# TSCH Multiflow Scheduling with QoS Guarantees:
## 1 Introduction
### RPL and Time Slotted Channel Hopping  
RPL is the standard routing protocol designed for Low-power and Lossy networks (LLN) devices, which have limitations in power processing, memory and energy consumption. According to the standard [21] this protocol represents the topology of routing through Destination-Oriented Directed Acyclic Graph (DODAG) oriented towards the root node or sink and is also the protocol used to build the topology in a WSN. To build it, it is necessary to exchange control messages, called DODAG Information Objects (DIO) and Destination Advertisement Object (DAO), which each node sends continuously. These messages contain information about the routing metric and the objective function to be used to find the parent node, which according to the objective function will be the optimal path to reach the root.  
The RPL process starts when the DODAG root node sends a DIO message with a range equal to 1. When it is received, the nodes update their range, which will be the distance to the root node. In addition, they use the objective function to select the best path to the root. This protocol is part of the 6 TiSCH stack Figure 1a, which includes the necessary protocols to adapt IPv6 to industry standards.  
The TSCH media access control protocol, is defined in the standard IEEE 802.15.4e, and is considered a suitable solution for providing real-time multi-hop transmissions in industrial environments [22]. TSCH offers guarantees in terms of latency and data delivery in IWSNs since it allows nodes to have a guaranteed space to send their packets. To do this, it divides the time into a fixed number of timeslots which are grouped into slotframes, which are cyclical, see Figure 1b. The most common duration of e  
Figure 1: (a) 6 TiSCH Protocol Stack (b) Time Slotted Channel Hopping Slotframe.  
enough time to be able to transmit a frame of the maximum size (127 Bytes), to wait for the Acknowledgement (ACK) from the receiver, and to process the packet. The frequency division is done with the assignment of a channel offset, which is used to determine the physical channel, therefore a timeslot and a channel offset must be defined for each transmission. The combination of these two parameters is a cell, which can be shared or dedicated. In the case of shared, any of the nodes can transmit if they have packets to send, otherwise the node goes to receiving mode.  
Therefore, in the shared slot the CSMA scheme is followed with backoff exponent to reduce collisions. On the other hand, if it is dedicated, a uni-directional link is established between two nodes and only the source node can transmit during the timeslot and in the specific offset channel. This time division requires that all nodes are synchronised so that they can exchange information. This is achieved with the Enhanced Beacon (EB) packet exchange, which includes information about the Absolute Sequence Number (ASN) which the nodes take as a time reference and determines the current timeslot. It also allows the clock drifts to be compensated. The ASN information is used with Channel offset to generate a rotation of the physical channels in each slotframe using Equation (1). This helps reduce the points of interference and path fading, thus increasing reliability.  
\[frequency=F[(ASN+Ch_{offset})modN_{ch}] \tag{1}\]  
To allocate each of the cells in the nodes, there must be a scheduling process to synchronise the transmission and reception states. However, the IEEE 802.15.4e standard does not include any specification on how to do this. For this reason, the following section explains different types of schedulers and details the operation of the most common of these.