## 5 Higher Layer Protocol Evaluation
### MAC Protocols  
In the first test we choose two MAC protocols: CSMA and CSMA/CA. Although they seem to be very similar approaches, CSMA it is a simple version with no signaling to handle a packet transmission, meanwhile CSMA/CA is a 4-way handshake protocol as defined in Section 3. Our purpose is to analyze how these MAC protocols tackle a network deployment with different power transmission policies, clearing up where it is worth to focus the efforts in terms of power consumption, throughput, packet delay, _etc._ The simulation parameters are the same as in Table 1 increasing the global load up to 12 packets/s. In Figure 12(a), we can see that CSMA-OH soon reaches its highest performance, and after saturation point its goodput performance degrades very quickly reaching a near network starvation state. However, CSMA-MH follows the same pattern with a smoother curve. This behavior in OH scenario can be easily explained because at lower loads the gateway receives more or less the same number of packets from all network nodes, while in MH scenarios the effect of hot-spot traffic  
Figure 11: Network operational modes. pattern leads to unbalance this behavior and as a consequence reduce network load in the gateway neighborhood. In turn the CSMA/CA evolution is similar in both strategies, quickly reaching its best performance and keeping it despite the increasing load; having a better overall result in the MH strategy due to the same reasons commented before. It is important to remark that at higher network loads, in all cases but especially in MH, sensors that are closer to the gateway have more chances to achieve successful data packet transmissions than farther nodes (no fair resource sharing due to hot-spot traffic pattern and the inherent large propagation delay). From these results we can state that the MH strategy has an overall better performance, being at the same time more energy efficient since it is able to reduce energy demands to half of the ones required by OH. Also, it was observed that those nodes located at the scenario surroundings will have less probability to successfully deliver packets to gateway, so this issue opens the way to define routing protocols that will balance the overall packet delivery rate between all the sensor nodes with independence of their location. Now, we take a look at the delay behavior shown at Figure 12(b). As stated in Equation (10) the CSMA/CA delay is the result of the acoustic propagation time and the transmission time of the different packets involved in the handshaking, meanwhile in CSMA we only send DATA packets so it is expected that it gets smaller delays. As shown in Figure 12(b) CSMA delays are slightly higher in MH than in OH. This result obeys to the fact that the signal propagation delay between a source node and the gateway is typically smaller than the sum of propagation delays of the paths followed to reach gateway node, plus the time required to send at least \(n\) data packets (where \(n\) is the number of hops to reach gateway) instead of one:  
\[ \begin{split} OH\text{-}Delay=T_{prop}(O)+T_{pkt}(DATA)\\ MH\text{-}Delay=T_{prop}(M1)+T_{prop}(M2)+2*T_{pkt}(DATA)\\ T_{prop}(O)\leq T_{prop}(M1)+T_{prop}(M2)\end{split}  \tag{12}\]  
Figure 12: Goodput (**a**) and delay (**b**) of selected MAC protocols in OH and MH modes. In Figure 13, we show an example involving two network nodes and the gateway. The propagation time from sensor 11 to gateway use to be smaller than the propagation from sensor 11 to 12 plus the propagation from sensor 12 to gateway. In the event where OH and MH strategies suffer the same propagation delay, MH mode would require two data transmission cycles, so the overall packet delay is always longer than in the OH strategy. This fact has a greater influence in ann-way handshaking protocol, like CSMA/CA, where for each data packet transmission (each hop) \(n\) packet propagation delays are required, increasing a lot the overall packet delay.