# 3 MSF: A Multi-Modal Adaptation of the 6 TiSCH Minimal Scheduling Function for the Industrial IoT
## 4 The Multi-Modal Minimal Scheduling Function
### TSCH Mode Abstraction  
As we wanted to use a Parent-Oriented (PO) approach (meaning devices are RPL neighbors) based on prior work ([PERSON] et al., 2017; [PERSON] et al., 2018) combined with a Tx-based routing metric (see Section 4.6), we needed a way to keep fresh inferred metrics without crowding the schedule. Hence, we devised an abstraction called \"TSCH modes\". A TSCH mode uniquely identifies which communication mode(s) to use in a (super)cell, and (potentially) when to do so. Firstly, we defined a unique non-simultaneous TSCH mode for each unique communication mode. In a supercell tagged with a non-simultaneous TSCH mode, transmission can only occur on the corresponding communication mode. Through a process we elaborate on in Section 4.3, one can determine the number of consecutive subcells required for a supercell tagged with a non-simultaneous TSCH mode. The timeslot timings (see ([PERSON] et al., 2018) (Section 6.5.4.2)) to use follow from this supercell length. Additionally, we defined a group of simultaneous TSCH modes. During simultaneous transmission, an identical copy of a packet (including MAC sequence number) is sent on all (available) communication modes pointed to by the simultaneous TSCH mode. The length \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multicolumn{2}{c}{**Fixed- or**} & \multirow{2}{*}{\begin{tabular}{c} **Multi-Mode** \\ **Duration Slots** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Simultaneous** \\ **Management** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Tx-Only** \\ **Runing Metric** \\ **Support** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Explicit Link** \\ **Reconfiguration** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **6 TiSCH** \\ **Compliant****a** \\ \end{tabular} } \\ \hline
[13] & fixed & — & — & ✓ & — & ✓ \\  & variable & ✗ & ✗ & ✓ & ✗ & ✗ \\ \hline
[14] & fixed b & ✗ & ✗ & ✓ & \(\pm\) c & \(\pm\) d \\ \hline
[16] & variable & \(\pm\) e & ✗ & ✗ & \(\pm\) c & ✓ \\ \hline This paper & variable & ✓ & ✓ & ✓ & \(\pm\) c & ✓ \\ \hline \hline \end{tabular}  
* We consider a solution compliant with 6 TiSCH if it can be integrated into a 6 TiSCH stack as is; b Multiple packets may be aggregated into a single timeslot; c Only (part of) unicast links are explicitly re-configurable; d 6 TiSCH compliance depends on the transmission acknowledgement scheme; e Autonomous supercells (and any management traffic sent on them) are single-mode.  
\end{table} 
Table 1: Overview of TSCH-based single-topology solutions for multi-modal Industrial Internet of Things (IIoT). Note that ✓ indicates full support, ✗ indicates no support, \(\pm\) indicates partial support, and — indicates “not applicable”. Adapted from ([PERSON] et al., 2017) with permission from the authors. of a supercell tagged with a simultaneous TSCH mode equates to the longest supercell length that would ordinarily be used for any non-simultaneous TSCH mode corresponding to the communication modes it points to. For example, if the non-simultaneous TSCH modes corresponding to its underlying communication modes require a supercell of length two and three, then a supercell tagged with a simultaneous TSCH mode has a supercell length of three. Simultaneous transmissions must adhere to the timeslot timings required for this longest underlying supercell length. Alternating TSCH mode is a special simultaneous mode that almost entirely evaluates to one of the other simultaneous TSCH modes (including its timings), depending on the starting ASN and the length of the slotframe (all 3 MSF slotframes are of equal length, see Section 4.2). The only difference is that the length of a supercell tagged with alternating mode is fixed to the longest required supercell length of any of the simultaneous TSCH modes to which it can evaluate. Alternating TSCH mode provides a means of cycling through all possible interface-modes towards a neighbor while occupying a minimal amount of time-frequency space.