# Dynamic Cooperative MAC Protocol for Navigation Carrier Ad Hoc Networks: A DiffServ-Based Approach
## 6 Performance Evaluation
### Simulation Results  
#### 6.2.1 Saturated Throughput and Delay in Periodic Communication  
This subsection evaluates the saturated throughput (ST) and average delay (AD) of DDC-MAC in PC scenario; the results are compared to the typical schedule-based MAC protocol (i.e., LMAC) and the typical contention-based MAC protocol (i.e., IEEE802.11b DCF, DMAC). The packet arrival rate (PAR) of each node is 20 _packets/sec_. To consider the impacts of fading effects and the influence of beam direction on saturation, the transmission rate was down-graded with the probabilities \(\beta\) and \(\xi\). Thus, we set \(\beta=0.20\) and \(\xi=0.20\), and increase the number of nodes from 40 to 160; the analysis of saturated throughput can be validated by observing the aggregate uplink throughput.  
Figure 9a compares the ST performance of four protocols with PC service. The results indicate that the throughputs of DDC-MAC, LMAC, and DMAC increase linearly with the increase of network size (\(N_{node}\in[40,160]\)). The reason is that LMAC and DDC-MAC adopt conflict-free TDMA schedule, while DMAC adopts a simple duty-cycle-based operation. Meanwhile, 802.11 DCF obtains the worst throughput performance. When \(N_{node}>80\), the throughput of 802.11 DCF stays at a constant value (\(Th_{def-pc}=780\) kbps), and then decreases slowly when \(N_{node}>100\). The reason for this is that the increasing pairs aggravate the contentions and collisions, and the capacity for communications is in saturation, i.e., 570 kbps when \(N_{node}=160\). Furthermore, these results can validate the analysis of saturation, which is presented in Section 5.2.  
Figure 8: Initial network topologies with 160 nodes.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **Values** & **Parameters** & **Values** \\ \hline Data rate (Mbps) & (1, 2, 5, 11) & _CW for Pri.S3_ & _CW\({}_{\min}=32\)_, _CW\({}_{\max}=1024\)_ \\ SIFS & 0.01 ms & ACK & 14 bytes \\ DIFS & 0.02 ms & DATA packet & 512 bytes \\ ACK time & 0.02 ms & MAC header & 34 bytes \\ SNR threshold & 15 dB & PHY header & 24 bytes \\ RTS & 20 bytes & Tx power (dBm) & 32(omni), 38(dir) \\ CTS & 14 bytes & Cluster Protocol & DMDG [2] \\ _CW for Pri.S1_ & _CW\({}_{\min}=8\)_, _CW\({}_{\max}=16\)_ & Routing Protocol & FMCQR [3] \\ _CW for Pri.S2_ & _CW\({}_{\min}=16\)_, _CW\({}_{\max}=32\)_ & — & — \\ \hline \hline \end{tabular}
\end{table}
Table 3: Analytical parameter settings.  
Figure 9b depicts the average latencies under PC service. It is clear that the average delay of DDC-MAC, LMAC, and DMAC increases slowly when the network size is in \(N_{node}\in[40,160]\), whereas the performance of 802.11 fluctuates due to its omni-directional transmission and CSMA-based mechanism. In addition, LMAC and DDC-MAC achieve a similar inclination as DMAC, but their average delays are much lower, as they enjoy the benefits of a TDMA-based mechanism, thus guaranteeing non-collision transmission. Moreover, DDC-MAC outperforms LMAC because its traffic period is shortened by cluster-based topology, and its average delay is lower than 55 ms when \(N_{node}=160\).