# Bell-X, An Opportunistic Time Synchronization Mechanism for Scheduled Wireless Sensor Networks
## 3 Related Work  
As the TSCH mechanism was included in the 2012 amendment of the IEEE 802.15.4 standard, different proposals have been conducted to improve different aspects of the aforementioned protocol. There are certain procedures that are not fully described in the IEEE 802.15.4 standard, either because there are different solutions that fit for different domain applications or because the optimization of different procedures are outside the scope of the standard. In this way, only those mechanisms that allow for TSCH to function properly, even if not optimized, are defined in the standard. The scheduling of the TSCH radio resources, in time and frequency, and the advertising message generation for time synchronization are some examples that have been studied in recent years. It is important to keep a minimum error between the drift clocks of different devices in order to maintain a precise synchronization between two devices. That is why different studies have been carried out [10; 11; 12; 13], evaluating the main problems that a poor estimation of the drift clock may cause. Figure 4: Enhanced Beacon transmission period triggered by RPL Trickle Tumer [6]. Figure 3: Routing Protocol for Low-Power and Lossy Networks (RPL) Trickle Tumer evolution over time. These authors propose an adaptive synchronization mechanism, which makes it possible to improve the estimates of drift clock in multi-hop networks. These implementations are widespread and they can be found integrated into WSN development environments, such as Contiki OS. However, although the nodes are able to maintain a very precise synchronization once they are connected, the pre-synchronization process is a problem widely analysed in the literature in recent years. The articles [14; 15] perform different analyses showing the problems and open fields of study on the TSCH mechanism, pointing out that the synchronization and maintenance of the network is still a subject of study. Different approaches have been adopted in the literature, such as enabling a dedicated scheduling to signalling traffic, increasing the number of nodes that send beacons, reducing the probability of collision by assigning guaranteed slots, using fewer channels in the scanning process or modifying the beacon transmission period to improve the synchronization process in TSCH networks. The authors in [16; 17; 18; 19; 20] propose different schemes for the TSCH scheduling to favour the connection process, enabling different slots for the signalling plane. [PERSON] et al. [16] propose two different schemes to speed up joining operations, allowing for the nodes to use a random channel or an advertising slot to send periodically EB frames. [PERSON] et al. [17] propose a different scheme in which the nodes can dynamically determine the number of EB needed for the synchronization. [PERSON] et al. [18] compare with the state of the art a new scheme that allows for beacons to be sent through all channels of the TSCH network without collision, improving the results proposed in [16]. Authors in [19] define another scheme to allow for the nodes to estimate the best channel to send EB, oriented toward being more efficient in noisy environments. [PERSON] et al. [20] propose a non-centralized EB scheduling that is free of collisions, even considering mobile nodes. All of the aforementioned solutions need to allocate more time slots in the scheduling of TSCH, which leads to greater energy consumption even when no message is transmitted. [PERSON] [21] propose a different technique that allows the EB rate to be increased without the need to allocate more timeslots, sending multiple EBs in different channels in a single timeslot. A performance analysis is carried out by the authors in [22], which shows a synchronization mechanism that randomly sends EB messages. They also analyse the influence of other parameters such as the number of channels involved in the synchronization process. [PERSON] et al.