Jason Redi

Ad hoc networking with directional antennas: a complete system solution

Directional antennas offer tremendous potential for improving the performance of ad hoc networks. Harnessing this potential, however, requires new mechanisms at the medium access and network layers for intelligently and adaptively exploiting the antenna system. While recent years have seen a surge of research into such mechanisms, the problem of developing a complete ad hoc networking system, including the unique challenge of real-life prototype development and experimentation has not been addressed. In this paper, we present utilizing directional antennas for ad hoc networking (UDAAN). UDAAN is an interacting suite of modular network- and medium access control (MAC)-layer mechanisms for adaptive control of steered or switched antenna systems in an ad hoc network. UDAAN consists of several new mechanisms-a directional power-controlled MAC, neighbor discovery with beamforming, link characterization for directional antennas, proactive routing and forwarding-all working cohesively to provide the first complete systems solution. We also describe the development of a real-life ad hoc network testbed using UDAAN with switched directional antennas, and we discuss the lessons learned during field trials. High fidelity simulation results, using the same networking code as in the prototype, are also presented both for a specific scenario and using random mobility models. For the range of parameters studied, our results show that UDAAN can produce a very significant improvement in throughput over omnidirectional communications.

Movement control algorithms for realization of fault-tolerant ad hoc robot networks

Autonomous and semi-autonomous mobile multirobot systems require a wireless communication network in order to communicate with each other and collaboratively accomplish a given task. A multihop communications network that is self-forming, self-healing, and self-organizing is ideally suited for such mobile robot systems that exist in unpredictable and constantly changing environments. However, since every node in a multihop (or ad hoc) network is responsible for forwarding packets to other nodes, the failure of a critical node can result in a network partition. Hence, it is ideal to have an ad hoc network configuration that can tolerate temporary failures while allowing recovery. Since movement of the robot nodes is controllable, it is possible to achieve such fault-tolerant configurations by moving a subset of robots to new locations. In this article we propose a few simple algorithms for achieving the baseline graph theoretic metric of tolerance to node failures, namely, biconnectivity. We formulate an optimization problem for the creation of a movement plan while minimizing the total distance moved by the robots. For one-dimensional networks, we show that the problem of achieving a biconnected network topology can be formulated as a linear program; the latter lends itself to an optimal polynomial time solution. For two-dimensional networks the problem is much harder, and we propose efficient heuristic approaches for achieving biconnectivity. We compare the performance of the proposed algorithms with each other with respect to the total distance moved metric using simulations.

Effect of overhearing transmissions on energy efficiency in dense sensor networks

Energy efficiency is an important design criterion for the development of sensor networking protocols involving data dissemination and gathering. In-network processing of sensor data, aggregation, transmission power control in radios, and periodic cycling of node wake-up schedules are some techniques that have been proposed in the sensor networking literature for achieving energy efficiency. Owing to the broadcast nature of the wireless channel many nodes in the vicinity of a sender node may overhear its packet transmissions even if they are not the intended recipients of these transmissions. Reception of these transmissions can result in unnecessary expenditure of battery energy of the recipients. We investigate the impact of overhearing transmissions on total energy costs during data gathering and dissemination and attempt to minimize them systematically. We model the minimum energy data gathering problem as a directed minimum energy spanning tree problem where the energy cost of each edge in the wireless connectivity graph is augmented by the overhearing cost of the corresponding transmission. We observe that in dense sensor networks, overhearing costs constitute a significant fraction of the total energy cost and that computing the minimum spanning tree on the augmented cost metric results in energy savings, especially in networks with non-uniform spatial node distribution. We also study the impact of this new metric on the well known energy-efficient dissemination (also called broadcasting) algorithms for multihop wireless networks. We show via simulation that through this augmented cost metric, gains in energy efficiency of 10% or more are possible without additional hardware and minimal additional complexity.

Coordinated flocking of UAVs for improved connectivity of mobile ground nodes

Unmanned aerial vehicles (UAV) have been used by the military for surveillance and reconnaissance operations for the past few decades. The recent proliferation of wireless networking technologies enables the equipment of UAVs with wireless transceivers, and that can in turn allow them to communicate with the friendly ground nodes as well as other UAVs. Since rugged ground terrain can result in significant signal attenuation, the ground network can be severely partitioned. However the lower propagation loss between ground and airborne nodes can be effectively utilized to connect the islands of connectivity on the ground to farm a unified ad hoc network. In this paper, we investigate the UAV placement and navigation strategies with the end goal of improving network connectivity. Since the ground nodes can be mobile, a fixed placement strategy is either inadequate or wasteful; hence, we propose to use local flocking rules that aerial living beings like birds and insects follow, to meet our goals. We show by simulation that a flocking based navigation strategy is adaptive to the motion of ground nodes and can indeed maintain high connectivity in a mobile ground network.

