# A MAC Protocol to Support Monitoring of Underwater Spaces +
## 2 Related Work  
Underwater acoustic sensor networks (UWASN) has several aspects to consider, which are not yet defined, like the physical layer selection, link layer protocols and network and transport layer protocols [6; 7; 8; 9]. The literature shows three main approaches to implement the medium access control protocol (MAC): frequency division multiple-access (FDMA), carrier sense multiple access with collision avoidance (CSMA/CA) and time division multiple access (TDMA) [10]. The first one is not suitable for underwater acoustic transmission, due to the narrow bandwidth in underwater acoustic channels and the vulnerability of limited band systems to fading and multi-path. CSMA/CA has been proposed in a previous work [11; 12; 13], but it is limited by the hidden and exposed station problem and the high propagation and variable delay. Long time periods are needed in each message transmission to guarantee that there have been no collisions degrading the overall performance. Finally, the TDMA schemes have problems with the synchronization, delay and unused bandwidth. However, for real-time transmissions, it is the only option that can provide bounded delays, although it is necessary to leave several slots empty. In this section, we discuss previous work mainly related to the medium access mechanism and its appropriateness for being used in underwater acoustic sensor networks. In [14], the authors proposed multiple access collision avoidance (MACA), a protocol based on the use of short messages request to send (RTS) and clear to send (CTS), followed by the sequence of messages DATA and ACK. This mechanism has been successfully used in the Seaweb project [15]. Before sending data, the node should reserve the channel by issuing an RTS, and after that, it has to wait for the CTS answer. Nearby stations also listen to the request and wait for the answer. In the case that a neighbor station does not listen to the CTS answer, it means there is no interference in the receiver and that the transmission may proceed solving in this way the exposed problem. Any station, other than the original RTS sender, on hearing CTS will defer its transmission. In the case of collisions of RTS messages, the nodes use a binary backoff algorithm to solve the conflict. In [16], the authors extend the MACA protocol and named it MACA for wireless (MACAW). They propose a less aggressive backoff algorithm for the link layer introducing the sequence RTS-CTS-DATA-ACK. In lightly-loaded scenarios, this mechanism has less performance, but with high loads, it has much better throughput and fairer allocation. In [17], another MACA-based protocol is proposed. In this case, the floor acquisition multiple access protocol requires that every transmitting station should acquire the floor control (in a wireless channel), before sending any data packet. Both the sender and the receiver should perform the collision avoidance to guarantee the control of the channel. In [18], the authors introduce the UW-MAC for acoustic underwater sensor networks. The MAC scheme is based on CDMA with a novel closed-loop distributed algorithm to set the optimal transmit power and code length to minimize the near-far effect. The algorithm compensates the multi-path effect by exploiting the time diversity in the underwater channel, thus achieving high channel reuse and a low number of packet retransmissions. The protocol works both in deep and shallow water and involving static or mobile nodes. Self-organizing TDMA (SO-TDMA) [19] and ad-hoc self-organizing TDMA (ASO-TDMA) [10] use a self-organized TDMA scheme for the access control. However, these protocols were conceived for VHF radio frequencies, and they are used for the localization of vessels and ships in the ocean. Finally, TDMA-based MAC underwater acoustic network protocols strongly need time synchronization to properly operate.