Son N. Le

Comparing underwater MAC protocols in real sea experiments

Underwater acoustic networks (UANs) have drawn significant attention from both academia and industry in recent years. Even though many underwater MAC protocols have been proposed and studied based on simulations and theoretical analysis, few work has been conducted to test and evaluate these protocols in a multi-hop real sea experiment. Due to the harsh acoustic channel condition caused by complex multi-path environment, fast varying acoustic channel and heterogenous channel quality, current simulators can hardly tell us how the protocols work in the real world. Along this direction, we conduced real sea experiments at Atlantic Ocean with 9 nodes deployed forming a multi-hop string network. In this experiment, the performance of three representative MAC protocols, random access based UW-Aloha, handshaking based SASHA, and scheduling based pipelined transmission MAC (PTMAC) are compared and analyzed at both packet behavior and node behavior levels. The end-to-end performance of these three protocols are also tested and studied in terms of throughput, delay, and packet delivery ratio. From field experiment results, the high packet loss rate and significant channel asymmetry, temporal and spatial transmission range uncertainty and delayed data transmissions are discovered to have evidential effects on the MAC performance. We provide some inspirations to address these observed issues in MAC design for real multi-hop networks.

SeaLinx: a multi-instance protocol stack architecture for underwater networking

Advancement in acoustic modem technology has gradually brought underwater network into real life. Despite high cost of sea testing, there have been initial realization effort, such as Sea-Web, Aqua-Net, SUNSET, etc. As the main developers of Aqua-Net, through extensive testing experiences at different scales, we have recognized shortcomings of Aqua-Net and decided to design and implement the next generation of it. In this paper, we introduce SeaLinx as a new protocol stack architecture for underwater networking which (i) enables users to exploit their hardware more efficiently by allowing applications to run simultaneously on a modem, and (ii) provide better support for cross-layer communication.

An OFDM based MAC protocol for underwater acoustic networks

In this paper, we investigate OFDM based MAC protocols for underwater acoustic networks. Due to the severe multi-path effects of underwater acoustic channels, the guard time between OFDM blocks becomes significant, which greatly degrades the system rate. In addition, the highly dynamic nature of underwater acoustic channels makes it hard to choose the optimal transmitting power and modulation mode. Furthermore, the bandwidth of most current OFDM modems is only around tens of kHz, which is much less than the available bandwidth in a network with short range communication links. All these challenges make existing OFDM based MAC protocols for designed terrestrial wireless networks unsuitable. In this work, we propose a new MAC protocol called TDM with FDM over OFDM MAC protocol (TFO-MAC), which smoothly couples TDM with FDM/OFDM for the uplink traffic in a cellular like underwater acoustic network. In TFO-MAC, the acoustic channel is divided into multiple sub-channels in frequency via FDM technology, and OFDM modulation is used in every sub-channel. In addition, time is partitioned into slots and every node can use different channels in different slots. Powerful base stations in the network are responsible for the dynamic channel assignment, power optimization and modulation method selection for all nodes in every slot. We formulate the problem as a mixed integer programming problem and propose an efficient greedy algorithm to solve it. Simulation results show that TFO-MAC can achieve high throughput with good fairness.

NAMS: A networked acoustic modem system for underwater applications

In this paper, we present a networked acoustic modem system (NAMS) by integrating a high-speed OFDM modem and a comprehensive underwater network protocol stack for underwater applications. This integrated system allows different underwater network protocols to run on top of the OFDM modem platform and can provide high-speed, reliable and efficient communications in underwater environments.

PMAC: a real-world case study of underwater MAC

In this paper we document the design, implementation and testing of a scheduling-based MAC protocol, called PMAC, for a 2012s sea trial, and experience gained from this process. In this experiment, the topology was a straight line, and PMAC is specifically design to take advantage of it: network nodes take turn to send and those that are three hops apart are allowed to do so simultaneously. The result shows that although PMACs performance is much better compared to other protocols in the same experiment, there is room for improvement by selecting an appropriate packet size and scheduling scheme. Besides that, the result reveals an important factor which we will show has significant impact on protocol design, yet is usually ignored in theoretical work: modem processing delay. The modem processing delay is in fact not negligible, and increasing with packet size. The paper also presents important lessons from this field test which are useful for similar future efforts.

