# Network Allocation Vector (NAV) Optimization for Underwater Handshaking-Based Protocols
## 1 Introduction  
Underwater communication has been receiving tremendous attention due to its application in underwater areas where it is difficult for humans to be constantly present. Such applications include marine resource exploration, pollution monitoring, surveillance, and oceanic environment observations. The unique characteristic of underwater channel limits the use of radio frequency (RF) waves in underwater networks. That is because RF waves experience extensive attenuation and fading losses in liquid media. Therefore, acoustic waves have been the preferred form of communication in underwater networks, as the acoustic waves are less affected by the aforementioned losses and can be used over long distances [1]. Generally, the networks that use acoustic waves are called underwater acoustic sensor networks (UWASNs). Nevertheless, acoustic waves are limited in operational bandwidth, suffer from a higher bit error rate (BER), and introduce a relatively large propagation delay compared to RF waves of terrestrial sensor networks [2]. To achieve higher throughputs in UWASNs, the requirement of an efficient underwater medium access control (MAC) protocol that can handle large propagation delays cannot be ignored [3; 4]. Contention-based MAC protocols are highly preferred for UWASNs as they have the advantage of utilizing the full bandwidth of the underwater channel [5]. They are classified into random-access protocols and handshaking protocols. Several underwater random-access protocols have been proposed such as the slotted ALOHA (S-ALOHA), ALOHA with carrier sense (ALOHA-CS), underwater wireless acoustic networks MAC (UWAN-MAC), and tone-Lohi (T-Lohi) [6; 7; 8; 9]. While these protocols are simple, collisions are still inevitable when multiple nodes try to access the channel simultaneously as they do not explicitly reserve the channel. On the other hand, handshakingprotocols reserve the channel by exchanging several control packets between nodes, thereby avoiding collisions. This makes handshaking protocols more suitable for UWASNs, and is evident from the large amount literature available--MACA for underwater (MACA-U), propagation-delay-tolerant collision avoidance protocol (PCAP), APCAP, receiver-initiated packet train (RIPT), delay-aware opportunistic transmission scheduling (DOTS), CS-MAC, reservation-based MAC (R-MAC), ROPA, Spatially fair MAC (SF-MAC), floor acquisition multiple access (FAMA), contention-based parallel reservation MAC (COPE-MAC), cascading multi-hop reservation and transmission (CMRT), hybrid sender- and receiver-initiated protocol (HSR), dynamic NAV (DYNAV) and carrier sense multiple access/collision avoidance for underwater communications (UW-CSMA/CA) [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. To avoid collisions, handshaking protocols rely on a system parameter called the network allocation vector (NAV). The NAV contains information on the duration for which the channel will be busy and allows a neighboring node to stay silent for that duration. If a neighboring node does not stay silent for that particular duration, then collisions may occur resulting from what is commonly known as the hidden-node problem. In terrestrial RF networks, the propagation delay is infinitesimally small and therefore the NAV duration is precisely equal to the communication time between a source and a destination [26]. However, in UWASNs where the propagation delay is large, the NAV duration must also include the propagation delay or else ignoring the propagation delay can result in an inaccurately small NAV duration. This may allow the hidden nodes to cause collisions as the hidden node will wake up before the existing communication is completed. A couple of studies have proposed a modified NAV that considers the propagation delays in underwater acoustic channels [24; 25]. In [25], the authors proposed UW-CSMA/CA (CSMA/CA for underwater communication) in which an NAV contains the maximum propagation delay of a network, thereby avoiding collisions.