Rahul Mangharam

RT-Link: A Time-Synchronized Link Protocol for Energy- Constrained Multi-hop Wireless Networks

We propose RT-link, a time-synchronized link protocol for real-time wireless communication in industrial control, surveillance and inventory tracking. RT-link provides predictable lifetime for battery-operated embedded nodes, bounded end-to-end delay across multiple hops, and collision-free operation. We investigate the use of hardware-based time-synchronization for infrastructure nodes by using an AM carrier-current radio for indoors and atomic clock receivers for outdoors. Mobile nodes are synchronized via in-band software synchronization within the same framework. We identify three key observations in the design and deployment of RT-link: (a) hardware-based global-time synchronization is a robust and scalable option to in-band software-based techniques, (b) achieving global time-synchronization is both economical and convenient for indoor and outdoor deployments, (c) RT-link achieves a practical lifetime of over 2 years. Through analysis and simulation, we show that RT-link outperforms energy-efficient link protocols such as B-MAC in terms of node lifetime and end-to-end latency. The protocol supports flexible services such as on-demand end-to-end rate control and logical topology control. We implemented RT-link on the CMU FireFly sensor platform and have integrated it within the nano-RK real-time sensor OS. A 42-node network with sub-20 mus synchronization accuracy has been deployed for 3 weeks in the NIOSH Mining Research Laboratory and within two 5-story campus buildings

The Wireless Control Network: A New Approach for Control Over Networks

We present a method to stabilize a plant with a network of resource constrained wireless nodes. As opposed to traditional networked control schemes where the nodes simply route information to and from a dedicated controller (perhaps performing some encoding along the way), our approach treats the network itself as the controller. Specifically, we formulate a strategy for each node in the network to follow, where at each time-step, each node updates its internal state to be a linear combination of the states of the nodes in its neighborhood. We show that this causes the entire network to behave as a linear dynamical system, with sparsity constraints imposed by the network topology. We provide a numerical design procedure to determine appropriate linear combinations to be applied by each node so that the transmissions of the nodes closest to the actuators will stabilize the plant. We also show how our design procedure can be modified to maintain mean square stability under packet drops in the network, and present a distributed scheme that can handle node failures while preserving stability. We call this architecture a Wireless Control Network, and show that it introduces very low computational and communication overhead to the nodes in the network, allows the use of simple transmission scheduling algorithms, and enables compositional design (where the existing wireless control infrastructure can be easily extended to handle new plants that are brought online in the vicinity of the network).

Voice over Sensor Networks

Wireless sensor networks have traditionally focused on low duty-cycle applications where sensor data are reported periodically in the order of seconds or even longer. This is due to typically slow changes in physical variables, the need to keep node costs low and the goal of extending battery lifetime. However, there is a growing need to support real-time streaming of audio and/or low-rate video even in wireless sensor networks for use in emergency situations and short-term intruder detection. In this paper, we present FireFly, a time-synchronized sensor network platform for real-time data streaming across multiple hops. FireFly is composed of several integrated layers including specialized low-cost hardware, a sensor network operating system, a real-time link layer and network scheduling which together provide efficient support for applications with timing constraints. In order to achieve high end-to-end throughput, bounded latency and predictable lifetime, we employ hardware-based time synchronization. Multiple tasks including audio sampling, networking and sensor reading are scheduled using the nano-RK RTOS. We have implemented RT-Link, a TDMA-based link layer protocol for message exchange on well-defined time slots and pipelining along multiple hops. We use this platform to support 2-way audio streaming concurrently with sensing tasks. For interactive voice, we investigate TDMA-based slot scheduling with balanced bi-directional latency while meeting audio timeliness requirements. Finally, we describe our experimental deployment of 42 nodes in a coal mine, and present measurements of the end-to-end throughput, jitter, packet loss and voice quality

GrooveNet: A Hybrid Simulator for Vehicle-to-Vehicle Networks

Vehicular networks are being developed for efficient broadcast of safety alerts, real-time traffic congestion probing and for distribution of on-road multimedia content. In order to investigate vehicular networking protocols and evaluate the effects of incremental deployment it is essential to have a topology-aware simulation and test-bed infrastructure. While several traffic simulators have been developed under the intelligent transport system initiative, their primary motivation has been to model and forecast vehicle traffic flow and congestion from a queuing perspective. GrooveNet is a hybrid simulator which enables communication between simulated vehicles, real vehicles and between real and simulated vehicles. By modeling inter-vehicular communication within a real street map-based topography it facilitates protocol design and also in-vehicle deployment. GrooveNets modular architecture incorporates mobility, trip and message broadcast models over a variety of link and physical layer communication models. It is easy to run simulations of thousands of vehicles in any US city and to add new models for networking, security, applications and vehicle interaction. GrooveNet supports multiple network interfaces, GPS and events triggered from the vehicles on-board computer. Through simulation, we are able to study the message latency, and coverage under various traffic conditions. On-road tests over 400 miles lend insight to required market penetration

