## 5 Evaluation Results: Influence of Each Layer to the Overall Performance
### NWK Layer  
This subsection studies how the overall network performance of the commonly used BeeStack routing metric compares to the performance of the HC, PLF and Path-DR routing metrics. The routing metric only is relevant for the FFDs that are participating in the routing. RFDs are not capable of routing; they are associated over a one-hop relationship to an FFD that is responsible for routing messages to and from the RFD. However, the routing metric also has a relevant impact on the performance of the end devices, as will be shown later, since all of the messages they are sending and receiving are routed by FFDs through the network. Figure 10 shows that the PLF metric always performs best in terms of overall PDR. The HC, BeeStack and Path-DR metrics follow in the given order, except for the low traffic case, where the Path-DR metric performs the second best. A strong degradation of the Path-DR performance can be observed for higher traffic loads. A higher PDR is correlated with a lower end-to-end delay, as can be seen in Figure 11. A similar dependency has been observed in the previous subsection between the PDR for the maximum MAC layer retransmission parameter and the resulting end-to-end delay. As explained in the analysis of the MAC layer results, the energy per bit should decrease with shorter end-to-end delay and higher PDR. The measurements confirm this assumption, as can be seen in Figure 12.  
Detailed investigations have been carried out to find out the reasons for the performance differences between the considered metrics and for the unexpected worse performance of Path-DR for the medium and high traffic scales. The main reason for the differences in the performance of the metrics lies in the characteristics of the chosen routes. The decision criteria of the routing metrics determine the number and the quality of the links of the resulting paths. In the experiments, the HC metric results in the minimum average path length, as expected and as seen in Figure 13, while the average length of routes chosen by Path-DR is the largest one. The average  
Figure 10: A comparison of the overall PDR for different routing metrics and traffic conditions. lengths of the routes chosen by the BeeStack and the PLF metrics lie between the ones chosen by the HC and Path-DR metrics. This can be explained by the fact that the BeeStack and PLF metrics apply an additive formula to calculate the path costs. Contrariwise, the Path-DR metric applies a multiplicative formula to calculate the expected PDR. Due to the multiplicative path cost calculation, Path-DR prefers longer routes with higher quality links over shorter routes with lower quality links. As an example, a route with a multitude of hops that have an expected LDR of almost 100% is preferred by the Path-DR metric over a single hop with a low expected LDR. This is unlikely for the BeeStack and PLF metrics, since each hop increments the path cost, making longer routes expensive. This behavior causes the routes chosen by the PLF and BeeStack metrics to be shorter than those selected by the Path-DR metric. Figure 11: A comparison of the average end-to-end delays with 95% confidence intervals for different routing metrics and traffic conditions. Figure 12: A comparison of the average energy efficiency for different routing metrics and traffic conditions. The end-to-end PDR of a route strongly depends on the link qualities along the route. Except for the HC metric, the routing metrics base their choice of routes on the measured link qualities. As indicators of the link qualities, LQI values are used, since a high LQI normally is correlated with a high LDR. Therefore, routing metrics that base their decision on measured link qualities expect a high end-to-end delivery ratio over these routes. The performance of the routing metrics depends on the mapping from LQI to link costs or expected LDR values.