefore, the cost on location update can be reduced due to the decreased frequency of location update. To reduce the paging delay, the paged mobile user will be searched in the macrocell tier only when the paging load is not high. Otherwise, it will be searched in the microcell tier, where a sequential searching method is applied. The operation for the scheme is simple, as the macrocell/microcell cellular network has the advantage because a mobile user can receive a signal from both a microcell and the overlaid macrocell. Analytical models have been built for cost and delay evaluation. Numerical results show that, at relatively low cost, the crossing-tier scheme also achieves low paging delay.

A low-cost, low-delay location update/paging scheme in hierarchical cellular networks

A low-cost, two-step location update/paging scheme in a macrocell/microcell network is proposed and investigated. To reduce operating cost, the location update is operated only in the macrocell tier. A callee will be paged in the macrocell tier first. If the paging delay in the macrocell tier is too high due to large queuing delay, the callee will then be paged in the microcell tier. Original searching method is used in the microcell tier paging. The operation for the scheme is simple, since the macrocell/microcell cellular network has the advantage that a mobile user in such a cellular network can receive a signal from both a macrocell and a microcell. The analytical results show that, along with the low location update/paging cost, the two-step paging scheme also achieves low paging delay.

Performance evaluation of multiple-rate mobile ad hoc networks

Existing wireless networks usually provide multiple data transmission rates. This paper presents a simulation study of the performance of multiple-rate mobile ad hoc networks (MANETs), based on an evolved ns-2 simulator. At the physical layer, realistic models such as the Walfisch-Ikagami radio propagation model and lognormal fading are implemented. At the link layer, a link adaptation algorithm is implemented to select an appropriate data transmission rate based on the receiving signal-to-noise ratio. At the transport and application layers, different data traffics, including constant bit rate, TCP, voice over IP, and video, are generated. We study the network performance such as throughput, delivery ratio, and end-to-end delay when position-based routing is used. We also study how node mobility and position error affect the performance. In addition, we investigate the impact of the link distance, namely the geographic distance for a hop, on the end-to-end network throughput. This work is a comprehensive simulation study on the impact of various factors on the performance of MANETs. It also provides guidelines for future protocol and algorithm design.

Call admission and handoff control in multi-tier cellular networks: algorithms and analysis

In this paper, we investigate a call-admission and handoff-control framework for multi-tier cellular networks. We first propose and compare Call-Admission Control (CAC) algorithms based on the cell-dwelling time, by studying their impact on the handoff-call dropping and new-call blocking probabilities and the channel partitioning between the two tiers. Our results show that a simple, cell-dwelling-time-insensitive algorithm performs better under various mixes of user mobilities and call types. Moreover, there is an optimal channel partition of the overall spectrum between the tiers which minimizes the dropping and blocking probabilities for the two different CAC algorithms studied in this paper. Once the call is admitted into the network, we propose and compare various handoff- queuing strategies to reduce the call dropping probability. We show that implementing a queuing framework in one of the tiers (especially the upper, i.e., macrocellular, tier), results in a significant reduction in the dropping probability.

A simulation study on multi-rate mobile ad hoc networks

This paper studies the performance of a multi-rate mobile ad hoc network (MANET) using an extended ns-2 simulator. A link adaptation algorithm is developed and tested. The multi-rate control algorithm is based on the channel access mechanism for IEEE 802.11 with modifications. Some realistic models for radio propagation, such as lognormal fading and Walfisch/Ikagami propagation model, are used. At transport and application layer, different kinds of data traffic, including constant bit rate, TCP, voice over IP, and video are tested. The effects due to position error and mobility are also examined. The simulation results show that link layer data rate control can greatly improve network performance. Components at different layers all contribute to the system performance of a MANET. It is also shown that multimedia data transmission over MANETs deserves future study.