Self-Stabilizing TDMA Algorithms for Dynamic Wireless Ad-hoc Networks

In dynamic wireless ad-hoc networks (DynWANs), autonomous computing devices set up a network for the communication needs of the moment. These networks require the implementation of a medium access control (MAC) layer. We consider MAC protocols for DynWANs that need to be autonomous and robust as well as have high bandwidth utilization, high predictability degree of bandwidth allocation, and low communication delay in the presence of frequent topological changes to the communication network. Recent studies have shown that existing implementations cannot guarantee the necessary satisfaction of these timing requirements. We propose a self-stabilizing MAC algorithm for DynWANs that guarantees a short convergence period, and by that, it can facilitate the satisfaction of severe timing requirements, such as the above. Besides the contribution in the algorithmic front of research, we expect that our proposal can enable quicker adoption by practitioners and faster deployment of DynWANs that are subject changes in the network topology

Self-stabilizing TDMA Algorithms for Wireless Ad-hoc Networks without External Reference

Time division multiple access (TDMA) is a method for sharing communication media. In wireless communications, TDMA algorithms often divide the radio time into timeslots of uniform size, $\xi$, and then combine them into frames of uniform size, $\tau$. We consider TDMA algorithms that allocate at least one timeslot in every frame to every node. Given a maximal node degree, $\delta$, and no access to external references for collision detection, time or position, we consider the problem of collision-free self-stabilizing TDMA algorithms that use constant frame size. We demonstrate that this problem has no solution when the frame size is $\tau < \max\{2\delta,\chi_2\}$, where $\chi_2$ is the chromatic number for distance-$2$ vertex coloring. As a complement to this lower bound, we focus on proving the existence of collision-free self-stabilizing TDMA algorithms that use constant frame size of $\tau$. We consider basic settings (no hardware support for collision detection and no prior clock synchronization), and the collision of concurrent transmissions from transmitters that are at most two hops apart. In the context of self-stabilizing systems that have no external reference, we are the first to study this problem (to the best of our knowledge), and use simulations to show convergence even with computation time uncertainties

Shared-object System Equilibria: Delay and Throughput Analysis

We consider shared-object systems that require their threads to fulfill the system jobs by first acquiring sequentially the objects needed for the jobs and then holding on to them until the job completion. Such systems are in the core of a variety of shared-resource allocation and synchronization systems. This work opens a new perspective to study the expected job delay and throughput analytically, given the possible set of jobs that may join the system dynamically. We identify the system dependencies that cause contention among the threads as they try to acquire the job objects. We use these observations to define the shared-object system equilibria. We note that the system is in equilibrium whenever the rate in which jobs arrive at the system matches the job completion rate. These equilibria consider not only the job delay but also the job throughput, as well as the time in which each thread blocks other threads in order to complete its job. We then further study in detail the thread work cycles and, by using a graph representation of the problem, we are able to propose procedures for finding and estimating equilibria, i.e., discovering the job delay and throughput, as well as the blocking time. To the best of our knowledge, this is a new perspective, that can provide better analytical tools for the problem, in order to estimate performance measures similar to ones that can be acquired through experimentation on working systems and simulations, e.g., as job delay and throughput in (distributed) shared-object systems

How to Stop Disagreeing and Start Cooperatingin the Presence of Asymmetric Packet Loss

We consider the design of a disagreement correction protocol in multi-vehicle systems. Vehicles broadcast in real-time vital information such as position, direction, speed, acceleration, intention, etc. This information is then used to identify the risks and adapt their trajectory to maintain the highest performance without compromising the safety. To minimize the risk due to the use of inconsistent information, all cooperating vehicles must agree whether to use the exchanged information to operate in a cooperative mode or use the only local information to operate in an autonomous mode. However, since wireless communications are prone to failures, it is impossible to deterministically reach an agreement. Therefore, any protocol will exhibit necessary disagreement periods. In this paper, we investigate whether vehicles can still cooperate despite communication failures even in the scenario where communication is suddenly not available. We present a deterministic protocol that allows all participants to either operate a cooperative mode when vehicles can exchange all the information in a timely manner or operate in autonomous mode when messages are lost. We show formally that the disagreement time is bounded by the time that the communication channel requires to deliver messages and validate our protocol using NS-3 simulations. We explain how the proposed solution can be used in vehicular platooning to attain high performance and still guarantee high safety standards despite communication failures

Brief Announcement: Self-stabilizing Virtual Synchrony

International audienceSystems satisfying the Virtual Synchrony (VS) [2] property provide message multicast and group membership services in which all system events, group membership changes, and incoming messages, are delivered in the same order. VS is an important abstraction, proven to be extremely useful when implemented over asynchronous, typically large-scale, message-passing distributed systems, as it simplifies the design of distributed applications, e.g., State Machine Replication (SMR). The VS property ensures that two or more processors that participate in two consecutive communicating groups should have delivered the same messages. Self-stabilizing systems [1,3] can tolerate transient faults that drive the system to an unpredicted arbitrary configuration. Such sys- tems automatically regain consistency from any such configuration, and then produce the desired system behavior ensuring it for a practically infinite number of successive steps, e.g., 264 steps. We present the first, to our knowledge, self-stabilizing virtual synchrony algorithm

