# A Joining Procedure and Synchronization for TSCH-RPL Wireless Sensor Networks
## 6 Results
### Probability of Connection Success  
The first batch of results aim to represent the probability of a successful connection to the network. Figure 8 shows the comparison of four different configurations when changing the number of channels that are used to establish communications (Test 1 shown in Table 2). These graphs show a representation of the physical topology of the nodes and their synchronization/connection success probability in the 15 min that were taken as a reference during the experiments. Specifically, these four configurations use the EB transmission mechanism based on the RPL Trickle Timer that is described in Figure 5 and provided in previous releases of the Contiki OS (i.e., the method described in Sections 4.1 and 4.3). Clearly, increasing the number of communication channels reduces the probability that a node is transmitting an EB in the same channel that another node is scanning. This behaviour can be observed, for instance, when 16 channels are used: more than 60% of the nodes have not connected after the 15-min simulation. A key observation is the impact of the RPL protocol in TSCH synchronization: when a node does not manage to establish a connection with its neighbours to become part of the topology, it does not start transmitting its own EBs, thus interrupting the propagation of synchronization to further nodes. This can be clearly seen in the configuration that uses a single channel. In this configuration, all the nodes managed to synchronize one of their neighbours; however, some nodes have not managed to join the RPL topology, thus damaging the nodes below. On the other hand, in the configuration that uses 16 channels, it is possible to see the effect of synchronization propagation: as the connection probability decreases at each level (see Figure 7), the nodes in the following levels are unable to synchronize because no EBs are transmitted downwards. In spite of these results, a logical next step in the configuration is to use fewer channels only during the scanning phase (Test 2 shown in Table 2). Therefore, nodes that are already connected transmit signalling messages, among other data, using four channels, and the nodes wanting to join would only scan in a smaller number of these channels. Figure 9 shows the results for a configuration in which four channels are assigned for the stable operation of the network, but that uses a different number of channels for the scanning phase. Figure 8: The probability of synchronization and connection success for different numbers of channels. These three configurations use the Trickle Timer to modify the beacon transmission period. As can be seen, the connection probability slightly improves when using less channels when scanning; however, the improvement is not significant enough to consider it an optimal configuration. Finally, to complete the configuration of the possible parameters that can lead to the optimization of the network joining procedure, the impact of the beacon transmission period has been analysed (Test 3 in Table 2), achieving interesting and practical results. Initially, the Trickle Timer was used to modify the beacon transmission period, as recommended in the Contiki implementation. However, this means that, once the network has started and some time has passed, not enough beacons will be sent since the Trickle Timer algorithm does not take into account the TSCH method's requirements and is only reset by the RPL protocol. This leads to the inclusion of other configurations, such as leaving a fixed beacon transmission period (for instance, the period initially set by the standard recommendation, which can be found in the OpenWSN OS), or the addition of a beacon transmission mechanism that varies with time and is independent of RPL. The proposed configuration, \"Custom Trickle Timer\", is designed to address the issues that were detected in the previous tests, and consists in transmitting beacons every 4 s during the first 2 min.