# A Joining Procedure and Synchronization for TSCH-RPL Wireless Sensor Networks
## 1 Introduction  
Nowadays, the concept of the Internet of Things (IoT) is awakening great interest and achieving great popularity in the scope of emerging technologies. IoT is at the core of many innovative applications and services supporting digital transformation, and takes advantage of the ever-increasing number of connected devices. This set of technologies encompasses a multitude of scenarios and applications, such as Smart Cities, wearables, smart agriculture, health services, and vehicle-to-vehicle communication. It is also a key enabling technology for the so-called Industry 4.0 revolution (IoT for industry or IIoT) [1]. Inside the Internet of Things' bag of assets, wireless sensor networks (WSN from now on) are fundamental for those digitalization services that require the agile deployment of sensing units, and network scalability and flexibility, in terms of reconfiguration and reorganization. The context in which this study is based is the development of a \"Deploy & Forget\" tool to facilitate the tasks of installing and operating a WSN in industrial scenarios and address the need to streamline the initial stages of synchronization and connection to the network. The objective of this tool is that personnel without prior knowledge of communications are able to deploy a WSN without requiring external tools or qualified assistance, thus facilitating the adoption of wireless systems in industrial sectors. Devices belonging to a WSN are characterized by constrained computing capabilities and power limitations, and are often battery-powered. Due to these characteristics, different protocols and standards have been proposed to implement low consumption communication systems. Among them,the IEEE 802.15.4 standard stands out, which serves as a base for other technologies that are commonly found in the industrial field, such as WirelessHart or ISA100.11a. The IEEE802.15.4e amendment of this standard defines a medium access mechanism called Time-Slotted Channel Hopping (TSCH), which targets aggressive scenarios, such as industrial environments. TSCH allows us to perform frequency channel jumps between different transmissions to mitigate the heavy interference from this type of scenario. However, although the mechanism that is used by TSCH is very effective in steady-state operation situations, reducing losses due to interference considerably, it also hinders network-joining and first synchronization tasks. A node, a priori, does not know the time instant nor the frequency in which the synchronization beacons (carrying the information necessary to join the network) are being transmitted. For this reason, it is necessary to perform a scanning process, listening to different channels to detect and receive these synchronization messages. In addition to the inconveniences that arise in the synchronization process using the TSCH mechanism, devices need to obtain information about the network topology once they have been synchronized. In order to choose the paths for routing the information to a WSN border router, this connection stage should be formed by a first phase of synchronization and a second phase of learning and connecting to the network topology. Therefore, a correct synchronization does not traduce directly or instantly in a node's joining successfully to the network. For this second part, the IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL from now on), as a de facto standard in terms of routing protocols for low consumption sensor networks, has been used. This combination of protocol (RPL) and standard (IEEE802.15.4e) has been widely extended in implementations of WSNs in industrial scenarios, although it still a subject of study as it can potentially be improved. In order to overcome these drawbacks, a deep study has been carried out on the different parameters that intervene in the joining process and the topology definition in RPL.