## 6 Simulation Results and Discussion
### DRiPLOF's Performance without External Interference  
Intuitively, any multi-interface solution should outperform a single-interface solution due to its added flexibility. However, the added flexibility of multiple interfaces can actually be a disadvantage when deployed in \"normal\" conditions (i.e., without cross-technology interference). For example, energy consumption may increase since control traffic needs to be distributed over multiple interfaces, resulting in additional overhead. As such, to be useful, any multi-interface solution should not perform (significantly) worse than a single-interface solution under normal conditions.  
Based on the evaluation metrics presented in Figure 9, the following observations can be made:  
1. DRiPLOF exceeds, or otherwise matches, the performance of the state-of-the-art POOF [9; 10] in all evaluation metrics. Although the per-source PDR (see Figure 9c) and the number of retransmissions (see Figure 9g) are comparable, DRiPLOF forms a more stable DODAG, as indicated by the number of parent changes (see Figure 9b). This can be explained by the fact that POOF is based on OF0 [16] and, thus, requires an additional stability mechanism, which we found to be inconsistent because it relies on infrequent OF calls (the occurrence of which varies between implementations).
2. DRiPLOF outperforms the single-interface approach using MRHOF with high-rate/short-range links (hereafter called \"high-rate MRHOF\") in all cases, except for end-to-end latency (Figure 9a) and per-node transmit energy (Figure 9e). While the higher transmit energy cost of DRiPL is partially caused by the additional control traffic required for multi-interface operation, it is mainly because with the high-rate MRHOF, nodes spend more time as orphans (which cannot send data packets to the root and, hence, consume less transmit energy) (Figure 9d). Likewise, the end-to-end latency is artificially kept low, since dropped packets and detached/orphaned nodes are not accounted for (since those cases would equate to infinite latency).
3. DRiPLOF matches, or is otherwise close to, the performance of the single-interface approach using MRHOF [11] with low-rate/long-range links (hereafter called \"low-rate MRHOF\"). Since DRiPLOF is based on MRHOF and because, with DRiPLOF, nodes generally prefer the low-rate interfaces, both solutions form relatively short and stable paths (Figure 9b,h). Although the transmit energy consumption of DRiPLOF is slightly higher due to control traffic duplication over both interfaces, in practice, this effect is negligible compared to the overall energy consumption of the devices. For all practical purposes, DRiPLOF matches the performance of the best single-hop solution we tested (when there is no cross-technology interference).  
As such, DRiPLOF not only outperforms POOF [9; 10] (i.e., a state-of-the-art multi-interface solution), but also has negligible overhead compared to single-interface solutions. Moreover, unlike a single-interface solution with a badly selected radio technology (e.g., in our simulations, using 2.4 GHz instead of 868 MHz), DRiPLOF automatically corrects for this mistake and switches to the most optimal interface. Note that, there is a point at which the inter-node spacing would be just small enough (or the transmit power high enough) for the end-to-end delivery rate of high-rate MRHOF to be comparable to low-rate MRHOF, POOF, and DRiPLOF. At that point, the end-to-end latency and per-node transmit energy of high-rate MRHOF would be much lower than any of its competitors, yet the per-node number of retransmissions would still be much higher. In future work, we may hence base DRiPLOF's preferred interface selection on more than lowest ETX such that it may also pick the high-rate interface even if its ETX is higher, that is if doing so would result in improved energy consumption/latency.  
Figure 9: _Cont_.