# TSCH Multiflow Scheduling with QoS Guarantees:
## 1 Introduction  
The adoption of Industrial Wireless Sensor Networks (IWSN) in industry requires devices and protocols that can guarantee physical operation and greater determinism in transmissions [1]. In order to satisfy the latency and reliability requirements of automation systems, even in converged environments, in which multiple information flows with different levels of priorities are involved. Therefore, the IEEE 802.15.4 standard, which specifies the physical and medium access control layers, has been updated to IEEE 802.15.4e. It maintains the physical layer, which consists of 16 channels in the 2.4 GHz band, and the medium access control replaces Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) with three new protocols [2] to support different types of industrially orientedapplications. These are Low Latency Deterministic Network (LLDN), Deterministic and Synchronous Multi-channel Extension (DSME) and Time Slotted Channel Hopping (TSCH). The protocols most adaptable to industry are DSME and TSCH, because they incorporate mechanisms to limit interference and increase determinism. They are based on time division to allocate transmission timeslots over different frequencies. However, the TSCH protocol is more widely used, because the slotframe it uses is simpler and has greater flexibility, as it allows the addition or removal of a single timeslot [3]. Because transmissions are made at a specific time, the transmitter and receiver must be synchronised, and they must be informed of the action to be taken at each instant of time. For this reason, a scheduling process is necessary to assign the transmission and reception timeslots as well as the channels on which they will be carried out. Currently, the standard does not define how to carry out this process, consequently, different proposals have emerged to generate TSCH scheduling. These schedulers can be classified as centralised, distributed or autonomous, depending on the entity that makes the decision to allocate the slots. In the centralised ones, a central element, called Path Computation Element (PCE) is in charge of assigning the scheduling to each node. Among the most representative of these are the Traffic Aware Scheduling Algorithm (TASA) [4] and Adaptive Multihop Scheduling (AMUS) [5]. In distributed scheduling, nodes negotiate with their neighbours to allocate slots. Among these are Decentralized Traffic Aware Scheduling (DeTAS) [6] and Wave [7]. Finally, the autonomous ones reuse information from other protocols and each node assigns its scheduling locally. Among these is Orchestra [8], along with certain variants such as Escalator [9] and Autonomous Link-based Cell scheduling (ALICE) [10]. Centralised schedulers allow more complete control of transmissions, as the PCE does the scheduling with full knowledge of the topology. However, the additional traffic between the nodes and the PCE decreases the available bandwidth at each node. This has led to the widespread use of autonomous and distributed solutions, which do not require this additional load of control traffic. One of the most widely used autonomous schedulers is Orchestra, due to its reliability, high performance and availability by default in the Contiki-NG ([[https://www.contiki-ng.org/](https://www.contiki-ng.org/)]([https://www.contiki-ng.org/](https://www.contiki-ng.org/)), accessed on 15 November 2021) operating system. In accordance with [8], this protocol achieves a packet delivery ratio of 99.9%. However, the constant increase in QoS requirements requires that other parameters such as end-to-end delay and deadline, be guaranteed in order for IWSN to adapt to the convergence of flows of different priorities. Satisfying these requirements with autonomous or distributed schedulers means increasing the complexity of the protocols, as their local operation results in sub-optimal solutions. For these reasons centralised schedulers are necessary.