# Parent and PHY Selection in Slot Bonding IEEE 802.15.4e TSCH Networks
## 5 Evaluation
### Simulator Results  
First, the heuristic for different \(\delta\) values was compared to the GA scheduler in the TSCH simulation experiments with 14 nodes. The results are shown in Figure 9. The PDR results were compared for different slotframe lengths of 120 ms, 200 ms, 280 ms and 360 ms, and for \(\delta\) values ranging from 0.2 to 1.0. When \(\delta=1.0\), the heuristic will pick for each possible parent the fastest available PHY. It is clear that the performance of the heuristic approximates the performance of the GA for all slotframe lengths. The PDRs of the GA  
Figure 8: The channel allocation for the 50 kbps and 1000 kbps PHYs in the 863–868 MHz band, as used in the evaluation. The former uses 3 channels of 200 kHz while the latter uses 2 channels of 1667 kHz.  
scheduler were 0.446 (120 ms), 0.694 (200 ms), 0.898 (280 ms) and 0.968 (360 ms). While the performance of all \(\delta\) values was very similar for all slotframe lengths, when \(\delta=0.6\) the PDRs were the highest with 0.431 (i.e., a decrease of 3.4% of the GA's performance), 0.666 (i.e., a 4% decrease), 0.843 (i.e., a 6.1% decrease) and 0.935 (i.e., a 3.4% decrease), respectively. The average deviation in PDR values, observed between the heuristic and its near-optimal counterpart, was 0.052 and the maximum of all deviations was 0.331, when including all slotframe sizes and all \(\delta\) values. When only taking into account \(\delta=0.6\), the average observed PDR difference was 0.044 and the maximum was 0.264. Figure 10 shows the reliability values for all links for the different \(\delta\) values (with the mean reliability value denoted by an x). As expected (see Algorithm 1), the results show that with increasing \(\delta\) values, the average link reliability value decreases. This decreasing reliability has an effect on the number of propagation failures, the number of needed retransmissions and, consequently, on the number of dropped packets (because of too many retransmission attempts). However, faster PHYs also need less transmission time and thus shorter bonded slots (i.e., less regular slots per bonded slot), leaving more free space in the schedule for extra retransmission slots or for other nodes to make slot allocations. As such, in this case, the best trade-off between using shorter/longer bonded slots and lower/higher reliability values was found when \(\delta=0.6\). This was confirmed by \(\delta=0.6\) consistently achieving the lowest total number of packet drops, including the dropped packets because of exceeding the retransmission threshold, but also the packets that were dropped immediately at the transmitter because there were not enough allocated slots to empty the queue on time.  
Figure 10: Mean link reliability values for different slotframe lengths, for different \(\delta\) values of the heuristic. The x denotes the mean value. The diamond shapes represent outliers that are values smaller than the minimum (i.e., the lower whisker) or larger than the maximum value (i.e., the top whisker).  
Figure 9: PDR simulation results for different \(\delta\) values for different slotframe lengths, comparing the heuristic with the GA scheduler. The x denotes the mean value. The diamond shapes represent outliers that are values smaller than the minimum (i.e., the lower whisker) or larger than the maximum value (i.e., the top whisker).