In the 802.11 frame encapsulation process, the MAC sublayer creates the MAC Protocol Data Unit (MPDU) and passes it to the Physical (PHY) layer. The PHY layer treats this incoming MPDU as its payload, which is called the PHY Service Data Unit (PSDU). The PHY layer's primary function is to prepare this data for transmission over the air. To do this, it prepends a Preamble (for synchronization) and a PHY Header (also called the PLCP Header, containing transmission parameters like data rate and length) to the PSDU. The resulting complete unit, ready for transmission, is the PHY Protocol Data Unit (PPDU).

A. LLC: The Logical Link Control (LLC) sublayer operates above the MAC sublayer and is not involved in the creation of the physical layer frame.

B. Network: The Network layer (Layer 3) handles logical addressing (e.g., IP) and routing, passing data down to the Data Link layer (MAC/LLC).

D. MAC: The MAC layer adds the MAC header to create the MPDU. It provides the PSDU to the PHY layer but does not add the PHY header itself.

1. IEEE Std 802.11™-2020 (Revision of IEEE Std 802.11-2016). Section 17.2.2, "PPDU format." The diagrams and descriptions in this section illustrate that the Physical Layer Convergence Procedure (PLCP) Preamble and PLCP Header are added to the PSDU at the physical layer to form the PPDU.

2. Carnegie Mellon University, ECE 18-759: Wireless Networks, Lecture 6: IEEE 802.11 Physical Layer. Slide 10, "PLCP Frame Format." The slide explicitly shows a diagram where the "PLCP Preamble" and "PLCP Header" are prepended to the "PSDU (from MAC)" by the PHY layer. This demonstrates the PHY layer's responsibility for adding its own header.