Zaihan Jiang

Toward practical MAC design for underwater acoustic networks

Recently, various medium access control (MAC) protocols have been proposed for underwater acoustic networks (UANs). These protocols have significantly improved the performance of MAC layer in theory. However, two critical characteristics, low transmission rates and long preambles, found in the commercial modem-based real systems, severely degrade the performance of existing MAC protocols in the real world. Thus, a new practical MAC design is demanded. Toward an efficient approach, this paper analyzes the impact of these two modem characteristics on the random access-based MAC and handshake-based MAC, which are two major categories of MAC protocols for UANs. We further develop the nodal throughput and collision probability models for representative solutions of these two MAC protocol categories. Based on the analyses, we believe time sharing-based MAC is very promising. Along this line, we propose a time sharing-based MAC and analyze its nodal throughput. Both analytical and simulation results show that the time sharing-based solution can achieve significantly better performance.

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.

Coding based multi-hop coordinated reliable data transfer for underwater acoustic networks: Design, implementation and tests

Motivated by some critical observations from field tests, in this paper, we design and implement a Coding based multi-hop Coordinated Reliable Data Transfer (CCRDT) protocol for underwater acoustic networks. CCRDT is based on hop-by-hop reliable data transfer with multi-hop coordination. For the per-hop data transfer, a GF(256) Random Linear Coding scheme and a coding ratio estimation approach are proposed to guarantee the reliability and efficiency of data transfer. In addition, a multi-hop coordination scheme is designed to enable transmission pipelining that allows multiple nodes to transmit simultaneously. By carefully scheduling the transmission of every node, our multi-hop coordination scheme effectively eliminates packet collisions, which further improves the end-to-end throughput. We implement CCRDT on a lab testbed and conducted extensive tests to evaluate its performance. Our results show that CCRDT achieves a higher end-to-end throughput than other existing reliable data transfer protocols which have been implemented in real systems.

Evaluating selective ARQ and slotted handshake based access in real world underwater networks

Medium Access Control (MAC) is an essential component of protocol stacks in Underwater Acoustic Networks (UANs). Numerous dedicated UAN MAC protocols have been proposed and studied via analysis and simulations. However, limited work has been done on evaluating these protocols in real ocean environments. To achieve a better understanding on how MAC protocols perform in real world UANs, we implemented Selective ARQ and Slotted Handshake based Access (SASHA) on UAN nodes. SASHA embraces some most essential and representative techniques in UAN MAC design, including selective ARQ, time slotting, handshake and collision avoidance. Moreover, a sea test was conducted at Atlantic Ocean to evaluate the performance of SASHA. With the experimental data, we are able to study how the aforementioned techniques affect the performance of SASHA. we also analyze the hop-by-hop and end-to-end behavior of SASHA. Specifically, we investigate the transmission delay and queuing delay of a data packet on one hop. From the findings, some issues are discovered and the corresponding design guidelines are emerged.

Exploring random access and handshaking techniques in underwater wireless acoustic networks

In this article, we study the medium access control (MAC) problem in underwater wireless acoustic networks. We explore the random access and handshaking (i.e., RTS/CTS) techniques in both single-channel and multi-channel network scenarios. We model and analyze these two approaches, and conduct extensive simulations to study their performance in various network conditions. Based on our results, we observe that the performance of both approaches are affected by many factors such as data rate, propagation delay and packet size. Our results show that the RTS/CTS approach is more suitable for dense networks with high date rate, whereas the random access approach is preferred in sparse networks with low data rate. Our results also demonstrate that multi-channel techniques can potentially help us combat the long delay feature of underwater acoustic channels. However, uncoordinated random channel access cannot fully exploit the advantages of the multi-channel network settings and it performs even worse than the single-channel random access protocol. Only with careful design and coordination such as multi-channel access with RTS/CTS handshaking process, can multi-channel MAC protocols greatly improve the system performance. We believe that this study will provide useful guidelines for efficient MAC design in underwater wireless acoustic networks.

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.