The contention-free protocols include time division multiple access (TDMA), frequency division multiple access (FDMA) and code division multiple access (CDMA), in which different time slots, frequency bands or codes are assigned to different users to avoid collisions among transmissions.FDMA divides the available frequency band into several sub-bands and assigns each sub-band to a node. Due to the limited available bandwidth of underwater channels, FDMA is not suitable for UASNs that consist of a large number of underwater sensors.In TDMA, in order to avoid the collision of packets from adjacent time slots, guard times are added to the time slot. The high propagation delay in underwater acoustic communication channels requires long guard times, which limit the efficiency of TDMA [30].

Moreover, TDMA systems require precise synchronization for proper utilization of the time slots.It is also known that CDMA-based protocols require a high complexity design for UASN. In addition, it is a challenging problem to assign pseudo-random codes to a large number of sensor nodes [2].On the other hand, contention-based protocols have received significant attention for UASN, due to their simplicity, acceptable throughput and energy efficiency [15�C23]. For example, the authors of [15] studied the performance of Aloha-based protocols in underwater networks and proposed two enhanced schemes that take advantage of the long propagation delay in the underwater acoustic channel and do not require handshaking or time synchronization.

It was also shown that, under the high and varying propagation delay in underwater acoustic channels, the performance of slotted Aloha becomes similar to that of pure Aloha [23]. The study in [16] proposed a propagation delay-tolerant Aloha protocol, where the authors address the space-time uncertainty by adding guard times to slotted Aloha.Another simple yet practical Aloha-based protocol, Aloha-CS (Aloha with carrier sensing), was also studied and evaluated in [15,18�C20]. In Aloha-CS, a node senses the carrier on the channel before it transmits data. The intended receiver sends an acknowledgment (ACK) packet to the source node to announce the successful reception. For unsuccessful transmissions, the retransmission mechanism with an exponential backoff can be also applied, i.e.

, the data packet can be retransmitted up to GSK-3 a maximum limit of retries unless an ACK packet is received at the source node. According to the results presented in these studies, Aloha-CS (Aloha with carrier sensing) [18�C20] can achieve high throughput and low latency without requiring time synchronization or handshaking.The authors of [21] proposed an extension of the FAMA protocol [31] for UASN, namely slotted FAMA. Slotted FAMA is also based on carrier sensing and handshaking prior to data transmission.