Multifrequency media access control has been well understood in general wireless ad hoc networks, while in wireless sensor networks, researchers still focus on single frequency solutions. In wireless sensor networks, each device is typically equipped with a single radio transceiver and applications adopt much smaller packet sizes compared to those in general wireless ad hoc networks. Hence, the multifrequency MAC protocols proposed for general wireless ad hoc networks are not suitable for wireless sensor network applications, which we further demonstrate through our simulation experiments. In this article, we propose MMSN, which takes advantage of multifrequency availability while, at the same time, takes into consideration the restrictions of wireless sensor networks. In MMSN, four frequency assignment options are provided to meet different application requirements. A scalable media access is designed with efficient broadcast support. Also, an optimal nonuniform back-off algorithm is derived and its lightweight approximation is implemented in MMSN, which significantly reduces congestion in the time synchronized media access design. Through extensive experiments, MMSN exhibits the prominent ability to utilize parallel transmissions among neighboring nodes. When multiple physical frequencies are available, it also achieves increased energy efficiency, demonstrating the ability to work against radio interference and the tolerance to a wide range of measured time synchronization errors.

This article presents the design, implementation, and evaluation of EnviroMic , a low-cost experimental prototype of a novel distributed acoustic monitoring, storage, and trace retrieval system designed for disconnected operation. Our intended use of acoustic monitoring is to study animal populations in the wild. Since a permanent connection to the outside world is not assumed and due to the relatively large size of audio traces, the system must optimally exploit available resources such as energy and network storage capacity. Towards that end, we design, prototype, and evaluate distributed algorithms for coordinating acoustic recording tasks, reducing redundancy of data stored by nearby sensors, filtering out silence, and balancing storage utilization in the network. For experimentation purposes, we implement EnviroMic on a TinyOS-based platform and systematically evaluate its performance through both indoor testbed experiments and an outdoor deployment. Results demonstrate up to a four-fold improvement in effective storage capacity of the network compared to uncoordinated recording.

This paper investigates resource allocation policies for achieving real-time content distribution with subsecond delay bounds on the current Internet. Resource allocation in real-time systems has been concerned primarily with meeting time constraints on single processors and multiprocessors. On a single processor, the main degree of freedom available for the real-time designer is the scheduling policy. On a (partitioned) multiprocessor, a second degree of freedom is the partitioning policy. This paper explores a third degree of freedom unique to large-scale (i.e., widearea) distributed systems with non-negligible communication delays among individual nodes. We call it the dominating set allocation policy. This policy is a primary determinant of schedulability in such systems. We present some initial steps towards understanding the properties of different dominating set allocation policies in terms of resulting task schedulability. We describe an optimal dominating set allocation algorithm (subject to certain design decisions), propose a number of simple heuristics, and evaluate them using realistic Internet measurements and HTTP workload. The key contribution of this paper lies in the practical applicability of the proposed heuristics in achieving delay guarantees (with a high probability) over best-effort wide-area networks.

The vulnerabilities that plague computers cause endless grief to users. Slammer compromised millions of hosts in minutes; a hit-list worm would take under a second. Recently proposed techniques respond better than manual approaches, but require expensive instrumentation, which limits deployment. Although spreading "antibodies" (e.g. signatures) ameliorates this limitation, hosts depending on antibodies are defenseless until inoculation; to the fastest hit-list worms this delay is crucial. Additionally, most recently proposed techniques cannot provide recovery to provide continuous service after an attack.

This work presents a TDMA based multi-channel MAC protocol called TMMAC for Ad Hoc Networks. TMMAC requires only a single half-duplex radio transceiver on each node. In addition to explicit frequency negotiation which is adopted by conventional multi-channel MAC protocols, TMMAC introduces lightweight explicit time negotiation. This two-dimensional negotiation enables TMMAC to exploit the advantage of both multiple channels and TDMA, and achieve aggressive power savings by allowing nodes that are not involved in communication to go into doze mode. Moreover, TMMAC dynamically adjusts its negotiation window size based on different traffic patterns, which further improves communication throughput and energy savings. In this paper, the performance of TMMAC is analyzed and evaluated. The evaluations show that TMMAC achieves up to 113% higher communication throughput while consuming 74% less per packet energy over the state-of-the-art multi-channel MAC protocols for single-transceiver wireless devices.

