## 4 Simulation Assessment of the ARounD Communication Scheme
### Results and Discussion
Figure 20: Cluster-tree and ARounD paths for one of physical topologies: (**a**) Scenario 1; (**b**) Scenario 2.  
queued data packets in the cluster-heads near the PAN coordinator [19], causing further delays to the newly arriving packets. However, as the ARounD scheme defines an alternative path using RP nodes (which are mainly leaf nodes), the end-to-end delay for the source-to-destination traffic is just slightly affected. As it was expectable, the average end-to-end delay is highly dependent on the background traffic load for the standard cluster-tree approach, as the source-to-destination traffic is sharing the same MAC queues with the background traffic. On the other hand, in the ARounD scheme, as the source-to-destination traffic almost does not share RP nodes with the background traffic, the average end-to-end delay keeps constant for the different data rates. Figure 22 illustrates the average end-to-end delays of the source-to-destination message stream according to the network load perspective (as a function of the traffic load imposed by sensor nodes). For this set of simulations, we just considered a data rate of 1 packet every 4 s. The rationale of this set of simulations is to evaluate the behaviour of source-to-destination traffic delays under different traffic loads imposed by sensor nodes (including a scenario with no background traffic). As it can be observed, the ARounD approach provides smaller and bounded end-to-end delays for the source-to-destination traffic, even under different traffic loads. On the other hand, the standard tree routing approach tends to increase as the background traffic increases. Note also that, even without background traffic (no interference), the end-to-end delays for the source-to-destination traffic is higher, when using the standard tree routing, due to the higher number of beacon intervals that are required to transmit data frames. Figures 23 and 24 illustrate the average end-to-end delay of the source-to-destination message stream for simulation _Scenario 2_ (five physical topologies, under different background and source-to-destination data rates), for both the application and network load perspectives. Also, the ARounD approach provides a smaller end-to-end delay when compared to the Standard approach for all background and source-to-destination data rates and traffic loads. Figure 22: End-to-end Delay for ARounD Approach vs. Standard Approach in Scenario 1, considering different background traffic loads. Figure 25 illustrates the packet loss rate due to buffer overflows as a function of the background traffic data rates for Scenario 1, while Figure 26 illustrates same metric as a function of the traffic load imposed by all sensor nodes. These results highlight that the ARounD scheme successfully delivers all the source-to-destination messages, while the standard cluster-tree path is prone to a high number of packet losses due to network congestion issues. The main reason is that, as the ARounD communication scheme operates during the inactivity period of the clusters, it does not need to contend for the channel access, which avoids packet losses caused by collisions. Additionally, as ARounD prioritises the use of leaf nodes to convey its source-to-destination packets, it avoids the use of more congested CH nodes and therefore the number of packet discards due to network congestion is negligible. Finally, note also that, although the availability of GTS mechanism to transmit time-sensitive packets without contention, the main weakness of this mechanism is its limited number of available slots [68]. Figure 23: End-to-end Delay for ARounD Approach vs. Standard Approach in Scenario 2, considering different data rates for the background traffic. Figure 24: End-to-end Delay for ARounD Approach vs. Standard Approach considering Scenario 2, considering different traffic loads imposed by sensor nodes.