# Escalator: An Autonomous Scheduling Scheme for Convergecast in TSCH
## 4 Proposed Autonomous Scheduling Scheme for Convergecast  
In this paper, we propose Escalator, an autonomous scheduling scheme for convergecast in TSCH-based WSNs with RPL. The basic idea of Escalator is to create a pipeline-like communication schedule. Each node allocates two timeslots for each descendant node included in its sub-graph. In one timeslot, a node receives a packet generated from a descendant node, and in the next timeslot, the node sends the received packet to its parent immediately. In addition, Escalator adjusts the schedule and determines the communication channel so that the packet can be continuously transmitted in chronological order from the source to the sink using the allocated timeslots. Escalator, operating on each node of the network, generates and manages timeslot and channel schedules for convergecast using only local routing information of the node without signaling or negotiation with other nodes. The generated schedules of Escalator on every nodes have two main characteristics: Firstly, it is conflict-free. Secondly, it is a consecutive timeslot schedule along the packet forwarding path, thereby reducing the end-to-end delay. Escalator uses routing topology information and hop counts for scheduling and operation. The hop count of a node means the hop distance to the DODAG root, and we assume that the node may know the hop count by the routing protocol. Since Escalator uses only the generated routing topology, the routing topology generation method does not affect the operation of Escalator. In other words, Escalator can be applied regardless of the Object Function (OF) of RPL. In Escalator, there are two types of slotframes with their own sizes: convergecast and baseline slotframes. The convergecast slotframe is used for carrying both TSCH beacons and the convergecast data of nodes, while the baseline slotframe is for RPL route control messages and downward data traffic (i.e., packets sent from the sink to nodes). Since RPL control messages are used to create and update the routing topology in real time, Escalator gives the baseline slotframe a higher priority than the convergecast slotframe. Figure 1 shows an example of a schedule generated by Escalator. Each timeslot in Figure 1b shows a transmission of data, and its background color matches with the color of the originating node of the transmitted data in Figure 1a. Convergecast and baseline slotframes are also shown in Figure 1b, separately. The size of the two slotframes are not necessarily the same. The generated schedules in the convergecast slotframe of Figure 1b are consecutive so that the data could be delivered to the sink from any node in a single convergecast slotframe. To explain the operation of Escalator, we define a network as follows. The network \(G=(V,E)\) consists of \(N\) nodes, and all of the nodes \(v_{i}\in V\) have unique ID \(\{i|1\leq i\leq N\}\). The ID of the node is pre-installed, and we assume that the node ID is managed by a central entity such as the DODAG root. Assigning a preconfigured unique ID to a node can be sufficiently assumed in a managed network, so it would not be a challenge in practical applications and implementations. All nodes in the network participate in DODAG using RPL, and the DODAG root acts as a sink. Each node \(v_{i}\) has its preferred parent \(p(v_{i})\), its direct child set \(C(v_{i})\) and its sub-graph's nodes set \(SG(v_{i})\). For nodes \(v_{i},v_{j}\in V\), \(l_{ij}\in E\) is defined as a link between the sender \(v_{i}\) and the receiver \(v_{j}\). \(H_{v_{i}}\) means the number of hops from the sink to node \(v_{i}\). The network performs channel hopping using \(Ch\) orthogonal channels. Data are generated from all nodes except the sink and are transmitted to the sink according to schedules specified for each node. Table 1 summarizes the main notations in this paper.