This paper presents EnviroMic, a novel distributed acoustic monitoring, storage, and trace retrieval system. Audio represents one of the least exploited modalities in sensor networks to date. The relatively high frequency and large size of audio traces motivate distributed algorithms for coordinating recording tasks, reducing redundancy of data stored by nearby sensors, filtering out silence, and balancing storage utilization in the network. Applications of acoustic monitoring with EnviroMic range from the study of mating rituals and social behavior of animals in the wild to audio surveillance of military targets. EnviroMic is designed for disconnected operation, where the luxury of having a basestation cannot be assumed. We implement the system on a Tiny OS-based platform and systematically evaluate its performance through both indoor testbed experiments and a preliminary outdoor deployment. Results demonstrate up to a 4-fold improvement in effective storage capacity of the network compared to uncoordinated recording.

This paper presents a new cooperative storage system for sensor networks geared for disconnected operation (where sensor nodes do not have a connected path to a basestation). The goal of the system is to maximize its data storage capacity by appropriately distributing storage utilization and opportunistically offloading data to external devices when possible. The system is motivated by the observation that a large category of sensor network applications, such as environmental data logging, does not require real-time data access. Such networks generally operate in a disconnected mode. Rather than focusing on multihop routing to a basestation, an important concern becomes (i) to maximize the effective storage capacity of the disconnected sensor network such that it accommodates the most data, and (ii) to take the best advantage of data upload opportunities when they become available to relieve network storage. The storage system described in this paper achieves the above goals, leading to significant improvements in the amount of data collected compared to non-cooperative storage. It is implemented in nesC for TinyOS and evaluated in TOSSIM through various application scenarios.

Sensor networks introduce a new application domain and set of challenges in distributed computing including new network-level programming languages, global system abstractions, and general-purpose communication protocols. These challenges are brought about by the tight integration of computation, communication, and distributed real-time interaction with the physical world. With the growing interest in interconnecting different sensor networks across a wide-area communication infrastructure, an overarching challenge becomes one of arriving at an agreed-upon global sensor network architecture that ensures interoperability. Unlike the Internet, where a layered communication stack (namely, the TCP/IP stack) defines the network architecture, a sensor network architecture must unify not only communication interfaces but also programming interfaces, since network communication and computation functions are tightly intertwined. In that sense, the sensor network architecture refers to a layered stack of distributed computing abstractions. This paper presents an architecture and key considerations in designing and interconnecting local and global sensor networks. Candidate protocols and middleware instantiations are described from the authors' ongoing work that meet the discussed considerations

Diagnosing production run failures is a challenging yet importanttask. Most previous work focuses on offsite diagnosis, i.e.development site diagnosis with the programmers present. This is insufficient for production-run failures as: (1) it is difficult to reproduce failures offsite for diagnosis; (2) offsite diagnosis cannot provide timely guidance for recovery or security purposes; (3)it is infeasible to provide a programmer to diagnose every production run failure; and (4) privacy concerns limit the release of information(e.g. coredumps) to programmers.

Abstract — Multi-frequency media access control has been well understood in general wireless ad hoc networks, while in wireless sensor networks, researchers still focus on single frequency solutions. In wireless sensor networks, hardware devices are equipped with very limited communication ability and applications adopt much smaller packet sizes compared to those in general wireless ad hoc networks. Hence, the multi-frequency MAC protocols proposed for general wireless ad hoc networks are not suitable for wireless sensor network applications, which we further demonstrate through our simulation experiments. In this paper, we propose MMSN, the first multi-frequency MAC protocol for wireless sensor networks. In the MMSN protocol, four frequency assignment options are provided to meet different application requirements. A scalable media access is designed with efficient broadcast support. Also, an optimal non-uniform backoff algorithm is derived and its lightweight approximation is implemented in MMSN, which significantly reduces congestion in the time synchronized media access design. Through extensive experiments, MMSN exhibits prominent ability to utilize parallel transmission among neighboring nodes. It also achieves increased energy efficiency when multiple physical frequencies are available. I.

