Analysis of MAC Strategies for Underwater Acoustic Networks

En esta tesis presentamos los protocolos MAC diseñados para redes acústicas subacuáticas, clasificándolos en amplias categorías, proporcionando técnicas de medición de rendimiento y análisis comparativo para seleccionar el mejor algoritmo MAC para aplicaciones específicas. Floor Acquisition Multiple Access (FAMA) es un protocolo MAC que se propuso para redes acústicas submarinas como medio para resolver los problemas de terminales ocultos y expuestos. Una versión modificada, Slotted FAMA, tenía como objetivo proporcionar ahorros de energía mediante el uso de ranuras de tiempo, eliminando así la necesidad de paquetes de control excesivamente largos en FAMA. Sin embargo, se ha observado que, debido al alto retraso de propagación en estas redes, el coste de perder un ACK es muy alto y tiene un impacto significativo en el rendimiento. Los mecanismos MultiACK y EarlyACK han sido analizados para el protocolo MACA, para mejorar su eficiencia. El mecanismo MultiACK aumenta la probabilidad de recibir al menos un paquete ACK al responder con un tren de paquetes ACK, mientras que el mecanismo EarlyACK evita la repetición de todo el ciclo de contención y transmisión de datos RTS / CTS enviando un ACK temprano. En esta investigación se presenta un análisis matemático de las dos variantes, los mecanismos MultiACK y EarlyACK, en Slotted FAMA. La investigación incluye las expresiones analíticas modificadas así como los resultados numéricos. Las simulaciones se llevaron a cabo utilizando ns-3. Los resultados han sido probados y validados utilizando Excel y MATLAB. La evaluación del rendimiento de S-FAMA con dos variantes mostró un factor de mejora del 65,05% en la probabilidad de recibir un ACK correctamente utilizando el mecanismo MultiACK y del 60,58% en la prevención de la repetición del ciclo completo, con EarlyACK. El impacto de este factor de mejora en el retardo, el tamaño del paquete de datos y el rendimiento también se analiza. La energía de transmisión desperdiciada y consumida en los mecanismos MultiACK y EarlyACK se analizan y comparan con S-FAMA. El rendimiento se ha evaluado, alcanzando una mejora en ambos casos, en comparación con S-FAMA. Estos mecanismos tendrán una utilidad práctica en caso de pérdida de ACK, al ahorrar energía y tiempo en períodos críticos. Fecha de lectura de Tesis Doctoral: 28 septiembre 2018.Esta tesis presenta una investigación sobre los protocolos MAC utilizados en la comunicación subacuática para explorar el mundo submarino. Los protocolos MAC ayudan en el acceso al medio compartido y la recopilación de datos de los océanos, para monitorizar el clima y la contaminación, la prevención de catástrofes, la navegación asistida, la vigilancia estratégica y la exploración de los recursos minerales. Esta investigación beneficiará a sectores como las industrias militares, de petróleo y gas, pesquerías, compañías de instrumentación subacuática, organismos de investigación, etc. El protocolo MAC afecta la vida útil de las redes inalámbricas de sensores. La eficiencia energética de las redes acústicas submarinas se ve gravemente afectada por las propiedades típicas de la propagación de las ondas acústicas. Los largos retrasos de propagación y las colisiones de paquetes de datos dificultan la transmisión de los paquetes de datos, que contienen información útil para que los usuarios realicen tareas de supervisión colectivas. El objetivo de este estudio es proponer nuevos mecanismos para protocolos MAC diseñados para funcionar en redes acústicas submarinas, con el propósito de mejorar su rendimiento. Para alcanzar ese objetivo es necesario realizar un análisis comparativo de los protocolos existentes. Lo que además sienta un procedimiento metodológicamente correcto para realizar esa comparación. Como la comunicación subacuática depende de ondas acústicas, en el diseño de los protocolos de MAC submarinos surgen varios desafíos como latencia prolongada, ancho de banda limitado, largas demoras en la propagación, grandes tasas de error de bit, pérdidas momentáneas en las conexiones, severo efecto multicamino y desvanecimientos. Los protocolos MAC terrestres, si se implementan directamente, funcionarán de manera ineficiente

