# Escalator: An Autonomous Scheduling Scheme for Convergecast in TSCH
## 1 Introduction  
Time Division Multiple Access (TDMA) and channel hopping-based MAC protocols are being developed for energy efficient and reliable communication in Wireless Sensor Networks (WSNs) composed of low-power wireless devices [1; 2; 3; 4]. These protocols perform synchronous and deterministic communications based on timeslot schedules in units of the slotframe (or superframe), a group of timeslots that continuously repeats over time. TDMA uses a deterministic schedule to reduce idle listening and collisions in packet transmission. Channel hopping mitigates packet transmission failures that might occur due to multipath fading or channel conflicts through frequency diversity [5]. These protocols perform channel hopping in timeslots along the hopping pattern and provide simultaneous communications in a single timeslot using a channel offset. These protocols provide a low duty cycle and high reliability compared to existing asynchronous protocols [6]. In the WSNs based on TDMA and channel hopping, routing and timeslot scheduling schemes are important because they determine the performance of the entire network. Routing determines the packet forwarding path of the network. Reliable link-based robust routing protocols provide reliability for the end-to-end packet delivery. The timeslot schedule of the TDMA network determines the end-to-end delay of the packet transmission and the bandwidth of the network. The timeslot schedules that generate consecutive timeslots along the packet forwarding path minimize the end-to-end delayof the packet transmission. Furthermore, the timeslot schedules that minimize the slotframe size (or length) increase the bandwidth of the network. Several scheduling schemes have been proposed for WSNs based on TDMA and channel hopping. Scheduling schemes could be classified by the entities that create and manage the schedules [7; 8]. Firstly, in centralized scheduling, a central entity performs the creation and management of the schedules. The central entity collects properties such as traffic requirements, network topology information and link status from the nodes to create schedules. Secondly, in the distributed scheduling, each node in the network performs the creation and management of the schedules through schedule information exchange and negotiations with neighbor nodes. Finally, in the autonomous scheduling, there is no entity that performs the creation and management of the schedules [8]. Autonomous scheduling determines the schedule according to the pre-established rules without information collections or negotiations among nodes for schedule creation and management, thereby reducing the collection overhead. One of the common communication patterns in WSNs is convergecast, which means the data collection from all nodes to the sink opposite to broadcast. In general, convergecast means that all nodes in the network transmit data generated to the sink periodically. Event-driven data collection is also a convergecast in a broad sense [9]. Because the data collection is one of the most typical WSN applications, the convergecast could be considered as the main communication pattern of WSNs. At the convergecast, all nodes must be able to forward packets to the sink until the next packet generation cycle. In this paper, we propose an autonomous scheduling scheme, named Escalator, for the convergecast in a TSCH network with the RPL [10] routing protocol. The main advantage of Escalator is that it creates a conflict-free schedule, which minimizes the end-to-end delay, without additional information exchange for scheduling. In Escalator, each individual node generates and manages a timeslot schedule in an autonomous manner for a convergecast based on the routing topology generated by RPL. That is, each node manages a timeslot schedule as soon as the node receives the RPL control messages such as DIO (Destination-Oriented Directed Acyclic Graph (DODAG) Information Object) or DAO (Destination Advertisement Object) when the network topology changes.