# A New Association Scheme for Handling Node Mobility in Cluster-Tree Wireless Sensor Networks
## 4 Simulation Assessment
### Results and Discussion
#### 4.2.1 Mobile Nodes at 1.4 m/s  
The first simulation experiments were performed with a set of mobile nodes (from 1 to 40) moving at 1.4 m/s. Figure 7 illustrates the average packet delivery rate. Using the standard association parameters, the average packet delivery rate was below 50%, even for a small number of active mobile nodes. This value remains more or less constant up to 15 mobile nodes, from this point the average packet delivery rate is considerably reduced, until the network became fully saturated. This behavior was already expected due to the CSMA-CA contention mechanism. These results, namely the reduction of the packet delivery rate to 50%, demonstrate the inefficiency of the association mechanism of the IEEE 802.15.4 standard when using mobile nodes \begin{table}
\begin{tabular}{c c} \hline
**Definition** & **Standard Value** \\ \hline Radio & cc2420 \\ Frequency & 2.4 ghz \\ Environment & 450 m \(\times\) 10 m \\ Packet rate & 4 pkt/s \\ Number of mobile nodes & 1 to 40 \\ Simulation time & 50,000 s \\ Packets per node & 1000 \\ Beacon Order & 8 \\ Superframe Order & 2 \\ \hline \end{tabular}
\end{table} 
Table 1: Simulation configuration. Figure 7: Average packet delivery rate at 1.4 m/s. with multi-hop topology networks. If alternative MAC parameters are used, there was no significant improvement in the packet delivery rate for a small number of mobile nodes. However, as the number of mobile nodes increases, the influence of managing the MAC parameters becomes visible. When configuring the network with the proposed improvements (anticipation of handover decision-making), the results were significantly improved. With up to five mobile nodes, the average packet delivery rate was greater than 90%. Similar values were obtained when in conjunction with the alternative MAC parameter strategy. The proposed scheme is clearly more efficient than the IEEE 802.15.4 standard basic association mechanism, for a network with up to 10 mobile nodes. Regarding the percentage of time during which the mobile nodes remain disconnected from the network (blackout interval), the use of the proposed anticipation mechanism clearly improves the behaviour of the IEEE 802.15.4 standard association mechanisms. Figure 8 illustrates the average percentage of time during which the mobile nodes remain disconnected from the network. The achieved results highlight that the proposed association mechanism allows mobile nodes to remain synchronized with the network nearly all the time. This behaviour is possible because the mobile nodes are always verifying the RSSI signal from the beacons received from the CHs. Therefore, a mobile node losing synchronization with the current CH, if it verifies that its RSSI signal has gone below the pre-established threshold and there is another CH with a better signal, it will immediately request an association to the new CH (make-before-break strategy). If the new association is not successful, the mobile node remains associated with the current CH. This effectively prevents mobile nodes from losing synchronization with the network, allowing them to send their messages during the active periods of their parent nodes (CHs). Even when facing a considerable increase in the number of mobile nodes, the proposed methodology is better. When using the IEEE 802.15.4 standard association mechanism, if the number of mobile nodes increases, the average time during which the nodes remain disconnected from the network clearly also increases. For example, with 40 mobile nodes, the average percentage of time during which the nodes remained disconnect from the network was 41.8%--that is, they did not have access to the network for almost half of the active time. When just adjustment of MAC parameters was used, the synchronization with the network was similar. Therefore, the reason why the mobile nodes kept synchronized with the network was the anticipation method of the handover decision. Figure 9 illustrates the average end-to-end communication delay.