# TSCH Multiflow Scheduling with QoS Guarantees:
## 1 Introduction
### TSCH Schedulers and Protocols
#### 1.2.3 Centralised  
Unlike the previous solutions where the nodes build their topology, in the centralised ones an external element known as PCE performs the cell allocation process of the entire network and sends this scheduling to the nodes. For this, the PCE collects topology information from specific messages created by each node. These messages contain the adjacent nodes and transmission statistics. This centralisation allows schedules with a high level of optimisation to be built, making them ideal for time-critical applications, as they can guarantee low latencies. This is the case for schedulers such as TASA [4], AMUS [5] and SDN WISE-TSCH [17]. The main problem with this approach is the traffic generated between the PCE and the nodes, which has a significant impact on network bandwidth. AMUS [5] is a TSCH scheduling protocol that provides reliability in terms of achieving low latency for applications with low latency requirements, as is the case with most industrial applications and processes. It has outperformed TASA [4], the centralised default protocol. In comparison AMUS performs better in terms of reliability and delays. Although AMUS is not available, the paper [5] contains a detailed explanation that has allowed it to be used in the comparison. This method is based on the reservation of the different scheduling cells (resources) along the different routes followed by the packets in a network. In this sense, the algorithm achieves this goal through a process consisting of three phases:  
(a) Collecting information from the network. Using routing and control protocols, such as RPL, network statistics information is collected in order to know parameters such as the number of packets generated by a node in a period, topologies, etc. (b) Cell assignment. For correct scheduling without collisions and with time guarantees, it is necessary to know in advance the sending sequences that are going to be carried out. This is because the nodes closest to the sink must route packets from nodes that are further away. Therefore, the cell in charge of transmitting the packets to the sink must be located in a timeslot after the reception of the packet to be transmitted. The process to be followed is as follows:  
* Knowledge of the routes that the packets of each traffic flow will take to the sink. * Construction of the Scheduling Sequence (SS) matrix: this is a \(2\cdot n\) (\(n\) = number of nodes that transmit packets) matrix, where the first row indicates the ID of the transmitting node and the second row indicates the traffic rate of each node. Each column of the SS matrix represents a traffic flow. * Construction of the Multi-hop Scheduling Sequence (MSS) matrix: this is a matrix composed of three rows, the first being the source node, the second the destination node and the third the traffic rate. Each column of the matrix represents a link between two adjacent nodes. * Scheduling: once the MSS matrix is obtained, the cells are assigned in column order, starting from the first free timeslot and the first Channel Offset. As many cells shall be assigned as indicated by the rate (third row). Cells should be allocated according to the following restrictions, to avoid collisions or interference:  
* Nodes can only have one state per timeslot (transmission or reception). * Transmissions from other links that may cause collisions or interference should be made on another channel of the same timeslot. * Links that do not have any conflict or interference can be allocated in parallel. In this case the same timeslot can be used. SDN WISE-TSCH is a highly deterministic system that guarantees industrial level requirements. It has a stack of protocols that allow centralised control of all processes in the nodes (MAC, Routing, QoS, Application). This is based on the SDN paradigm, which consists of managing networks by separating the control plane and the data plane [27].