In this paper we present a balanced data replication scheme that provides real-time latency bounds on content retrieval in content distribution networks. Many network applications have ever-increasing requirements on latency sensitive data services. Data replication services have been widely used as an important performance enhancement mechanism to reduce data access latency and throughput. We investigate the problem of provisioning an underlying balanced data replication service to provide a global latency bound on data retrieval in content distribution networks. The solution involves constructing an overlay network based on the given latency bound, and a mechanism to assign content objects to the network nodes so that the workload of all the network nodes is balanced. Our evaluation results drawn from detailed simulations show the efficacy of our load-balancing scheme in meeting the latency bound requirements with high confidence under heavy load.

This paper investigates the performance of a content distribution network designed to provide bounded content access latency. Content can be divided into multiple classes with different configurable per-class delay bounds. The network uses a simple distributed algorithm to dynamically select subsets of its proxy servers for different classes such that a global per-class delay bound is achieved on content access. The content distribution algorithm is implemented and tested on PlanetLab, a world-wide distributed Internet testbed. Evaluation results demonstrate that, despite Internet delay variability, subsecond delay bounds (of 200-500 ms) can be guaranteed with a very high probability at only a moderate content replication cost. The distribution algorithm achieves a four to fivefold reduction in the number of response-time violations compared to prior content distribution approaches that attempt to minimize average latency. To the authors' knowledge, this paper presents the first wide-area performance evaluation of an algorithm designed to bound maximum content access latency, as opposed to optimizing an average performance metric.

With the proliferation of location dependent applications in sensor networks, location awareness becomes an essential capability of sensor nodes. Because coarse accuracy is sufficient for most sensor network applications, solutions in range-free localization are being pursued as a cost-effective alternative to more expensive range-based approaches. In this paper, we present APIT, a novel localization algorithm that is range-free. We show that our APIT scheme performs best when an irregular radio pattern and random node placement are considered, and low communication overhead is desired. We compare our work, via extensive simulation, with three state-of-the-art range-free localization schemes to identify the preferable system configurations of each. In addition, we provide insight into the impact of localization accuracy on various location dependent applications and suggestions on improving their performance in the presence of such inaccuracy.

This paper investigates the performance of a content distribution network designed to provide bounded content access latency. Content can be divided into multiple classes with different configurable per-class delay bounds. The network uses a simple distributed algorithm to dynamically select a subset of its proxy servers for different classes such that a global per-class delay bound is achieved on content access. The content distribution algorithm is implemented and tested on PlanetLab, a world-wide distributed Internet testbed. Evaluation results demonstrate that despite Internet delay variability, subsecond delay bounds (of 200-500ms) can be guaranteed with a very high probability at only a moderate content replication cost. The distribution algorithm achieves a 4 to 5 fold reduction in the number of response-time violations compared to prior content distribution approaches that attempt to minimize average latency. This paper presents the first wide-area performance evaluation of an algorithm designed to bound maximum content access latency, as opposed to optimizing an average performance metric.

Wireless Sensor Networks have been proposed for a multitude of location-dependent applications. For such systems, the cost and limitations of hardware on sensing nodes prevent the use of range-based localization schemes that depend on absolute point-to-point distance estimates. Because coarse accuracy is sufficient for most sensor network applications, solutions in range-free localization are being pursued as a cost-effective alternative to more expensive range-based approaches. In this paper, we present APIT, a novel localization algorithm that is range-free. We show that our APIT scheme performs best when an irregular radio pattern and random node placement are considered, and low communication overhead is desired. We compare our work via extensive simulation, with three state-of-the-art range-free localization schemes to identify the preferable system configurations of each. In addition, we study the effect of location error on routing and tracking performance. We show that routing performance and tracking accuracy are not significantly affected by localization error when the error is less than 0.4 times the communication radio radius. 1.