In the IEEE 802.11 architecture, data encapsulation occurs as data moves down the protocol stack. The Physical (PHY) layer receives a data unit from the MAC layer, which is referred to as the PHY Service Data Unit (PSDU). The PHY layer's Physical Layer Convergence Procedure (PLCP) sublayer then prepends a preamble and a PHY header to this PSDU. This newly created data unit, which includes the PHY-level framing and is ready for transmission over the wireless medium, is called the PHY Protocol Data Unit (PPDU).

A. MPDU: The MAC Protocol Data Unit (MPDU) is created by the MAC layer, not the PHY layer, by adding a MAC header to the MSDU.

B. PSDU: The PHY Service Data Unit (PSDU) is the payload that the PHY layer receives from the MAC layer; it is not the final framed unit created by the PHY.

C. MSDU: The MAC Service Data Unit (MSDU) is the data payload from the upper network layers before any 802.11 MAC or PHY headers are added.

1. IEEE Std 802.11™-2020 (Revision of IEEE Std 802.11-2016), IEEE Standard for Information Technology—Telecommunications and Information Exchange between Systems—Local and Metropolitan Area Networks—Specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.

Section 17.2.2 (PLCP sublayer): States, "The PLCP sublayer adds a preamble and a PHY header to the MPDU to create the PLCP protocol data unit (PPDU)." (Note: The PSDU is the MPDU from the MAC layer's perspective).

Figure 17-2 (PPDU general frame format): Visually depicts the PPDU as being composed of the PLCP Preamble, PLCP Header, and the Data field (which is the PSDU).

2. Kurose, J. F., & Ross, K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson.

Chapter 7, Section 7.3 (IEEE 802.11 Wireless LANs): This chapter describes the 802.11 frame structure, explaining that the PHY layer takes the MAC frame (MPDU) and adds its own header and preamble to create the physical-layer frame (PPDU) for transmission. This aligns with the standard's definition of encapsulation.

3. University of California, Santa Barbara, Courseware (CS 176A, Introduction to Computer Communication Networks).

Lecture Slides on "Wireless and Mobile Networks": These materials typically illustrate the 802.11 frame encapsulation process, showing the progression from MSDU to MPDU at the MAC layer, and finally to PPDU at the PHY layer, clarifying that the PPDU is the unit created by the PHY layer for transmission.