# A Joining Procedure and Synchronization for TSCH-RPL Wireless Sensor Networks
## 3 Related Work  
Ever since the IEEE Computer Society released the IEEE 802.15.4e amendment in 2012, which included the TSCH mechanism, network synchronization has been a popular topic in Wireless Sensor Networks research. The following works [14; 15; 16; 17] consider problems in synchronization due to inaccurate estimates of the deviation between two slots. To improve the synchronization process, adaptive synchronization mechanisms have been proposed, obtaining very accurate synchronizations, reducing traffic, and saving energy. Their solutions allow us to improve the drift clock estimations;  
Figure 2: The exchange of Routing Protocol for Low-Power and Lossy Networks (RPL) control messages to build the topology. DIO, Destination-Oriented Directed Acyclic Graph (DODAG) Information Object; DAO, Destination Advertisement Object. (**a**) Sink node starts the dissemination of the RPL information with DIO messages; (**b**) Connected nodes forward the information downwards; (**c**) Connected nodes send upwards a DAO message. however, no mechanism for advertising beacons that minimizes the network joining time has been developed. Duquennoy et al. [18] summarize the main challenges and difficulties that arise when using TSCH in Contiki, highlighting that very precise synchronization can be achieved with the current implementation. In 2015, the Internet Engineering Task Force (IETF) presented an informative document [19] detailing some of the issues with, and objectives for, the TSCH mechanism, among which is the formation and maintenance of the network. The 6 TiSCH working group has also been putting its best efforts into improving the mechanism to make it possible to use IPv6 in TSCH networks. A minimal mode of operation has been defined in [20], which specifies the recommended configuration of some protocols that are related to TSCH; for instance, the routing protocol RPL. However, a mechanism that improves the network joining process or the relationship between RPL and TSCH has yet to be proposed. Vogli et al. propose and improve in [21; 22] a series of mechanisms that improve the speed with which the nodes join the network, studying each of them using an analytical model. Duy et al. [23; 24] carry out another analysis on the problems of long delays in connecting to the network and propose a new slotframe-planning scheme to improve the efficiency of the TSCH union mechanism. This solution allows a device to dynamically determine the number of synchronization messages that is needed. Khoufi et al. [25; 26] evaluate the time needed to form a TSCH-based network using the NS3 simulator, proposing a new algorithm for sending Enhanced Beacons. Kim et al. [27] propose a new scheme to join the network, based on channel quality estimates, that is more efficient in environments where interference is more severe. The works described in [21; 22; 23; 24; 25; 26; 27] propose different solutions based on EB transmission schemes that improve synchronization times and enable advertising-based slots in the TSCH schedule. These solutions are based on statically improving the planning, and assume that with a higher number of synchronized nodes the synchronization is faster because there are more messages with information on synchronization. Although this is true, networks do not always have enough nodes to improve synchronization in this way. De Guglielmo et al. [28] evaluate a TSCH announcement algorithm that is based on the random transmission of synchronization messages. Their results show that the joining time of a node is mainly influenced by the number of channels that is used for the transmission of beacons on the nodes that are already part of the network. Wang et al. [29] evaluate the impact of different parameters during the joining phase, demonstrating the impact of EB transmission and their possible collision in large networks.