# An Efficient Scalable Scheduling MAC Protocol for Underwater Sensor Networks +
## 2 Related Work
Delay-aware Opportunistic Transmission Scheduling (DOTS) [34] is another handshaking-based approach, which utilises the network topology information and the handshaking mechanism to improve the protocol's performance. R-MAC [6] is another handshaking-based method mainly designed for energy efficiency and fairness. In this technique, packet collision is entirely avoided by accurately scheduling the transmissions of control and data packets. R-MAC not only avoids data packet collision, but can solve the exposed terminal problem inherited from the RTS/CTS-based protocols. By solving this problem,it can ideally save more energy together with supporting the fairness. In this protocol, instead of using RTS/CTS exchange control packets to avoid data packets' collisions, the transmission of these packets is scheduled between both sender and receiver sides [37]. Moreover, to reduce energy consumption in the idle state and overhearing, every node operates in the listen and sleep modes periodically [2; 8]. R-MAC trades off high end-to-end delay for energy efficiency and fairness, hence collision avoidance is achieved, but, when the offered traffic increases, only several data packets are transmitted [6]. Accordingly, the random access-based group usually allows sender nodes to transmit packets randomly or after an initial one-way contention [38]. UW-ALOHA [39] is a typical random access-based protocol for UWSNs. The unique features are the automatic repeat request (ARQ) and back-off schemes, which are used to improve the performance of traditional ALOHA in underwater environments. In [16] two enhanced designs for ALOHA in UWSNs are proposed. In ALOHA collision avoidance (ALOHA-CA) protocol, every packet is segmented into two separate parts: a header segment and a data segment. In this model, a sensor node can extract both transmitter and receiver information through a short overhearing. ALOHA with advanced notification (ALOHA-AN) is another random access-based approach, which includes sending a short data packet before the actual data transmission within the information of both sender and receiver [40]. A similar approach called T-Lohi is proposed in [41], which employs a tone-based contention mechanism to detect collisions. Technically, it uses the tone signal, which is short enough to avoid collisions. UWAN-MAC [17] is also a random access-based process, which leverages local synchronisation to determine the time-line of each sensor node for energy efficiency improvement. It provides a suitable method by extending the sleep mode rather than the idle listening mode. This is because the former consumes less energy than that in the latter. In particular, UWAN-MAC is an energy efficient MAC protocol designed for UWSN. For example, when the source node transmits a packet, it informs the destination node when it is assumed to transmit the next packet; then, the neighbourhoods will overhear the packet to prevent any possible collisions [42]. The main issue with this technique is that the spatial-temporal uncertainty and the hidden terminal problem are not properly detected; therefore, it consumes significantly more energy because of the collisions and retransmissions.