# 3 MSF: A Multi-Modal Adaptation of the 6 TiSCH Minimal Scheduling Function for the Industrial IoT
## 3 Related Work  
Other researchers have also contemplated choosing between either fixed- or variable-duration timeslots. For example, [PERSON] et al. [13] envisioned using multi-mode TSCH slotframes to address application-specific performance requirements, mitigate the impact of interference, etc. For this purpose, the authors put forward two design strategies. Disregarding the authors' fixed-duration solution (which only served to characterize the medium), their variable-duration alternative uses a single low-rate PHY for TSCH beacons while sending all other traffic across a different/high-rate PHY. In other words, rather than being allowed to switch between communication modes (at run-time) on a per-link basis, devices must do so based on traffic type instead. [PERSON] et al. [14] demonstrated that fixed-duration timeslots can achieve considerable throughput. Hence, they saw value in the reduced implementational complexity of a fixed-duration approach and enabled the aggregation of multiple transmissions in one timeslot. The authors proposed two strategies for aggregating transmissions in unicast links, i.e., sending an acknowledgement in response to (1) individual unicast packets; or (2) a burst of unicast packets. Both of the authors' strategies are based on an autonomous SF called Orchestra [15], employing a single low-rate PHY for broadcast and beacon slots while enabling unicast slots to utilize either the low-rate or a different high-rate PHY. As the duration of a timeslot allows for one low-rate transmission, aggregating transmissions is only allowed in unicast slots. From a throughput perspective, transmission aggregation is most advantageous for unicast traffic anyway. [PERSON] et al. [14] explicitly reconfigured unicast links to use a certain PHY. Beginning from the PHY with the lowest data rate, a link's receiving endpoint assesses the quality of the link upon packet reception and, through a rate-limiting mechanism, determines whether switching to a PHY with a higher data rate (i.e., beginning with the next occurrence of a timeslot for the link) is appropriate or not. The decision is subsequently sent to the transmitting endpoint by repurposing bits within the returned acknowledgement(s). To prevent link configuration inconsistencies (and by extension, dropped packets) due to a lost acknowledgement, if the transmitter fails to receive an acknowledgement for four Tx attempts in a row (on a unicast link), it will automatically transition to the PHY with the next highest data rate. Proper configuration of the reconfiguration rate-limiting mechanism is essential, as cycling through PHYs is costly. [PERSON] et al. [16] based their variable-duration slot approach on MSF (see Section 2.1.2), which required several adaptations. Firstly, the minimal slotframe contains a minimal supercell for each unique communication mode. Nodes use the TSCH Slotframe and Link Informational Element (IE) [2] (Figure 7-52) carried in TSCH EBs to reserve groupings of subcells with consecutive slot offsets (starting from slot offset 0). Each subcell grouping forms the minimal supercell for a given mode. Additionally, the three Most Significant Bits (MSBs) of a LinkOptions field ([PERSON], 2016) (Figure 7-55), which are ordinarily reserved for future use, now indicate the communication mode to use in minimal supercells/subcells. Secondly, a node still has at most one autonomous Tx supercell towards any of its neighboring devices (\(\
eq\)RPL neighbors, in this case), and all autonomous supercells use the same communication mode. Thirdly, the authors proposed using the three MSBs of the CellOptions([PERSON], 2016) (Section 3.2.3) field present in certain 6P requests, such as the one depicted in Figure 4, to indicate the communication mode to use in the negotiated supercells that are the subject of said 6P management traffic. [PERSON] et al. (2016) identified a similar need (see Section 2.2) for annotating cells with communication modes.