layer1 and layer2

At the beginning, I saw the seven-layer network model of osi. No, it should be 8 layers. The last layer is people. It feels really too complicated. However, for hardware engineers, we are only exposed to the bottom four layers:
PHY
DATA
NETWORK
TRANSPORT.
PHY is divided into two sub-layers: PLCP (physical layer convergence protocol), PMD (physical media dependent),
DATA is divided into: LLC (Logical link control), MAC (media access control).
LLC receives data from layers 3-7, called It is MSDU (MAC service data unit), and it is delivered to the MAC layer.
The MAC layer adds the second layer of information, such as RA, TA, BSSID. It is delivered to the PHY layer and is an MPDU (MAC payload unit).
PMD converts MPDU to SYMBOL.
PLCP adds PLCP header. Like MODULATION, Length, CRC. In addition, PlCP will add PREAMBLE at the end, which is a string of modulated SYMBOL sequences. The receiver is used to adjust the gain of the receiver and synchronize SYMBOL TIMING.