# Multiple Mobile Sinks for Quality of Service Improvement in Large-Scale Wireless Sensor Networks
## 4 Results and Discussion
### Packets Latency Time (End-to-End Delay) Evaluation  
In this simulation scenario, we compared packet latency (the percentage of packets that arrive at the sink with less than 1 ms delay) by varying the number of mobile and static sinks monitoring the network to study the impact of the use of multiple mobile sinks on packet latency in LS-WSNs.  
The simulation results illustrated in Figure 6a show that the use of a mobile sink, whatever the model, increases the mean percentage of fast packets (packets with delay less then 1 ms) compared to that of a single fixed sink from +8% for RWP up to +28% for RD, and the best result is obtained with the RD model and the LEACH routing protocol +73%.  
Figure 6: Packets latency with different routing protocols by using multiple statics and mobiles sinks, (**a**) Single Sink for the entire network, (**b**) two Sinks for two sub-networks, (**c**) four Sinks for four sub-networks, (**d**) eight Sinks for eight sub-networks.  
The simulation results shown in the other figures show that the use of multiple mobile sinks increases the mean percentage of fast packets compared to the use of a single static sink (Figure 6a):  
* By using two mobile sinks (Figure 6b) from +21% for RWP up to +43% for RD, the best result is obtained with the RD model and the LEACH routing protocol +99.5%.
* By using four mobile sinks (Figure 6c) from +43% for RWP up to +69% for RD, the best result is obtained with the RD model and the LEACH routing protocol +153%.
* Using eight mobile sinks (Figure 6d) from 51% for RWP to 78% for RD, the best result is obtained with the RD model and the LEACH routing protocol +175%.  
We can draw from the previous results that the best mobility model is RD, which offers almost triple results and 2.73 times faster packets compared to a single fixed sink by using eight mobile sinks. Therefore, we conclude that using multiple mobile sinks enhances the packet latency, particularly in smaller areas.  
More precisely, the RD moves in the network by approaching them in a realistic way towards the SN clusters to avoid packet retransmission, decreases the distance between the sink and the CH, and consequently increases the number of packets that reach sinks with low delays due to the single hop routing technique used by LEACH.  
On the other hand, by comparing the latency scenarios, we note that the use of four mobile sinks gives results very close to that of eight mobile sinks. So, we can conclude that in terms of profitability, using four mobile sinks establishes a good tradeoff between the latency time and the cost of multiple mobile sinks. So, we can obtain a better latency of +153% for fast packets with only four mobile sinks by using the RD model and the LEACH routing protocol.  
In conclusion, when the set of static or mobile sinks increases their proximity to sensor nodes, fewer nodes are associated with each sink. This results in the reduction of the packet interference and the un-saturation of the sink buffer. This avoids packet retransmission and subsequently improves the number of packets that hold their sinks without additive delay.  
Comparing the effect of static and mobile sinks, we can deduce that mobile sinks offer gains in energy conservation of \(-\)59.5%, throughput of +90.2%, reliability of +140% and latency of 175% with eight mobile sinks with the best RD the best mobility model for our scenarios. The use of eight mobile sinks offers a non-significant gain compared to the use of four mobile sinks. However, supplementary expenses were incurred due to the use of a high number of mobile sinks. So, we can just be satisfied with four mobile sinks with the RD mobility model, seeing the high costs of eight mobile sinks. Otherwise, it will be a waste.