Latency-Optimized and Energy-Efficient MAC Protocol for Underwater Acoustic Sensor Networks: A Cross-Layer Approach

Considering the energy constraint for fixed sensor nodes and the unacceptable long propagation delay, especially for latency sensitive applications of underwater acoustic sensor networks, we propose a MAC protocol that is latency-optimized and energy-efficient scheme and combines the physical layer and the MAC layer to shorten transmission delay. On physical layer, we apply convolution coding and interleaver for transmitted information. Moreover, dynamic code rate is exploited at the receiver side to accelerate data reception rate. On MAC layer, unfixed frame length scheme is applied to reduce transmission delay, and to ensure the data successful transmission rate at the same time. Furthermore, we propose a network topology: an underwater acoustic sensor network with mobile agent. Through fully utilizing the supper capabilities on computation and mobility of autonomous underwater vehicles, the energy consumption for fixed sensor nodes can be extremely reduced, so that the lifetime of networks is extended

LTDA-MAC v2.0 : Topology-Aware Unsynchronized Scheduling in Linear Multi-Hop UWA Networks

This paper investigates the use of underwater acoustic sensor networks (UASNs) for subsea asset monitoring. In particular, we focus on the use cases involving the deployment of networks with line topologies, e.g., for monitoring oil and gas pipelines. The Linear Transmit Delay Allocation MAC (LTDA-MAC) protocol facilitates efficient packet scheduling in linear UASNs without clock synchronization at the sensor nodes. It is based on the real-time optimization of a packet schedule for a given network deployment. In this paper, we present a novel greedy algorithm for real-time optimization of LTDA-MAC schedules. It produces collision-free schedules with significantly shorter frame duration, and is 2–3 orders of magnitude more computationally efficient than our previously proposed solution. Simulations of a subsea pipeline monitoring scenario show that, despite no clock synchronization, LTDA-MAC equipped with the proposed schedule optimization algorithm significantly outperforms Spatial TDMA

Scalable adaptive networking for the Internet of Underwater Things

Internet of Underwater Things (IoUT) systems comprising tens or hundreds of underwater acoustic communication nodes will become feasible in the near future. The development of scalable networking protocols is a key enabling technology for such IoUT systems, but this task is challenging due to the fundamental limitations of the underwater acoustic communication channel: extremely slow propagation and limited bandwidth. The aim of this paper is to propose the JOIN protocol to enable the integration of new nodes into an existing IoUT network without the control overhead of typical state-of-the-art solutions. The proposed solution is based on the capability of a joining node to incorporate local topology and schedule information into a probabilistic model that allows it to choose when to join the network to minimize the expected number of collisions. The proposed approach is tested in numerical simulations and validated in two sea trials. The simulations show that the JOIN protocol achieves fast convergence to a collision-free solution, fast network adaptation to new nodes, and negligible network disruption due to collisions caused by a joining node. The sea trials demonstrate the practical feasibility of this protocol in real UAN deployments and provide valuable insight for future work on the trade-off between control overhead and reliability of the JOIN protocol in a harsh acoustic communication environment

Self-organizing Fast Routing Protocols for Underwater Acoustic Communications Networks

