# Parent and PHY Selection in Slot Bonding IEEE 802.15.4e TSCH Networks
## 2 Related Work  
The introduction of the IEEE 802.15.4g amendment and the commercial availability of IEEE 802.15.4g-compliant transceivers has spiked research on alternative PHYs and their potential for 6 TiSCH networks. There are many works carrying out experimental evaluations including range tests, interference robustness and the overall suitability of different IEEE 802.15.4g PHYs, tested outdoors, in smart buildings or in industrial scenarios [8; 9; 10; 11; 12; 13; 14]. In our previous work, the slot bonding technique was proposed to combine multiple PHYs with various data rates in a single TSCH network. It allows the combining of multiple TSCH time slots of regular length into a bonded slot with a length large enough to transmit or receive a packet using the data rate of the PHY selected for the link [7]. By adapting the length of the bonded slot as closely as possible to the duration required by the data rate of the selected PHY, slot bonding avoids wasting valuable airtime. Subsequently, the TSCH slot problem was described formally in terms of optimizing the packet delivery ratio while keeping the radio on time to a minimum. To be able to solve this slot bonding problem, a Genetic Algorithm (GA) was introduced. The GA could find the parent, PHY and number of slot selections in a multi-PHY slot bonding TSCH network in a near-optimal manner. As this GA was computationally too complex to be used in a real-time TSCH network, in this work, a computationally efficient parent and PHY heuristic is proposed. Currently, research that investigates specifically how different PHYs can be combined in a single TSCH network, is scarce. [PERSON] et al. adjusted the OpenWSN firmware to support Offset-Quadrature Phase Shift Keying (O-QPSK) (2.4 GHz at 250 kbps), Frequency Shift Keying (FSK) (868 MHz option 1 at 50 kbps) and Orthogonal Frequency Division Multiplexing (OFDM) (868 MHz option 1 MCS3 at 800 kbps) [6]. For a fair comparison, the authors used a 40 ms time slot that could facilitate all implemented PHYs. After evaluating the different PHYs on a 42 node office testbed, the authors conclude that no PHY outperforms the other PHYs for all metrics, and therefore the combination of different PHYs should be considered on a frame-by-frame basis (depending on the changing propagation characteristics of the link) in a 6 TiSCH architecture. Defining a policy on how to select the appropriate PHY was not in the scope of the article. In later work, [PERSON] et al. proposed 6 DYN, an extension of the 6 TiSCH stack with heterogeneous slot durations for different PHYs, implemented in OpenWSN [15]. The authors provide three different minimal cells in the TSCH schedule, one for each employed PHY. Additionally, they provide a separate row in the neighbor table for each different PHY of the same neighbor. These decisions allow for a seamless integration of the PHY and MAC layers in the routing layer and could be merged with the parent and PHY selection mechanism proposed in this work. 6 DYN is evaluated in a rather small experiment setup of four motes as a more elaborate evaluation is kept for future work. [PERSON] et al. propose different policies on packet replication and the use of multiple IEEE 802.15.4g modulations for different packets, that is, modulation diversity, to increase network reliability [16]. While how to exactly integrate the different modulations in TSCH is not discussed, the proposed policies should be considered as PHY selection mechanisms that can be applied on top of our slot bonding approach. Another recent interesting approach is presented by [PERSON] et al., who take into account the different data rates possible for different PHYs [4]. The authors defined two alternative timeslot structures allowing multiple packet transmissions to increase the throughput for higher data rate PHYs while maintaining a fixed slot duration. This approach is different from the slot bonding, in which only one packet is transmitted, but the slot length is adapted to the chosen PHY [7].