As an example, in Fig. 3, three sensors S1, S2, and S3 have
residual voltages of 2.6, 2.3, and 3.0 V, respectively. Since the
voltage of S2 has reached a pre-set threshold, 2.3 V, this node
sends out the RFE packet requesting for immediate charging.
RF-MAC, through the access priority mechanism (Section V-C),
ensures that S2 wins the channel access for energy transfer
earlier than S1 and S3 that have data packets to deliver. At this
time, S1 and S3 are forced to freeze their backoff timers and
get into charging mode, as data communication is infeasible.
Once RFE packet is received by the ETs, they reply back with
the CFE, which is a short energy pulse.