To address this problem, in this thesis we propose a cross-layer proactive routing initialization mechanism that does not require additional measurements and, at the same time, is energy efficient. Two routing protocols are proposed: Self-Organized Fast Routing Protocol for Radial Underwater Networks (SOFRP) for radial topology and Self-organized Proactive Routing Protocol for Non-uniformly Deployed Underwater Networks (SPRINT) for a randomly deployed network. SOFRP is based on the algorithm to recreate a radial topology with a gateway node, such that packets always use the shortest possible path from source to sink, thus minimizing consumed energy. Collisions are avoided as much as possible during the path initialization. The algorithm is suitable for 2D or 3D areas, and automatically adapts to a varying number of nodes. In SPRINT the routing path to the gateway is formed on the basis of the distance, measured by the signal strength received. The data sending node prefers to choose the neighbor node which is closest to it. It is designed to achieve high data throughput and low energy consumption of the nodes. There is a tradeoff between the throughput and the energy consumption: more distance needs more transmission energy, and more relay nodes (hops) to the destination node affects the throughput. Each hop increases the packet delay and decreases the throughput. Hence, energy consumption requires nearest nodes to be chosen as forwarding node whereas the throughput requires farthest node to be selected to minimize the number of hops. Fecha de lectura de Tesis Doctoral: 11 mayo 2020Underwater Wireless Sensor Networks (UWSNs) constitute an emerging technology for marine surveillance, natural disaster alert and environmental monitoring. Unlike terrestrial Wireless Sensor Networks (WSNs), electromagnetic waves cannot propagate more than few meters in water (high absorption rate). However, acoustic waves can travel long distances in underwater. Therefore, acoustic waves are preferred for underwater communications, but they travel very slow compare to EM waves (typical speed in water is 1500 m/s against 2x10^8 m/s for EM waves). This physical effect makes a high propagation delay and cannot be avoided, but the end-to-end packet delay it can be reduced. Routing delay is one of the major factors in end-to-end packet delay. In reactive routing protocols, when a packet arrives to a node, the node takes some time to select the node to which the data packet would be forwarded. We may reduce the routing delay for time-critical applications by using proactive routing protocols. Other two critical issues in UWSNs are determining the position of the nodes and time synchronization. Wireless sensor nodes need to determine the position of the surrounding nodes to select the next node in the path to reach the sink node. A Global Navigation Satellite System (GNSS) cannot be used because of the very short underwater range of the GNSS signal. Timestamping to estimate the distance is possible but the limited mobility of the UWSN nodes and variation in the propagation speed of the acoustic waves make the time synchronization a challenging task. For these reasons, terrestrial WSN protocols cannot be readily used for underwater acoustic networks

Network protocols and time synchronization for underwater acoustic networks

Ph.DDOCTOR OF PHILOSOPH

Medium Access Control in Distributed Networks with Large Propagation Delay

Most of the Earth is covered by water, so underwater acoustic networks (UWANs) are becoming increasingly popular in a variety of undersea applications. The needs to understand the underwater environment and exploit rich undersea resources have motivated a further development of UWANs. Underwater acoustic signals suffer from more difficult physical channel phenomena than terrestrial radio signals due to the harsh underwater environment, such as sound absorption, time-varying multipath spread, man-made and ambient noise, temperature and pressure dependent refraction, scattering and Doppler shift. Among all the challenges, the large ratio of propagation delay to packet duration (relative propagation delay (a)) is arguably the most difficult one to address in the Medium Access Control (MAC) layer. In this dissertation we focus on the examination and improvement of the MAC layer function in UWANs, based on a critical examination of existing techniques. Many MAC techniques have been proposed in recent years, however most of them assume the ratio of the propagation delay to the packet duration is negligibly small (a>1), these protocols perform poorly. This is because the large a leads to both a large negotiation delay in handshaking based protocols and the space-time uncertainty problem as the packets do not arrive at each node contemporarily. Some underwater-oriented protocols have been proposed which attempt to address these issues but the more successful rely on master nodes or a common understanding of geometry or time. We show by analysis and simulation that it is possible to eliminate collisions in ad-hoc networks with large relative propagation delay (a>>1) as well as improving the channel utilisation, without a common understanding of geometry or time. This technique is generally applicable, even for truly ad-hoc homogeneous peer-to-peer networks with no reliance on master nodes or other heterogeneous features. The mechanism is based on future scheduling with the inclusion of overhearing of RTS messages and allowing third-party objections to proposed transmissions. This MAC mechanism is immediately applicable in underwater acoustic networks (UWANs), and may find other uses, such as in space or very high rate terrestrial wireless networks. In summary, the key contributions of this study are: investigating the causes of the poor performance of existing MAC protocols in ad-hoc UWANs with large relative propagation delay, fully detailing the problem in order to propose analytic solutions, demonstrating how the MAC layer of an ad-hoc UWAN can eliminate packet collisions as well as improve channel utilisation without time synchronization or a network’s self-configuring phase to gain knowledge of the geometry, and verifying the utility of the proposals via both theoretical analysis and simulations

TIME DOMAIN MEDIUM ACCESS CONTROL PROTOCOLS FOR UNDERWATER ACOUSTIC NETWORKS

Ph.DDOCTOR OF PHILOSOPH