# An Efficient Scalable Scheduling MAC Protocol for Underwater Sensor Networks +
## 2 Related Work  
Packet retransmission is the major source of energy waste, i.e., sending a packet twice, leads to increased energy consumption [22]. Many underwater MAC protocols have recently emerged to overcome this and to cope with the features of underwater acoustic communications. These protocols can be classified into two categories: contention-free and contention-based [23; 24; 25]. In the contention-free category, communication channels are separated into time, frequency or code domains, such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and Code Division Multiple Access (CDMA) [26]. In TDMA, the channel is divided into a number of time slots. All sensor nodes have to remain synchronised to keep reliable transmission schedules by incurring additional control and scheduling packets [27]. UnderWater FLASHR (UW-FLASHR) [28], Spatial-Temporal MAC (ST-MAC) protocol [5], and Staggered TDMA Underwater MAC Protocol (STUMP) [29] are typical TDMA-based protocols for UWSNs. Some other TDMA-like approaches have also been proposed [30; 31]. FDMA divides the frequency band into sub-bands. However, the narrow band of an acoustic channel leads to a low throughput because of the diffuse fading in underwater areas. CDMA is robust against frequency fading and able to improve the network throughput. Thus, receivers can distinguish between signals concurrently transmitted by multiple sensor nodes, which increases channel utilisation and decreases the packet retransmissions. Nevertheless, CDMA is not appropriate for UWSNs because it is difficult to set pseudo-random codes to many sensor nodes. In contrast, the contention-based category can be more manageable and effective with dynamic network topologies and therefore more suitable for UWSNs. This type of protocol can be further classified into two groups, namely handshake-based and random access-based MAC protocols [9; 25; 32]. Protocols in different groups have notable performance on throughput, delay or energy efficiency. In the first group, handshaking-based, source and destination nodes exchange control packets before sending data packets, attempting to avoid any possible collision. In the second group, random access-based, nodes randomly attempt to access the medium. When a data packet arrives at the destination node, if there is no packet coming from other nodes, this node can receive the packet successfully. However, most of the efforts of the MAC protocol design for UWSNs have focused on the handshaking group such as S-FAMA [12], DACAP [33], DOTS [34], and R-MAC [6]. Some variances based on the handshaking group have also been proposed [35; 36]. Slotted Floor Acquisition Multiple Access (S-FAMA) [12] is a handshaking-based protocol, which combines both carrier sensing (CS) and an initiate phase between the source and destination before data transmission. During the initiate phase, RTS/CTS control packets are exchanged between the source and the destination nodes to avoid two or more simultaneous transmissions. This means that S-FAMA uses these kinds of packets (e.g., RTS, CTS, DATA and ACK) before the data packets, which should also be sent at the beginning of a single slot, to schedule sender and receiver sides properly. Despite S-FAMA avoiding data packet conflicts without relying on the size of the packet, it needs a clock synchronisation between sensor nodes, which is difficult to obtain in UWSNs. Moreover, S-FAMA consumes more energy because of idle listening and overhearing. In [33], a Distance-Aware Collision Avoidance Protocol (DACAP) is another handshaking-based protocol, which also merges CS and exchanging RTS/CTS control packets before sending data packets. However, it does not need any concurrence between nodes. Nevertheless, exchanging such control packets, between the source and destination to eliminate data packet collisions, consumes a significant amount of energy in UWSNs.