Recently, various Medium Access Control (MAC) protocols have been proposed for underwater acoustic networks. 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, drastically degrade the performance of existing MAC protocols in the real world. A new practical MAC design is demanded. Toward a proper approach, this paper analyzes the impact of the two newly found modem characteristics on the random access-based MAC and handshakebased MAC, which are two major types of MAC protocols for underwater acoustic networks. We further develop the nodal throughput and collision probability models for representative solutions of these two MAC protocol types. Based on the analysis, 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.

The underwater acoustic channel is characterized by a path loss that is dependent on both the distance and the frequency of communication. Given this dependence, it has been previously demonstrated that for a given communication distance, there is an optimal operating frequency, where conditions for signal propagation and noise are most favorable. In this work, we consider extending this optimal frequency concept to scenarios in which the frequencies that can be employed by the system are constrained. Such constraints are important considerations for practical system design. The first problem we study is to find a single frequency that minimizes the energy over a number of links of varying lengths. An approximate model for this frequency is proposed that is very close to the true optimal. We then generalize this problem to finding the best frequency band, within which the frequency can be tuned for different link lengths. We demonstrate how our model is applied to a 2-D network scenario. We simulate random node placement for such a network, and we observe that the optimal frequencies are very close to the proposed model.

In this paper, we investigate the multichannel MAC problem in underwater acoustic sensor networks. To reduce hardware cost, only one acoustic transceiver is often preferred on every node. In a single-transceiver multichannel long-delay underwater network, new hidden terminal problems, namely, multichannel hidden terminal and long-delay hidden terminal (together with the traditional multihop hidden terminal problem, we refer to them as "triple hidden terminal problems”), are identified and studied in this paper. Based on our findings, we propose a new MAC protocol, called CUMAC, for long-delay multichannel underwater sensor networks. CUMAC utilizes the cooperation of neighboring nodes for collision detection, and a simple tone device is designed for distributed collision notification, providing better system efficiency while keeping overall cost low. Analytical and simulation results show that CUMAC can greatly improve the system throughput and energy efficiency by effectively solving the complicated triple hidden terminal problems.

In this paper, we investigate the multi-channel MAC problem in underwater acoustic sensor networks. To reduce hardware cost, only one acoustic transceiver is often preferred on every node. In a single-transceiver multi-channel long-delay underwater network, new hidden terminal problems, namely multi-channel hidden terminal and long-delay hidden terminal (together with the traditional multi-hop hidden terminal problem, we refer to them as "triple hidden terminal problems"), are identified and studied in this paper. Based on our findings, we propose a new MAC protocol, called CUMAC, for long delay multi-channel underwater sensor networks. CUMAC utilizes the cooperation of neighboring nodes for collision detection, and a simple tone device is designed for distributed collision notification, providing better system efficiency while keeping overall cost low. Analytical and simulation results show that CUMAC can greatly improve the system throughput and energy efficiency via effectively solving the complicated triple hidden terminal problems.

All-IP based wireless WAN can be a system beyond 3G. A simplified all-IP wireless WAN architecture is defined in this paper. Some existing MAC protocols, which are classified into serial transmission MAC protocol and parallel transmission MAC protocol, can be potential MAC for all-IP wireless WAN. This paper analyzes these protocols and proposes a prioritized parallel transmission MAC protocol as a better candidate than those existing ones.