The DARPA WNaN network architecture

The warfighter network of the future needs to be low-cost, instantly deployable, self-organizing, robust, and scale with both size and density. DARPAs Wireless Network after Next (WNaN) meets these challenges using an architecture that combines several innovative features for the first time in a real functional system: dynamic spectrum access (DSA), adaptive multi-transceiver frequency assignment, multi-channel medium access, highly scalable routing and disruption-tolerant networking (DTN). The WNaN system has been successfully demonstrated in real-world military experiments for up to 100 nodes - the largest military MANET demonstration on record. We present the network architecture of the WNaN system, focusing mainly on the medium access and sub-network layers. We briefly describe how key protocols work in tandem, and how they are implemented on the objective platform. Finally, we discuss some future plans.

On the use of directional antennas for sensor networks

Directional antennas have been shown to have the potential to provide dramatic increases in throughput and reduction in delay, while simultaneously requiring lower transmit power and increasing LPI/LPD and A/J qualities. Among military systems, sensor systems have a strong need for such characteristics due to their inability to move, length of deployment time, as well as a need for stealth operation. In this paper we describe and analyze significant issues for contention-based MACs for directional antennas which are of particular interest to sensor systems. We also provide a MAC design that overcomes these difficulties.

JAVeLEN - An ultra-low energy ad hoc wireless network

Wireless networks are often very lightly used. Some wireless networks, most notably sensor networks, are also energy-constrained - that is, the period of time during which the network is operational depends on battery lifetime. We have designed and simulated a novel design for a mobile ad hoc network with a low offered load (of approximately 1% average loading) that uses dramatically less (often 300 times or 99.7% less) power than industry standard protocols and yet achieves higher delivery reliability, handles substantially greater node densities, supports mobility, and has the ability to perform well even under high offered loads. Several innovations were required to achieve this efficiency, most notably the design of a dual-radio transceiver and careful redesign of the protocol stack (physical, media access, routing and transport protocols) to make effective use of the power of the radio transceivers.

Optimizing parameters of a mobile ad hoc network protocol with a genetic algorithm

Mobile ad hoc networks are typically designed and evaluated in generic simulation environments. However the real conditions in which these networks are deployed can be quite different in terms of RF attentution, topology, and traffic load. Furthermore, specific situations often have a need for a network that is optimized along certain characteristics such as delay, energy or overhead. In response to the variety of conditions and requirements, ad hoc networking protocols are often designed with many modifiable parameters. However, there is currently no methodical way for choosing values for the parameters other than intuition and broad experience. In this paper we investigate the use of genetic algorithms for automated selection of parameters in an ad hoc networking system. We provide experimental results demonstrating that the genetic algorithm can optimize for different classes of operating conditions. We also compare our genetic algorithm optimization against hand-tuning in a complex, realistic scenario and show how the genetic algorithm provides better performance.

Scalability of Mobile Ad Hoc Networks: Theory vs practice

Over the past decade, the theoretical or asymptotic scalability of Mobile Ad Hoc Networks (MANETs) has been extensively studied. However, the implication of these asymptotic results on finite, brigade-sized networks with reallife assumptions is not well-understood. We present a two-pronged study on the scalability of military networks with assumptions and goals pertinent to such networks: 1) we investigate the traffic distribution characteristics in a typical military network and show that it follows a power law which exhibits very good scaling properties; 2) we introduce the notion of “in practice” scalability and derive an expression for the in-practice scalability of a simple example network. Our study indicates that MANETs may well be adequately scalable in practice even if they are asymptotically unscalable, and that military MANETs may also even be asymptotically scalable by virtue of their traffic characteristics.

An Energy Efficient and Accurate Slot Synchronization Scheme for Wireless Sensor Networks

Existing slotted channel access schemes in wireless networks assume that slot boundaries at all nodes are synchronized. In practice, relative clock drifts among nodes cause slot misalignment over time and can result in catastrophic data loss in such systems. We propose a simple network-wide slot synchronization scheme (Slot-Sync and Slot-Resync) suitable for duty-cycling wireless sensor networks. The proposed scheme attains high accuracy by circumventing dominant sources of error inherent in traditional time synchronization protocols. At moderate duty-cycle frequencies, the proposed scheme also has the unique advantage of eliminating re-synchronization (re-sync) overhead completely, thereby achieving slot re-sync essentially for free. We provide an energy efficient slot guard time and re- sync interval design for the proposed scheme and analyze several spanning tree structures for slot-alignment message propagation. In addition, we derive upper bounds on the synchronization (sync) error for a family of trees. Through simulations, we compare the spanning trees we propose to those used for time sync in literature and show up to 80% reduction in sync error and up to 70% reduction in energy needed for slot-alignment message propagation by choosing appropriate tree structures.