Tina Nolte

Consensus and collision detectors in wireless Ad Hoc networks

We consider the fault-tolerant consensus problem in wireless ad hoc networks with crash-prone nodes. We develop consensus algorithms for single-hop environments where the nodes are located within broadcast range of each other. Our algorithms tolerate highly unpredictable wireless communication, in which messages may be lost due to collisions, electromagnetic interference, or other anomalies. Accordingly, each node may receive a different set of messages in the same round. In order to minimize collisions, we design adaptive algorithms that attempt to minimize the broadcast contention. To cope with unreliable communication, we augment the nodes with collision detectors and present a new classification of collision detectors in terms of accuracy and completeness, based on practical realities. We show exactly in which cases consensus can be solved, and thus determine the requirements for a useful collision detector.We validate the feasibility of our algorithms, and the underlying wireless model, with simulations based on a realistic 802.11 MAC layer implementation and a detailed radio propagation model. We analyze the performance of our algorithms under varying sizes and densities of deployment and varying MAC layer parameters. We use our single-hop consensus algorithms as the basis for solving consensus in a multi-hop network, demonstrating the resilience of our algorithms to a challenging and noisy environment.

The virtual node layer: a programming abstraction for wireless sensor networks

The Virtual Node Layer (VNLayer) programming abstraction provides programmable, predictable automata--virtual nodes--emulated by the low-level network nodes. This simplifies the design and rigorous analysis of applications for the wireless sensor network setting, as the layer can mask much of the uncertainty of the underlying components. In this paper, we define a general VNLayer architecture, and then use this framework to design a practical VNLayer implementation, optimized for real-world use. We then discuss our experience deploying this implementation on a testbed of hand-held computers, and in a custom-built packet-level simulator, and present a sample application--a virtual traffic light--to highlight the power and utility of our abstraction. We conclude with a survey of additional applications that are well-suited to this setting.

Consensus and collision detectors in radio networks

We consider the fault-tolerant consensus problem in radio networks with crash-prone nodes. Specifically, we develop lower bounds and matching upper bounds for this problem in single-hop radios networks, where all nodes are located within broadcast range of each other. In a novel break from existing work, we introduce a collision-prone communication model in which each node may lose an arbitrary subset of the messages sent by its neighbors during each round. This model is motivated by behavior observed in empirical studies of these networks. To cope with this communication unreliability we augment nodes with receiver-side collision detectors and present a new classification of these detectors in terms of accuracy and completeness. This classification is motivated by practical realities and allows us to determine, roughly speaking, how much collision detection capability is enough to solve the consensus problem efficiently in this setting. We consider nine different combinations of completeness and accuracy properties in total, determining for each whether consensus is solvable, and, if it is, a lower bound on the number of rounds required. Furthermore, we distinguish anonymous and non-anonymous protocols—where “anonymous” implies that devices do not have unique identifiers—determining what effect (if any) this extra information has on the complexity of the problem. In all relevant cases, we provide matching upper bounds.

Motion Coordination using Virtual Nodes

We describe how a virtual node abstraction layer can be used to coordinate the motion of real mobile nodes on a 2D plane. In particular, we consider how nodes in a mobile ad hoc network can arrange themselves along a predetermined curve in the plane, and can maintain themselves in such a configuration in the presence of changes in the underlying mobile ad hoc network, specifically, when nodes may join or leave the system or may fail. Our strategy is to allow the mobile nodes to implement a virtual layer consisting of mobile client nodes, stationary Virtual Nodes (VNs) for predetermined zones in the plane, and local broadcast communication. The VNs coordinate among themselves to distribute the client nodes between zones based on the length of the curve through those zones, while each VN directs its zone’s local client nodes to move themselves to equally spaced locations on the local portion of the target curve.

Self-stabilizing mobile node location management and message routing

We present simple algorithms for achieving self-stabilizing location management and routing in mobile ad-hoc networks. While mobile clients may be susceptible to corruption and stopping failures, mobile networks are often deployed with a reliable GPS oracle, supplying frequent updates of accurate real time and location information to mobile nodes. Information from a GPS oracle provides an external, shared source of consistency for mobile nodes, allowing them to label and timestamp messages, and hence aiding in identification of, and eventual recovery from, corruption and failures. Our algorithms use a GPS oracle. Our algorithms also take advantage of the Virtual Stationary Automata programming abstraction, consisting of mobile clients, virtual timed machines called virtual stationary automata (VSAs), and a local broadcast service connecting VSAs and mobile clients. VSAs are distributed at known locations over the plane, and emulated in a self-stabilizing manner by the mobile nodes in the system. They serve as fault-tolerant building blocks that can interact with mobile clients and each other, and can simplify implementations of services in mobile networks. We implement three self-stabilizing, fault-tolerant services, each built on the prior services: (1) VSA-to-VSA geographic routing, (2) mobile client location management, and (3) mobile client end-to-end routing. We use a greedy version of the classical depth-first search algorithm to route messages between VSAs in different regions. The mobile client location management service is based on home locations: Each client identifier hashes to a set of home locations, regions whose VSAs are periodically updated with the client’s location. VSAs maintain this information and answer queries for client locations. Finally, the VSA-to-VSA routing and location management services are used to implement mobile client end-to-end routing.

