# Dynamic Cooperative MAC Protocol for Navigation Carrier Ad Hoc Networks: A DiffServ-Based Approach
## 6 Performance Evaluation
### Simulation Results
#### 6.2.2. Saturated Throughput and Delay in 1-Hop on-Demand Communication  
This subsection evaluates the ST and AD performance of DDC-MAC in 1-hop OC mode under single-channel (SC) and multi-channel (MC) environments, respectively. We assume that each node in 802.11 DCF is equipped with an omni-directional antenna, and the node in 802.11 EDCA and DDC-MAC is equipped with the adaptive antenna array proposed in Section 3.2. To verify the proposed Diffserv mechanism, the results of different service priorities, namely Pri.S1, Pri.S2, and Pri.S3, are depicted in Figure 10. As shown in the results, in general, the service with higher service priority can achieve the performance of throughput and average delay superior to those of lower priority services (Pri.S1 > Pri.S2 > Pri.S3), whereas the performances in MC environments outperform those of SC environments.  
Figure 10a,b depicts the ST comparison of three protocols under SC and MC environments. In Figure 10a, the throughput of 802.11 DCF increases slowly due to its OT transmission mode, and thus results in a high probability of contentions and collisions. Meanwhile, 802.11 EDCA and DDC-MAC explore the opportunities of spatial reuse effectively by using adaptive antenna array; thus, their throughputs increase along with the number of nodes. Finally, compared to 802.11 DCF, 802.11 EDCA and DDC-MAC achieve approximately 45% and 70% of throughput gain, respectively. When \(N_{node}>70\), the throughput growth of 802.11 EDCA and DDC-MAC decelerates and holds a constant value when \(N_{node}=100\) (\(Th_{dmax-sc}=7540\) kbps, \(Th_{ddmax-sc}=8760\) kbps). In a SC environment, DDC-MAC cannot use ACBS for distributing communicating nodes over multiple channels; thus, its throughput performance is similar to 802.11 EDCA. Figure 10b depicts the ST of three protocols in an MC environment, in which 802.11 DCF and 802.11 EDCA use a random channel selection policy, and the proposed DDC-MAC uses an ACBS policy. The throughputs of three protocols are similar when the number of pairs is few (\(N_{node}\leq 30\)). However, when the number of pairs increases (\(N_{node}>30\)), the throughputs of 802.11 EDCA and DDC-MAC are superior to that of 802.11 DCF because of the exploitation of spatial reuse opportunities by means of directional antenna and ACBS mechanisms. Moreover, DDC-MAC outperforms 802.11 EDCA because its traffic congestion  
Figure 9: Performance of MAC protocols in PC mode: (a) Throughput; (b) End-to-end average delay.  
is alleviated by the DMDG-based cluster protocol. The ST performance in an MC environment outperforms that of SC environment by 20% to 32%.  
Figure 10c,d describes the impact of the number of nodes on packet delay. The packet delay is measured by the duration from the memory buffer to the time when the packet has been sent successfully. Figure 10c shows the comparison of the three protocols in SC environment. Generally, the AD increases along with the number of nodes, owing to increasing of contentions and collisions. The 802.11 DCF adopts an omni-directional antenna, which blocks parallel communications, and thus its packet delay increases the fastest. Meanwhile, the AD of 802.11 EDCA and DDC-MAC is compensated for by exploiting spatial reuse opportunities by directional antennas. Compared to Figure 10c, the performance in multi-channel environments is superior in every respect, with the average delay decreasing by approximately 40-50%. As shown in Figure 10d, the ADs of three protocols are constant (about 2.5 ms) when \(N_{node}\leq 30\). Since there are three data channels in use, the probability of collision and retransmission is low when \(N_{node}\) is small. However, the AD of 802.11 DCF increases rapidly with the increase of \(N_{node}\), and reaches 0.42 s when \(N_{node}=100\). Meanwhile, the AD of 802.11 EDCA and DDC-MAC increases slower than 802.11 DCF, and keeps a constant value when \(N_{node}=100\) (62.3 ms for DDC-MAC, 92.6 ms for 802.11 EDCA).