## 5 Performance Evaluation
### Comparative Evaluation  
In order to compare ROR with other protocols, we have chosen to use SCEEM [23; 24], because, until now, SCEEM is the only work that has properly addressed routing from the perspective of CRSN. Other works that have been cited in the review are not considered for comparison, because OSDRP [22] does not consider the unique constraints of CRSN, while SER [29] did not present a concise implementation of the MAC considered for the design. Similarly, ECR [28], which presents a similar clustering network model, does not mention how the clustering process is implemented. Thus, the best option will be to compare our approach with SCEEM [23; 24]. Figure 12: Goodput _vs._ number of CRSN nodes in the network. In Figure 14, the throughput at the sink is compared for both approaches. The x-axis shows the PU arrival pattern. It can be observed that the ROR strategy ensures that more packets arrive at the sink, as against SCEEM. The reason behind this performance for SCEEM can be explained thusly. In SCEEM, cluster heads aggregate packets to be sent to the sink from all cluster members. In turn, a path to the sink is negotiated with the cluster heads serving as intermediate routers. This strategy easily creates a bottleneck in the network, especially when the data meant for the sink is high. This attribute becomes multiplied with increased PU activity on the data channel. Thus, the protocol will experience more packet drops, as illustrated in Figure 15. This is contrary to decentralized routing strategy of ROR, which creates other paths among virtual cluster members to reduce packet dropping by avoiding the creation of potential hot spots in the network. Another important metric responsible for the performance of ROR is the SNR reliability metric integrated into the forwarding metric, as discussed below. Figure 14: Throughput _vs_. primary user (PU) occupancy. Figure 13: End-to-end latency _vs_. number of CRSN nodes in the network. In Figure 16, the goodput of both strategies is compared. It can be observed that an average of a 20% improvement is recorded by ROR in the reception rate when compared with SCEEM. Although, as we have already discussed above, the SNR reception metric is responsible for this performance by ROR, the prioritized receiver contention module also helps to reduce the contention and subsequently reduce collision during forwarding the packet towards the sink. On the other hand, although SCEEM also reduces the forwarding contention among nodes by clustering, the lack of instantaneous channel information often necessitates multiple re-tries in sending the packet to the next hop. This, in turn, has an adverse effect on the latency experienced by networks running SCEEM, as discussed next. In Figure 17, the end-to-end latency of both approaches is compared. ROR outperforms SCEEM in this respect, basically because of the time taken to synchronize the SCEEM nodes in the TDMA communication-based clusters. Latency incurred in this process is fully part of the routing process and  
Figure 16: Goodput _vs._ PU occupancy. Figure 15: Loss rate _vs._ PU occupancy. not part of the initialization of the network. The reason behind this is that the cluster head can switch to the data channel for packet forwarding within unused frame periods. Alongside this is the effect of maintaining a single path towards the sink to service accumulated packets from cluster members, which has an adverse effect on the queuing time of packets. On the other hand, ROR reduces latency via its geographical routing technique, which always seeks the best route that is closest to the sink, _i.e._, hop length extension. Moreover, there is no extra latency incurred for VCG formation, other than the normal initialization period for standard AODV. Furthermore, the queuing time is controlled by allowing other eligible nodes to route pending packets towards the sink. In Figure 18, the energy consumed per packet of both approaches is compared. Again, the energy consumed per packet for SCEEM exceeds that consumed in ROR.