# Bell-X, An Opportunistic Time Synchronization Mechanism for Scheduled Wireless Sensor Networks
## 4 Problem Statement and Proposed Advertising Method  
The TSCH medium access method has proven to be one of the most relevant solutions that are included within the IEEE 802.15.4 standard, thanks to the fact that it is aimed at providing the systems with a deterministic behaviour that guarantees a certain degree of reliability and robustness, which are essential characteristics in industrial settings. For this reason, many proprietary communications standards for wireless sensor networks rely on this same dynamic [26]. Although it is true that once the nodes are synchronized TSCH provides great reliability to communications, the mechanism has certain difficulties when it comes to forming the network in a fast and energy efficient way, as mentioned in related works.  
The RPL protocol is also an indispensable tool in the formation of sensor networks, providing connectivity and routing capabilities to the devices once synchronization has been acquired. These two protocols were not originally designed to work with each other, and some procedures, such as sending DIS, are relegated to the background when working with a deterministic scheduling, such as that used in TSCH. With the aim of improving the synchronization phase, these two protocols have also combined some of their characteristics. In Contiki OS, it is possible to configure the EB transmission period so that it follows the same evolution as the timer used in the Trickle Timer of RPL, obtaining the same behavior when transmitting beacons than when sending RPL control messages. Although this combination can help to reduce traffic and consumption, the value of the Trickle Timer evolves so quickly that it reaches its maximum value in a few minutes, thus hindering the synchronization process of the nodes if it is not carried out during the first instance of the deployment.  
A dynamic beacon transmission mechanism was proposed during the work done in [6]. This mechanism was focused on improving the synchronization process during the initial deployment phases of the network, setting up a beacon22 transmission period low enough for the nodes to connect quickly, then changing to a much smaller advertising traffic. This fast connection allowed to save time and energy, since during the scanning phases the duty cycle of the radio is close to 100%, so long scanning times can significantly affect the battery level of the devices. In addition, if a node synchronizes quickly during the deployment phases, the Trickle Timer will not yet have reached its maximum value, so the connection with the RPL topology will also be faster.  
The conclusions of this work showed that a dynamic mechanism for network set-up could improve the work of deploying a network if we compare it with both configurations, the fixed transmission period and the period modified by the Trickle Timer, since messages that are oriented to synchronization in TSCH have different requirements than the RPL control messages. However, there are situations in which new nodes are added to the network when it is in a stable state, such as restarted nodes, replacement of devices or batteries, or simply an expansion of the network in which several nodes are added to an existing network. This flexibility is a main feature that should be highlighted in wireless networks and the main reason to design a new mechanism that addresses these network needs. Moreover, it is of special interest after assessing the poor results of active scan mechanisms in these networks. In the next section, the proposed operation of a dynamic and configurable EB transmission mechanism is presented.