A Virtual Node-Based Tracking Algorithm for Mobile Networks

We introduce a virtual-node based mobile object tracking algorithm for mobile sensor networks, VINESTALK. The algorithm uses the virtual stationary automata programming layer, consisting of mobile clients, virtual timed machines distributed at known locations in the plane, called virtual stationary automata (VSAs), and a communication service connecting VSAs and mobile clients. VINESTALK maintains a data structure on top of an underlying hierarchical partitioning of the network. In a grid partitioning, operations to find a mobile object distance d away take O(d) time and communication to complete. Updates to the tracking structure after the object has moved a total of d distance take O{d*log network diameter) amortized time and communication to complete. The tracked object may relocate without waiting for VINESTALK to complete updates for prior moves, and while a find is in progress.

Timed virtual stationary automata for mobile networks

We define a programming abstraction for mobile networks called the Timed Virtual Stationary Automata programming layer, consisting of mobile clients, virtual timed I/O automata called virtual stationary automata (VSAs), and a communication service connecting VSAs and client nodes. The VSAs are located at prespecified regions that tile the plane, defining a static virtual infrastructure. We present a self-stabilizing algorithm to emulate a timed VSA using the real mobile nodes that are currently residing in the VSAs region. We also discuss examples of applications whose implementations benefit from the simplicity obtained through use of the VSA abstraction.

Brief announcement: virtual stationary automata for mobile networks

The task of designing algorithms for constantly changing networks is difficult. We focus on mobile ad-hoc networks, where mobile processors attempt to coordinate despite minimal infrastructure support. We develop new techniques to cope with this dynamic, heterogeneous, and chaotic environment. We mask the unpredictable behavior of mobile networks by defining and emulating a virtual infrastructure, consisting of timing-aware and location-aware machines at fixed locations, that mobile nodes can interact with. The static virtual infrastructure allows appplication developers to use simpler algorithms — including many previously developed for fixed networks. Virtual Stationary Automata programming layer. Our programming abstraction consists of a static infrastructure of fixed, timed virtual machines with an explicit notion of real time, called Virtual Stationary Automata (VSAs), distributed at known locations over the plane, and emulated by the real mobile nodes in the system. Each VSA represents a predetermined geographic area and has broadcast capabilities similar to those of the mobile nodes, allowing nearby VSAs and mobile nodes to communicate with one another. This programming layer provides mobile nodes with a virtual infrastructure with which to coordinate their actions. Many practical algorithms depend significantly on timing, and it is reasonable to assume that many mobile nodes have access to reasonably synchronized clocks. In the VSA programming layer, the virtual automata also have access to virtual clocks, guaranteed to not drift too far from real time. Our virtual infrastructure differs in key ways from others that have previously been proposed for mobile ad-hoc networks. The GeoQuorums algorithm [2] was the first to use virtual nodes; the virtual nodes in that work are atomic objects at fixed geographical locations. More general virtual mobile automata were suggested in [1]; our automata are more powerful than those in [1] in that ours include timing capabilities, which are important for many applications. Also, our automata are stationary, and are arranged in a connected pattern that is similar to a traditional wired ne-

Self-stabilization and virtual node layer emulations

We present formal definitions of stabilization for the Timed I/O Automata (TIOA) framework, and of emulation for the timed Virtual Stationary Automata programming abstraction layer, which consists of mobile clients, virtual timed machines called virtual stationary automata (VSAs), and a local broadcast service connecting VSAs and mobile clients. We then describe what it means for mobile nodes with access to location and clock information to emulate the VSA layer in a self-stabilizing manner. We use these definitions to prove basic results about executions of self-stabilizing algorithms run on self-stabilizing emulations of a VSA layer, and apply these results to a simple geographic routing algorithm running on the VSA layer.

Reconciling the theory and practice of (un)reliable wireless broadcast

Theorists and practitioners have fairly different perspectives on how wireless broadcast works. Theorists think about synchrony; practitioners think about backoff. Theorists assume reliable communication; practitioners worry about collisions. The examples are endless. Our goal is to begin to reconcile the theory and practice of wireless broadcast, in the presence of failures. We propose new models for wireless broadcast and use them to examine what makes a broadcast model good. In the process, we pose some interesting questions that help to bridge the gap.