Cyber–Physical Modeling of Implantable Cardiac Medical Devices

The design of bug-free and safe medical device software is challenging, especially in complex implantable devices that control and actuate organs in unanticipated contexts. Safety recalls of pacemakers and implantable cardioverter defibrillators between 1990 and 2000 affected over 600 000 devices. Of these, 200 000 or 41% were due to firmware issues and their effect continues to increase in frequency. There is currently no formal methodology or open experimental platform to test and verify the correct operation of medical device software within the closed-loop context of the patient. To this effect, a real-time virtual heart model (VHM) has been developed to model the electrophysiological operation of the functioning and malfunctioning (i.e., during arrhythmia) heart. By extracting the timing properties of the heart and pacemaker device, we present a methodology to construct a timed-automata model for functional and formal testing and verification of the closed-loop system. The VHMs capability of generating clinically relevant response has been validated for a variety of common arrhythmias. Based on a set of requirements, we describe a closed-loop testing environment that allows for interactive and physiologically relevant model-based test generation for basic pacemaker device operations such as maintaining the heart rate, atrial-ventricle synchrony, and complex conditions such as pacemaker-mediated tachycardia. This system is a step toward a testing and verification approach for medical cyber-physical systems with the patient in the loop.

GrooveSim: a topography-accurate simulator for geographic routing in vehicular networks

Vehicles equipped with wireless communication devices are poised to deliver vital services in the form of safety alerts, traffic congestion probing and on-road commercial applications. Tools to evaluate the performance of vehicular networks are a fundamental necessity. While several traffic simulators have been developed under the Intelligent Transport System initiative, their primary focus has been on modeling and forecasting vehicle traffic flow and congestion from a queuing perspective. In order to analyze the performance and scalability of inter-vehicular communication protocols, it is important to use realistic traffic density, speed, trip, and communication models. Studies on multi-hop mobile wireless routing protocols have shown the performance varies greatly depending on the simulation models employed. We introduce GrooveSim, a simulator for geographic routing in vehicular networks to address the need for a robust, easy-to-use realistic network and traffic simulator. GrooveSim accurately models inter-vehicular communication within a real street map-based topography. It operates in five modes capable of actual on-road inter-vehicle communication, simulation of traffic networks with thousands of vehicles, visual playback of driving logs, hybrid simulation composed of real and simulated vehicles and easy test-scenario generation. Our performance results, supported by field tests, establish geographic broadcast routing as an effective means to deliver time-bounded messages over multiple-hops.

Toward patient safety in closed-loop medical device systems

A model-driven design and validation of closed-loop medical device systems is presented. Currently, few if any medical systems on the market support closed-loop control of interconnected medical devices, and mechanisms for regulatory approval of such systems are lacking. We present a system implementing a clinical scenario where closed-loop control may reduce the possibility of human error and improve safety of the patient. The safety of the system is studied with a simple controller proposed in the literature. We demonstrate that, under certain failure conditions, safety of the patient is not guaranteed. Finally, a more complex controller is described and ensures safety even when failures are possible. This investigation is an early attempt to introduce automatic control in clinical scenarios and to delineate a methodology to validate such patient-in-the-loop systems for safe and correct operation.

Model-Driven Safety Analysis of Closed-Loop Medical Systems

In modern hospitals, patients are treated using a wide array of medical devices that are increasingly interacting with each other over the network, thus offering a perfect example of a cyber-physical system. We study the safety of a medical device system for the physiologic closed-loop control of drug infusion. The main contribution of the paper is the verification approach for the safety properties of closed-loop medical device systems. We demonstrate, using a case study, that the approach can be applied to a system of clinical importance. Our method combines simulation-based analysis of a detailed model of the system that contains continuous patient dynamics with model checking of a more abstract timed automata model. We show that the relationship between the two models preserves the crucial aspect of the timing behavior that ensures the conservativeness of the safety analysis. We also describe system design that can provide open-loop safety under network failure.

The Swarm at the Edge of the Cloud

Mobile devices such as laptops, netbooks, tablets, smart phones and game consoles have become our de facto interface to the vast amount of information delivery and processing capabilities of the cloud. The move to mobility has been enabled by the dual forces of ubiquitous wireless connectivity combined with the increasing energy efficiency offered by Moores law.

MEERA: cross-layer methodology for energy efficient resource allocation in wireless networks

In many portable devices, wireless network interfaces consume upwards of 30% of scarce system energy. Reducing the transceivers power consumption to extend the system lifetime has therefore become a design goal. Our work is targeted at this goal and is based on the following two observations. First, conventional energy management approaches have focused independently on minimizing the fixed energy cost (by shutdown) and on scalable energy costs (by leveraging, for example, the modulation, code-rate and transmission power). These two energy management approaches present a tradeoff. For example, lower modulation rates and transmission power minimize the variable energy component, but this shortens the sleep duration thereby increasing fixed energy consumption. Second, in order to meet the quality of service (QoS) timeliness requirements for multiple users, we need to determine to what extent each system in the network may sleep and scale. Therefore, we propose a two-phase methodology that resolves the sleep-scaling tradeoff across the physical, communications and link layers at design time and schedules nodes at runtime with near optimal energy-efficient configurations in the solution space. As a result, we are able to achieve very low run-time overheads. Our methodology is applied to a case study on delivering a guaranteed QoS for multiple users with MPEG-4 video over a slow-fading channel. By exploiting runtime controllable parameters of actual RF components and a modified 802.11 medium access controller, system lifetime is increased by a factor of 3-to-10 in comparison with conventional techniques