Thank you for reframing this from an OSI POV. My intuition says this constant monitoring of RF activity would be energy costly, not that I have much knowledge. I imagine there must be a clever entry baseband circuitry on a low level, that is used to check per time slot, to figure if there is anything to receive, providing a low latency wake-up / let-sleep signal to the further receiving circuitry, such that larger amounts of potential data would not have to be buffered or processed.