# Parent and PHY Selection in Slot Bonding IEEE 802.15.4e TSCH Networks
## 1 Introduction  
Industrial applications that are deployed on low-power wireless sensor networks often demand a trade-off between data rate, energy consumption, latency and reliability. To fit the exact requirements, IPv6 over the TSCH mode of IEEE 802.15.4e (6 TiSCH) networks allow the tuning of the network characteristics at different layers of the network stack, starting from the IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) Medium Access Control (MAC) layer up to the application layer [1, 2]. TSCH was introduced to combat external interference and multi-path propagation effects in challenging wireless sensor network environments. To do so, it uses a tightly synchronized time-slotted schedule and pseudo-random channel hopping to provide high reliability while maintaining low-power operation. Nevertheless, the 6 TiSCH Industrial Internet of Things (IIoT) stack is still limited by the characteristics of the chosen physical (PHY) layer, which is often challenged by the harsh conditions of industrial environments filled with other wireless devices andmetal obstacles. To overcome this limitation, the TSCH and 6 TiSCH community is shifting research efforts to the support of multiple PHY layers [3; 4; 5; 6]. Employing different PHYs in a single TSCH network also means embracing the different characteristics of each PHY, such as its data rate, bandwidth, computational complexity, energy consumption and wireless reliability. Integrating multi-PHY support in a 6 TiSCH sensor network therefore implies, on the one hand, dealing with the different transmission lengths and bandwidth requirements to fit the PHY allocation in the TSCH schedule and, on the other hand, defining policies on how to select the appropriate PHY for each network link. In our previous work, the question of how different PHYs can be fitted into a TSCH schedule was answered by introducing the concepts of slot and channel bonding [7]. The former allows the combination of multiple TSCH time slots of regular length into a larger bonded slot. The length of the resulting bonded slot is large enough to transmit or receive a packet using the data rate of the PHY selected for the link. The concept of the latter is similar, but now multiple frequency channels are bonded together to a channel wide enough to communicate using the selected PHY. Slot and channel bonding are considered resource-efficient as both adapt the resources (i.e., the time slots and channels) exactly to the requirements of the selected PHYs. In this work, to facilitate the simultaneous use of multiple PHYs in a single TSCH sensor network, the slot bonding technique is used. In particular, the parent and PHY selection for each network node is investigated, related to making correct routing decisions based on the available PHYs and being able to schedule those PHY resources in the TSCH schedule. The contributions of this article are two-fold. First, a computationally efficient heuristic is proposed that is a step towards a distributed PHY and parent selection mechanism for a TSCH network that applies slot bonding. The performance of this heuristic is initially evaluated via simulation results, by comparing it against a near-optimal, but computationally complex, multi-PHY scheduler. Additionally, a proposal on how this heuristic can be implemented in the Routing Protocol for Low-power and Lossy network (RPL) is also presented. Second, an easy-to-configure TSCH slot bonding implementation is proposed in the latest version of the Contiki-NG IIoT operating system. Subsequently, this slot bonding implementation is used to extensively evaluate the proposed heuristic on a real hardware sensor testbed. The remainder of this article is structured as follows: first, the related work on using multi-PHY TSCH is discussed in Section 2. Second, in Section 3, the parent and PHY selection heuristic is introduced. In Section 4, the TSCH slot bonding implementation in the Contiki-NG firmware is described. Afterwards, the heuristic is evaluated in Section 5.