Aloha-based MAC Protocols with Collision Avoidance for Underwater Acoustic Networks Nitthita Chirdchoo, Wee-Seng Soh, Kee Chaing Chua Department of Electrical & Computer Engineering National University of Singapore, Singapore Email: {g0500102, elesohws, eleckc}@nus.edu.sg Abstract— Unlike terrestrial networks that mainly rely on radio waves for communications, underwater networks utilize acoustic waves, which have comparatively lower loss and longer range in underwater environments. However, the use of acoustic waves pose a new research challenge in the networking area. While existing network schemes for terrestrial sensor networks are mainly designed for negligible propagation delay and high data rate, underwater acoustic communications are characterized by high propagation delay and low data rate. These terrestrial schemes, when directly applied to the underwater channel, will under-utilize its already limited capacity. We investigate how the underwater channel’s throughput may be enhanced via medium access control (MAC) techniques that consider its unique characteristics. Specifically, we study the performance of Aloha-based protocols in underwater networks, and propose two enhanced schemes, namely, Aloha with collision avoidance (Aloha-CA), and Aloha with advance notification (Aloha-AN), which are capable of using the long propagation delays to their advantage. Simulation results have shown that both schemes can boost the throughput by reducing the number of collisions, and, for the case of Aloha-AN, also by significantly reducing the number of unproductive transmissions. I. I NTRODUCTION Unlike the terrestrial wireless sensor networks that mainly rely on radio waves for communications, underwater sensor networks utilize acoustic waves, which present a much harsher environment for both the physical and the data-link layers. Acoustic waves appear to be a good choice for underwater communications because of their low loss when compared to radio waves. However, one major disadvantage is that acoustic waves travel at approximately 1500 m/s, which is five orders of magnitude slower than radio waves. Moreover, the underwater acoustic channel’s bandwidth is very limited, typically in the order of several kilohertz. These undesirable characteristics are most significant at the data-link layer, because of the long propagation delay and packet transmission time. Currently, the research efforts in underwater MAC protocols are still in their infancy stage. Some work in the literature, such as [1], has adopted a centralized control approach, which requires a master node to configure the data scheduling, and pass the control messages to its slaves. On the other hand, the distributed control approach, in which each node decides on its own whether to send out a packet, appears to be more attractive. In [2], Rodoplu and Park propose a MAC protocol that achieves energy efficiency by reducing the number of collisions. Each node schedules by itself the time to transmit the next packet, and broadcasts this information by attaching it to the current data packet. Upon hearing the broadcast, the other nodes will know when to wake up for the subsequent packet. However, in order to operate at a low collision rate, each node requires a small duty cycle, which makes throughput low. In [3], Morns et al. propose two scheduling protocols to control data packet transmission and arrival times. One protocol is based on CDMA, while the other one is based on TDMA. However, both protocols require clock synchronization between all the nodes. Also, the time slot allocation for individual nodes becomes hard to manage when the number of nodes grow. Guo et al. introduce the propagation-delay-tolerant collision avoidance protocol (PCAP) in [4], which is a handshaking-based protocol. It also requires clock synchronization between neighboring nodes. Besides the requirement of request-to-send (RTS) and clear- to-send (CTS) frames, the uniqueness is that it allows a sender to perform other actions during the long wait between the RTS and CTS frames. Although its maximum throughput is 20% higher than what the conventional handshaking protocol can achieve in underwater, this is merely comparable to Aloha’s throughput. Molins and Stojanovic propose in [5] a slotted random access MAC protocol, which, yet again, requires clock synchronization. It is also handshaking-based, but an RTS or CTS frame can only be transmitted at the beginning of each time slot. Although the protocol achieves guaranteed collision avoidance for its data packets, the long slot length requirement and the handshaking mechanism itself affect the throughput. This is also supported by the work in [6]. The protocols above have generally focused on reducing or eliminating packet collisions, but have placed little emphasis on achieving high throughput. Those that employ handshaking inevitably amplify the effect of long propagation delay, which restricts the throughput. On the other hand, those that rely on time slot allocation generally require slot lengths that are larger than the maximum propagation delay, which again affect the throughput, in addition to problems due to clock drift. These led us to ponder whether simpler MAC protocols may be, in fact, more capable of achieving high throughput and low collision rate, in the face of peculiar underwater acoustic properties. In this paper, we study Aloha-based variant proto- cols and propose two Aloha-based random access MAC pro- tocols, namely, Aloha with collision avoidance (Aloha-CA), and Aloha with advance notification (Aloha-AN).