# An Efficient Scalable Scheduling MAC Protocol for Underwater Sensor Networks +
## 1 Introduction  
Underwater sensor networks (UWSNs) have a promising future in the area of information collection with increasingly more applications in recent years. These applications vary from environmental monitoring and oceanographic data collection to early warning systems, tactical surveillance, assisted navigation, and resource discovery [1; 2; 3; 4]. Similar to other ad hoc networks, a UWSN consists of a number of sensors that are deployed in water to collaboratively perform their tasks over a given area [5]. Unlike terrestrial sensor nodes that rely on radio signals to communicate with each other, the nodes use the acoustic channel as the communication medium with 1500 m/s speed, which is five orders of magnitude lower than that of radio signals. As a result of the lower propagation speed, higher propagation delays occur in communication [6; 7]. In addition, the available bandwidth of acoustic channels is typically less than 15 kHz, which is much narrower compared with that of terrestrial channels. Apart from long propagation delay and narrow bandwidth, energy-efficiency is another primary concern due to difficulties of replacing or recharging batteries. Furthermore, UWSNs usually cover a very largearea of the ocean environment, which leads to sparse deployment [8; 9]. Due to these characteristics of underwater environment, the design of Medium Access Control (MAC) protocols has become a challenging task [10; 11]. The most critical challenge of underwater MAC protocols is perhaps the spatial-temporal uncertainty. In UWSNs, uncertainty in the receiving time which depends on the transmitting time (time uncertainty) as well as the propagation delay to the destination (space uncertainty) [8]. Thus, transmissions time and location (e.g., propagation delay) of sensors should be considered in order to make a collision-free scheduling of transmissions and receptions. Moreover, a collision can also be caused by a hidden terminal. This case occurs when a sensor node cannot sense other nodes that interfere with its transmission. To enable collision-free scheduling of transmissions and receptions, these problems should properly be addressed. To solve these issues, some handshake-based MAC protocols have been proposed [6; 12]. In this approach, a Request-To-Send/Clear-To-Send (RTS/CTS) mechanism is used to reserve the channel. Floor Acquisition Multiple Access (FAMA) [13] lengthens the transmission delays of RTS/CTS control packets to let Multiple Access with Collision Avoidance (MACA) [14] operate in the networks with long propagation delay such that in UWSNs. However, it consumes high energy by transmitting such long control packets. To reduce the energy consumption, S-FAMA [12] introduces a slot technique to FAMA by employing short RTS/CTS control packets while collision in UWSNs is still avoided. For the RTS/CTS mechanism, a one-time process usually reserves the channel for only one sender-receiver pair. Although energy efficiency is extremely improved in this way, the network throughput is usually low because of the high delay in the handshaking phase. To improve the network throughput, random access-based MAC protocols, such as ALOHA and Slotted ALOHA, have been explored [15]. To achieve better performance, several random access-based MAC protocols are particularly designed for UWSNs, such as Aloha with Advance Notification (ALOHA-AN) [16] and UWAN-MAC [17], which employ short control packets without coordination. It means that when a data packet arrives at a receiver, if the receiver is not receiving any other packets and there is no other packet coming in this period, the packet transmission is successful. Thus, collision avoidance is totally probabilistic because they cannot handle the hidden terminal problems [9]. While the above-mentioned classes of MAC protocols are expected to achieve high performance, some recent observations, however, have reported that long propagation delay, narrow bandwidth, and high bit error rate make the contention-based MAC protocols costly [18; 19; 20].