## 3 Design of MCSS
### Multiple Concurrent Slofframes  
The MCSS uses three slotframes to handle heterogeneous traffic types (i.e., CM traffic, inter-cluster traffic, and intra-cluster traffic) in the WPT-enabled WSN, as follows:  
1. The CM slotframe is used to send and receive EB frames, which notify the MCSS schedule of the HAP cluster. It is also used to exchange other CMs, such as RPL messages and 6P messages. In the CM slotframe, one shared cell (TxRxS) capable of both transmission and reception is allocated, through which all CMs are exchanged between neighbors. The length of the CM slotframe is set in consideration of the EB transmission period. 2. The HAP slotframe is used to deliver data traffic (i.e., inter-cluster traffic) between HAPs, for which the HAP allocates at least one data transmission cell (DTx) to transmit data traffic to its parent. In addition, multiple data reception cells (DRxs) can be allocated to the HAP slotframe. The number of data reception cells equals the number of child HAPs of the HAP. The HAP slotframe length is set shorter than other concurrent slotframes to minimize the delivery latency of data traffic. 3. The WPT slotframe supports the transmission of power and data traffic (i.e., intra-cluster traffic) within the HAP cluster. In the WPT slotframe, multiple power transmission cells (PTxs) and data reception cells (DRxs) can be allocated by the HAP and sensor nodes through the three-step 6P transaction described in Section 3.2. The length of the WPT slotframe can be set differently for each HAP cluster and is determined by the HAP. The MCSS establishes communication schedules by projecting the multiple concurrent slotframes onto a single plane. This operation may cause overlap between cells allocated to different slotframes. To reduce the overlapped cells, MCSS sets the number of timeslots within each slotframe to be a different prime number, making the length of the slotframes mutually prime. In addition, the transmission priority for each slotframe is set to the following priority order considering the traffic characteristics: CM slotframe > HAP slotframe > WPT slotframe. Accordingly, if overlap occurs between cells, MCSS selects the cells allocated to a slotframe with the highest transmission priority. The WPT slotframe has the lowest transmission priority among slotframes. To compensate for the overlapped cells of the WPT slotframe, MCSS determines the length of the WPT slotframe for each HAP cluster using the minimum number of power and data cells and the number of overprovisioned cells. The length determination for the WPT slotframe is described in Section 3.2 in detail. Figure 2(a) presents an example of a WPT-enabled WSN topology consisting of three HAPs (HAP 0, 1, and 2) and two sensor nodes (sensor node 0 and 1). Figure 2(b)-d represent the CM, HAP, and WPT slotframes, and their lengths are 19, 5, and 11, respectively. In addition, HAP 1 can broadcast its EB frame in timeslots 0 and 19 (CM TxRxS) within the CM slotframes. In addition, it can exchange the RPL and 6P messages with its parent (i.e., HAP 0) or child nodes (i.e., HAP 2 and sensor node 1) in timeslots 0 and 19. Moreover, HAP 1 receives data packets from HAP 2 in timeslots 2, 7, 12, and 17 (DRxs) and transmits data packets to HAP 0 in timeslots 3, 8, 13, and 18 (DTxs) within the HAP slotframes. In the WPT slotframe, HAP 1 transmits power to sensor node 1 using power transmission cells (PTxs) and receives data packets from sensor node 1 using data reception cells (DRxs). Figure 4 illustrates the MCSS schedule of HAP 1 in Figure 2(a). In the figure, multiple concurrent slotframes of HAP 1 are projected onto one plane, and as a result, only one cell is allocated to each timeslot. Specifically, the data reception cell (DRx) of the HAP slotframe and data reception cell (DRx) of the WPT slotframe overlap in the cell (1 and 2) of the MCSS schedule. In this case, the data reception cell (DRx) of the HAP slotframe is allocated to the MCSS schedule of HAP 1 because the HAP slotframe has a higher priority than the WPT slotframe.