# Bell-X, An Opportunistic Time Synchronization Mechanism for Scheduled Wireless Sensor Networks
## 1 Introduction  
The increasing popularity of applications that are derived from the concept of Industry 4.0 is leading to the emergence of new requirements in the scope of connectivity and data collection. Giving support to this need for massive data and flows of information, interconnecting everything and enabling the digital transformation, is the role of technologies labelled under the Industrial Internet of Things denomination (IIoT). Among these technologies, a favourite candidate is the Wireless Sensor Network (WSN) [1], because it provides a faster, more flexible, scalable, and cheaper solution to deploy than its wired counter-part. Nevertheless, the adoption of WSNs, especially for industrial applications, raises several challenges that need to be addressed before considering it a mature solution that is capable of hitting the market successfully. Any network deployed to operate in an industrial setting requires a high degree of reliability, which translates into enhanced connection times and avoidance of downtimes due to power or traffic loss. Traditionally, wireless communications have not been widely adopted or popular in industrial scenarios due to the harsh environment and its impact on these type of communications. Radio interferences or signal fading due to multi-path propagation and metal and other material obstacles are common issues that hinder transmissions, which, actually, must be received with the according reliable delivery QoS (Quality of Service), and not with best-effort service behaviour. In response to these requirements, in the scope of WSN and IEEE 802.15.4 protocol [2] (a standard for low-power wireless personal area networks that operate in short to middle range, with distances up to 100m per hop in mesh networks), the IEEE802.15.4e amendment (first approved in 2012) defines a group of different medium access control modes. Among the described modes, the Time Slotted Channel Hopping (TSCH) [3] stands out as a solution that enables frequency channel jumps to palliate interferences. These TSCH based networks, when combining a limited number of nodes with deterministic time behaviour that helps to limit latency and lost traffic, show promising results to be a suitable candidate for manufacturing instrumentation and monitoring tasks [4], which, using IEC 62264 (ANSI/ISA 95) nomenclature, can be defined as:  
* Flow based process monitoring (latency 100 ms. loss probability below 10%-6%, and number of devices below 30) and Supervisory control (latency 10s. loss probability below 10%-6%, and number of devices below 100). * Job based factory monitoring (latency 100 ms. loss probability below 10%-7%, and number of devices below 10). On the other hand, the IPv6 Routing Protocol for Low-Power and Lossy Networks, known as RPL [5], is one of the most adopted solution in WSNs, and it can used over IEEE802.15.4e or even other link layer options to achieve a robust deployment with networking capabilities, mesh and multi-path support, and integration and interoperability with IP networking. Nevertheless, this deterministic behaviour has certain drawbacks during the initial deployment stages and recurrent situations during the whole network time operation. When there is a lack of synchronization between the joining node and the rest of the network, successfully connecting (i.e. not only synchronizing clocks, but also joining the network effectively) can be challenging. This problem does not only show up during initial deployment, but also after replacement of batteries, after unexpected disconnections due to external factors, etc. This issue might appear due to several factors that the authors analysed previously in [6], although this study focused on presenting a solution for achieving better connectivity success without compromising energy consumption, for the initial deployment stage.