Underwater delay tolerant routing in action

Underwater Sensor Networks (UWSN) have several applications in aquatic environments ranging from target tracking to environmental monitoring. Harsh nature of underwater environments and different conditions and properties of the medium compared to on-ground environment pose some challenges when it comes to the design of a robust system. One of these challenges is the continuously moving aquatic flows, and at sometimes the autonomously moving nodes, that lead to a continuously varying network topology. When the nodes within the network are sparse, this variation requires special attention when designing a data routing protocol for such networks. Delay tolerant networks (DTN) account for this variation through the allowance of some permissible amounts of delay when transferring the data from a source node to a destination node. In this work, a DTN routing protocol for UWSN is designed. During the design process, the main properties of UWSN were taken into consideration. For example, continuous motion of nodes was dealt with through the use of state tables to keep the one-hop neighbor and hop count information. Additionally, collision avoidance was alleviated through the use of random time intervals between control packet transmissions. The protocol was successfully implemented in Aqua-Net architecture. Lab tests were also conducted successfully using Benthos acoustic modems in the University of Connecticuts UWSN lab. Results demonstrated that data packets can be successfully sent from source to destination with the assistance of auxiliary control packets within permissible delays.

Aqua-3D: An underwater network animator

Research on developing Underwater Sensor Networks (UWSNs) has increased over the last few years. Due to the high costs of deployment and dangerous ocean environments, real-world field tests are not always a viable option. Therefore, a majority of researchers rely on simulations to perform experiments. Visualization of simulation results and field test experiments provide researchers with a vivid animation of the events and conditions that occurred, allowing for a more intuitive understanding of the system. There are many simulators and animators for terrestrial networks but few such tools exist for UWSNs. In this paper, we present a 3D underwater sensor network animator called Aqua-3D. This visualization tool has the ability to read trace files, which can be generated by an existing UWSN simulator or from field test experiments and animate the simulations in full 3D graphics. The accuracy of Aqua-3Ds visualizations have been verified through test scenarios generated from field tests. Aqua-3D is a robust visualization tool with the ability to correctly visualize trace files of underwater network simulations, while providing a number of additional features.

Deployment framework for mobile underwater wireless networks with node reuse

A mobile underwater wireless sensor network is a system of mobile underwater sensors which are acoustically networked. Existing ocean studies using independent mobile sensors have led to many interesting discoveries. Therefore, the networking capability will extend the capability of existing approaches and enable novel applications because it provides more control and coordination of the sensors. However, network mobility and environment uncertainty create numerous challenging issues which need to be addressed before such a paradigm becomes reality. Network deployment is among the most important problems because it has a significant impact on other research areas. In this paper, we consider the reuse-based deployment scheme in which a mobile underwater sensor network operates in a bounded area and a sensor, when reaching beyond the area, is deployed back into it. We propose a deployment framework to study the connectivity and coverage of such networks and show its effectiveness through two case studies. Our framework is based on the idea that the reuse of network nodes stabilizes these network metrics. By modeling them, we can estimate the number of nodes needed to achieve a requirement on network coverage or network connectivity only by solving algebraic equations rather than running more simulations.

Aqua-OS: An Operating System for Underwater Acoustic Networks

Underwater acoustic networks have recently emerged as a promising approach for oceanic applications such as exploration and surveillance. This new type of networks differs from terrestrial wireless sensor networks in that the network nodes are powerful and well equipped with many resources for diverse applications in challenging environments. Existing operating systems for terrestrial wireless sensor networks may not be able to fully utilize the resources available in underwater networks or work efficiently in the underwater environment with diverse application requirements. This calls for a new operating system design for underwater acoustic networks. Motivated by this, we propose a plan to implement an operating system for underwater acoustic networks: Aqua-OS. Aqua-OS is going to be robust, highly customizable and energy efficient to tackle the harsh underwater environment, fully utilize the hardware resources and meet the diverse application requirements.

A control system for OFDM networked autonomous underwater vehicles

Autonomous Underwater Vehicle (AUV) networks are becoming increasingly popular in scientific, commercial, and military applications. AUVs are used in undersea exploration and environmental monitoring for tasks such as detection of oilfields and marine life, distributed tactical surveillance for offshore and seaport defense and mine reconnaissance. AUV networks are also becoming an important interest in an effort to enhance the capabilities of underwater sensor networks (UWSNs). In this paper we propose a control system for networked autonomous underwater vehicles that includes both hardware and software modules. The system integrates various communication and sensor devices, such as an IMU and an OFDM acoustic modem and contains a graphical user interface for optional manual remote control and monitoring of AUVs in the network from a base station.