# Network Allocation Vector (NAV) Optimization for Underwater Handshaking-Based Protocols
## 2 Problem Statement  
Figure 1 shows the NAV determination along with the handshaking procedure in a typical carrier sense multiple access/collision avoidance (CSMA/CA) protocol such as in UW-CSMA/CA and DYNAV. \(\tau(i,j)\) and \(\tau_{max}\) denote the propagation delay between nodes \(i\) and \(j\), and the maximum propagation delay in the network, respectively. The handshaking takes place by exchanging packets in an orderly manner--RTS, clear-to-send (CTS), data (DATA), and acknowledgement (ACK). On overhearing the control packets such as RTS or CTS, the neighbors establish the NAV based on the transmission timing information contained in the packets, and later revise the NAV on overhearing a new control packet. The propagation delay between the source and destination is required to determine the NAV. However, because the exact propagation delay between the source and destination is unknown, the neighbors should estimate the value. In case of UW-CSMA/CA, the maximum propagation delay \(\tau_{max}\) is used as shown in the Figure 1, where the neighbor sets the NAV on overhearing the CTS (point A) as:  
\[NAV_{UW-CSMA/CA}^{CTS}=3\tau_{max}+T_{DATA}+T_{ACK} \tag{1}\]  
where \(NAV_{Y}^{X}\) denotes the NAV calculated by overhearing the packet X of scheme Y, \(3\tau_{max}\) corresponds to the sum of the three-way propagation delays associated with the CTS, data, and ACK, and \(T_{X}\) denotes the transmission time of packet X. \(\tau_{max}\) is the maximum propagation delay that can exist between two nodes in the network. It is shown that the NAV is unnecessarily long, which tends to degrade the channel utilization and throughput. In an effort to reduce the NAV, DYNAV [24] replaces \(\tau_{max}\) with the source-destination propagation delay \(\tau(S,D)\). Thus,  
\[NAV_{DYNAV}^{CTS}=3\tau(S,D)+T_{DATA}+T_{ACK} \tag{2}\]  
The use of \(\tau(S,D)\) definitely reduces NAV. However, considering the long propagation delay in the acoustic channel, the NAV is still too large compared to the communication time and should be reduced further. Let us consider a time instant, denoted by point B in Figure 1, such that the neighbor may transmit a new RTS even during data overhearing without causing collisions at the source or destination [27; 28]. That is, the neighbor can wake up before the NAV expires. In this paper, we utilize this feature to reduce the NAV.  
Figure 1: Example of network allocation vector (NAV) determination in conventional schemes and a highlight on the possibility of an early wake-up for a neighbor (denoted by point B). RTS: request-to-send; CTS: clear-to-send; DATA: data; ACK: acknowledgement; \(\tau_{max}\): maximum propagation delay; \(\tau(S,D):\) source-destination propagation delay; DYNAV: dynamic NAV; UW-CSMA/CA: carrier sense multiple access/collision avoidance for underwater communications.  
Another issue caused by the long propagation delay in underwater acoustic channels is the unfairness between nodes. Figure 2 shows a situation where the two neighbors 1 and 2 are waiting to access the channel. In case of a constant NAV, Neighbor 2, located farther away from the destination, overhears the CTS later than Neighbor 1, and accordingly, wakes up later. Thus, as long as Neighbor 1 has data to transmit, it occupies the channel, before Neighbor 2. This causes a severe unfairness problem.