MSDU and MPDU
. . . .

---

MSDU and MPDU

MSDU (MAC Service Data Unit)

• When the Network layer (layer 3) sends data to the Data-Link layer, that data is handed off to the LLC and becomes known as the MAC Service Data Unit (MSDU).
• The payload of a 802.11 data frame is the layer 3–7 information known as the MSDU.
• A simple definition of the MSDU is that it is the data payload that contains the IP packet plus some LLC data.
• The upperlayer information that is contained in the body of an 802.11 wireless data frame is called a MAC Service Data Unit (MSDU).
• The forwarding of the MSDU is the switchlike intelligence that exists in either standalone APs or WLAN controllers. The intelligence that is often compared to a CAM table is known as the distribution system services
• The job of the integration service is to remove the 802.11 header and trailer and then encase the MSDU VoIP payload inside an 802.3 frame.
  

MPDU (MAC Protocol Data Unit)

• When the LLC sublayer 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 802.11 MPDU is that it is an 802.11 frame.
• As shown in Figure 9.1, an 802.11 MPDU consists of the following three basic components:
  

Figure 9.1 802.11 MPDU

---

Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.