The 802.11 Data-Link layer is divided into two sublayers:
The upper portion is the IEEE 802.2 Logical Link Control (LLC) sublayer, which is identical for all 802-based networks.
The bottom portion of the Data-Link layer is the Media Access Control (MAC) sublayer, which is identical for all 802.11-based networks. The 802.11 standard defines operations at the MAC sublayer. The MAC sublayer acts as an interface between the lower layer PHY and the upper LLC sublayer.
When the Network layer (layer 3) sends data to the Data-Link layer (layer 2), the data is handed off to the LLC and becomes known as the MAC Service Data Unit (MSDU). The MSDU contains data from the LLC and layers 3–7. A simple definition of the MSDU is that it is the data payload that contains the IP packet plus some LLC data.
When the LLC sends the MSDU to the MAC sublayer, the MAC header information is added to the MSDU to identify it. The MSDU is now encapsulated in a MAC Protocol Data Unit (MPDU). A simple definition of an MPDU is that it is an 802.11 frame.
Similar to the way the Data-Link layer is divided into two sublayers, the Physical layer (PHY) is also divided into two sublayers. The upper portion of the Physical layer is known as the Physical Layer Convergence Procedure (PLCP) sublayer, and the lower portion is known as the Physical Medium Dependent (PMD) sublayer. The PLCP sublayer prepares the frame for transmission by taking the frame from the MAC sublayer and creating the PLCP Protocol Data Unit (PPDU). The PMD sublayer then modulates and transmits the data as bits.
When you are at a door, it could be the entrance or the exit. It depends on what side of the door you are on, but either way, it is the same door. The PLCP Service Data Unit (PSDU) is a view of the MPDU from the other side. The MAC layer refers to an 802.11 frame as the MPDU, while the Physical layer refers to this same exact 802.11 frame as the PSDU. The only difference is which side of the door you are on, or, in the OSI model, from which layer of the model you are looking at the frame.
When the PLCP receives the PSDU, it then prepares the PSDU to be transmitted and creates the PLCP Protocol Data Unit (PPDU). The PLCP sublayer adds a preamble and PHY header to the PSDU. The various PHY headers will be discussed in detail in Chapter 2. The preamble is used for synchronization between transmitting and receiving 802.11 radios. After the PPDU is created, the PMD sublayer takes the PPDU and modulates the data bits and begins transmitting.
source: internet / cwap