# Time Slotted Channel Hopping and ContikiMAC for IPv6 Multicast-Enabled Wireless Sensor Networks
## 6 Results
### Delay per Hop  
Another parameter used to analyze the performance of TSCH and ContikiMAC and the two forwarding modes, LL unicast and LL broadcast, is the delay per hop. The results for the delay are illustrated in Figure 11 for topologies without interference, in Figure 12 for topologies with interference between same rank nodes, and in Figure 13 for topologies with a random placement of nodes. We also studied the influence of LL unicast retransmission mechanism on the delay, as shown in Figure 14.  
The results for topologies without interference and LL unicast, shown in Figure 11a,c and with idealized interference in Figure 12a,c, respectively, indicate that the delay introduced by ContikiMAC was lower than the delay of TSCH. This was due to the unicast period of TSCH being higher than the wake-up period in ContikiMAC, together with the fact that topologies with little interference are advantageous for ContikiMAC.  
However, for topologies with a random placement of nodes and LL unicast, ContikiMAC showed much larger delays (Figure 13c) than TSCH (Figure 13a). This could be explained by the fact that TSCH benefits from the scheduling of packet transmissions, which avoids collisions and associated retransmission. It can also be observed that with ContikiMAC the number of interested children had an influence on the delay, whereas this was not the case with TSCH because in TSCH, the schedule of every neighbor is chosen such that there is minimal overlap.  
Figure 9: PDR comparison between LL unicast and LL broadcast and between ContikiMAC and TSCH on topologies with interference between same rank nodes. (**a**) Results for LL unicast in BMRF over TSCH. (**b**) Results for LL broadcast in BMRF over TSCH. (**c**) Results for LL unicast in BMRF over ContikiMAC. (**d**) Results for LL broadcast in BMRF over ContikiMAC.  
For the topologies where the same rank nodes interfered with each other (Figure 12), and for the topologies with the random placement of nodes (Figure 13), when using BMRF LL broadcast forwarding on top of ContikiMAC, the delay increased with more interference (Figures 12d and 13d), especially for higher hops when compared to LL unicast. This was because the gain obtained from the first hop faded away when the subsequent nodes added an artificial delay before further broadcasting an incoming packet (this avoided collisions with the other broadcasters). However, for the same topologies with BMRF in LL broadcast on TSCH, the delay remained approximately the same, as each broadcaster had its own slot from the customized rule, as shown in Figures 12b and 13b. We therefore can conclude that for placements resulting in trees deeper than 1 hop and with a lot of interference between nodes, TSCH is the better solution when using LL broadcast.  
Figure 10: PDR comparison between LL unicast and LL broadcast and between ContikiMAC and TSCH on topologies with a random placement of nodes. (**a**) Results for LL unicast in BMRF over TSCH. (**b**) Results for LL broadcast in BMRF over TSCH. (**c**) Results for LL unicast in BMRF over ContikiMAC (**d**) Results for LL broadcast in BMRF over ContikiMAC.  
The delay in BRMF over TSCH-enabled networks was lower for LL broadcast forwarding (Figure 11b, Figure 12b, and Figure 13b) compared to LL unicast forwarding (Figure 11a, Figure 12a, and Figure 13a). The high delay obtained when running TSCH with BMRF in the unicast mode configuration can be explained by the retransmission mechanism. In the case of a negative ACK, the packet is retransmitted. We ran new simulations on the topologies with interference, and we set the number of retransmissions to zero. In Figure 14b, it is shown that the difference in delay was indeed due to retransmissions. The effect of reducing the number of retransmissions to zero is illustrated in Figure 14a, where it is shown that the PDR dropped to below 55% and the delay of LL unicast became similar to the one of LL broadcast.