# 3 MSF: A Multi-Modal Adaptation of the 6 TiSCH Minimal Scheduling Function for the Industrial IoT
## 1 Introduction  
Industrial Internet of Things (IIoT) devices increasingly possess multiple radio interfaces, each supporting multiple physical layers (PHYs) [1]. Nonetheless, higher protocol layers have not co-evolved to exploit these multi-modal capabilities, generally resulting in a single static link configuration being used throughout the network. Limiting all network devices to the same, statically configured communication mode (i.e., a given configuration of a given PHY) leads to sub-optimal performance, especially in dynamic environments. Additionally, this strategy often necessitates extensive manual setup when deploying IIoT networks across different sites. Moreover, extremely heterogeneous environments such as petrochemical installations may not even be fully serviceable with a single communication mode, leading to gaps in network coverage. As such, a network architecture that dynamically engages the multi-modal capabilities of modern IIoT devices on a per-link basis is needed to meet the demands of diverse operational landscapes.  
Over the past decade, a distinct subset of open protocol layers defined by the Internet Engineering Task Force (IETF) for use with the IEEE 802.15.4 PHY and Medium Access Control (MAC) layers [2] has become synonymous with IIoT-related research. Conceivably, this \"IETF stack\" partially owes its popularity to the promise of seamless integration into existing Internet Protocol (IP) infrastructure. However, besides the convenience of end-to-end IP connectivity, it also offers technical benefits specific to industrial applications, especially since the IEEE 802.15.4e amendment [3] introduced simultaneous Time-Division Multiple Access (TDMA) and Frequency-Division Multiple Access (FDMA) through the Time-Slotted Channel Hopping (TSCH) MAC mode, which has recently resulted in several IETF Requests For Comments (RfCs) related to an IP version 6 (IPv6) over the TSCH mode of IEEE 802.15.4 (6 TSCH) architecture [4, 5].  
Nonetheless, 6 TiSCH (the latest iteration of the IETF stack) still suffers from its MAC and routing layers typically not being configured to accommodate multi-modal operation. This is unfortunate, especially since both TSCH and the Routing Protocol for Low-Power and Lossy Networks (LLNs) (RPL) [6] (the 6 TiSCH routing layer) offer considerable implementational flexibility through TSCH Scheduling Functions (SFs) and RPL Objective Functions (OFs), respectively.  
To overcome these limitations, we designed the Multi-Modal Minimal Scheduling Function (3 MSP), which is partially based on (1) the 6 TiSCH Minimal Scheduling Function (MSF) [7]; (2) a state-of-the-art solution from literature; and (3) design considerations for a multi-modal 6 TiSCH architecture we identified in prior work [1]. In conjunction with a slightly modified version of an RPL OF previously designed for a non-TSCH-based version of the IETF stack [8], this constitutes our attempt at a multi-modal 6 TiSCH architecture capable of dynamic link reconfiguration. While (few) others (see Section 3) have made similar attempts, our solution is the first to fully harness the potential of modern IIoT devices through a combination of, among other things, variable-duration timeslots, simultaneous transmission, and allowing all management traffic to be sent on more than one communication mode, all while staying compliant with 6 TiSCH.  
The remainder of this paper is structured as follows. We first explain the basic working principles of 6 TiSCH and outline several considerations for multi-modal operation in Section 2 to better understand the related work in Section 3. Next, we break down the main concepts behind our new TSCH SF in Section 4 before describing a set of well-chosen simulation scenarios and examining their results in Section 5. Finally, Section 6 summarizes our findings.