Hovering data clouds: A decentralized and self-organizing information system

Abstract. With ever-increasing numbers of cars, traffic congestion on the roads is a very serious economic and environmental problem for our modern society. Existing technologies for traffic monitoring and management require stationary infrastructure. These approaches lack flexibility with respect to system deployment and unpredictable events (e.g., accidents). Moreover, the delivery of traffic reports from radio stations is imprecise and often outdated. In the project AutoNomos we aim at developing a decentralized system for traffic monitoring and managing, based on vehicular ad-hoc networks (VANETs). Our objective is to design a system for traffic forecasting that can deliver faster and more appropriate reactions to unpredictable events. In our design, cars collect traffic information, extract the relevant data, and generate traffic reports. A key concept are so-called Hovering Data Clouds (HDCs), which are based on the insight that many crucial structures in traffic (e.g., traffic jams) lead an existence that is independent of the individual cars they are composed of. The result is an elegant, robust and self-organizing distributed information system. In this paper we demonstrate first experimental results

Interacting Urns Processes for Clustering of Large-Scale Networks of Tiny Artifacts

We analyze a distributed variation on the Pólya urn process in which a network of tiny artifacts manages the individual urns. Neighboring urns interact by repeatedly adding the same colored ball based on previous random choices. We discover that the process rapidly converges to a definitive random ratio between the colors in every urn. Moreover, the rate of convergence of the process at a given node depends on the global topology of the network. In particular, the same ratio appears for the case of complete communication graphs. Surprisingly, this effortless random process supports useful applications, such as clustering and computation of pseudo-geometric coordinate. We present numerical studies that validate our theoretical predictions

Changing lanes on a highway

We study a combinatorial optimization problem that is motivated by the scenario of autonomous cars driving on a multi-lane highway: some cars need to change lanes before the next intersection, and if there is congestion, cars need to slow down to make space for those who are changing lanes. There are two natural objective functions to minimize: (1) how long does it take for all traffic to clear the road, and (2) the total number of maneuvers. In this work, we present an approximation algorithm for solving these problems in the two-lane case and a hardness result for the multi-lane case.Peer reviewe

Self-stabilizing Uniform Reliable Broadcast

International audienceWe study a well-known communication abstraction called Uniform Reliable Broadcast (URB). URB is central in the design and implementation of fault-tolerant distributed systems, as many non-trivial fault-tolerant distributed applications require communication with provable guarantees on message deliveries. Our study focuses on fault-tolerant implementations for time-free messagepassing systems that are prone to node-failures. Moreover, we aim at the design of an even more robust communication abstraction. We do so through the lenses of self-stabilization|a very strong notion of fault-tolerance. In addition to node and communication failures, self-stabilizing algorithms can recover after the occurrence of arbitrary transient faults; these faults represent any violation of the assumptions according to which the system was designed to operate (as long as the algorithm code stays intact). This work proposes the first self-stabilizing URB solution for time-free message-passing systems that are prone to node-failures. The proposed algorithm has an O(bufferUnitSize) stabilization time (in terms of asynchronous cycles) from arbitrary transient faults, where bufferUnitSize is a predefined constant that can be set according to the available memory. Moreover, the communication costs of our algorithm are similar to the ones of the non-self-stabilizing state-ofthe-art. The main differences are that our proposal considers repeated gossiping of O(1) bits,messages and deals with bounded space (which is a prerequisite for self-stabilization). Specifically, each node needs to store up to bufferUnitSize n records and each record is of size O(v + n log n) bits, where n is the number of nodes in the system and v is the number of bits needed to encode a single URB instance

Relocation analysis of stabilizing MAC algorithms for large-scale mobile ad hoc networks

11 This work improves the understanding on impossibility results, the possible trade-offs and the analysis 12 of fault-tolerant algorithms in MANETs. As the first result in the paper and as motivation for the ones 13 that follow, we show that there is no efficient deterministic MAC algorithm for MANETs. Moreover, we 14 prove a lower bound of the throughput in the radical settings of complete random relocation between 15 every two steps of the algorithm. The lower bound matches the throughput of a strategy that is oblivious 16 to the history of broadcasts. 17 Subsequently, we focus on the analysis of non-oblivious strategies and assume a bound on the rate 18 by which mobile nodes relocate, i.e., randomly changing their neighborhoods. Our analysis is the first to 19 demonstrate a novel throughput-related trade-off between oblivious and non-oblivious strategies of MAC 20 algorithms that depends on the relocation rate of mobile nodes. We present a non-oblivious strategy 21 that yields a randomized, fault-tolerant algorithm that can balance between the trade-offs. The studied 22 algorithm is the first of is kind because it is a &quot;stateful&quot; one that quickly converges to a guaranteed 23 throughput