What is MPDU and MSDU in WIFI?

In Wi-Fi communication, MPDU (MAC Protocol Data Unit) and MSDU (MAC Service Data Unit) are terms associated with the Medium Access Control (MAC) layer, defining different stages of data encapsulation and transmission. These units play a vital role in organizing and managing the flow of data between devices within a Wi-Fi network.

MPDU (MAC Protocol Data Unit):

  1. Definition:
    • An MPDU, or MAC Protocol Data Unit, represents the frame structure used by the MAC layer for the transmission of data over the wireless medium. It encapsulates data into a format suitable for transmission within the Wi-Fi network.
  2. Structure:
    • The MPDU includes various fields, such as the MAC header, Frame Control field, Address fields (for source and destination), Frame Body (containing the actual data payload), and a Frame Check Sequence (FCS) for error detection.
  3. MAC Header:
    • The MAC header within the MPDU contains control information, frame type, duration information, and other parameters necessary for the proper functioning of the MAC layer.
  4. Frame Control Field:
    • The Frame Control field is a key component of the MPDU, indicating the type of frame (data, management, or control), addressing information, and other control parameters.
  5. Usage:
    • MPDUs are used for the transmission of data frames within a Wi-Fi network. Data frames carry the payload of the communication, such as user data, and are an integral part of the overall communication process.
  6. MPDU Processing:
    • The MPDU undergoes processing at the MAC layer, including addressing, frame control validation, and error checking. Once prepared, it is handed over to the Physical (PHY) layer for modulation and transmission over the wireless channel.

MSDU (MAC Service Data Unit):

  1. Definition:
    • An MSDU, or MAC Service Data Unit, represents the data payload before it is encapsulated into an MPDU. The MSDU is the higher-layer data that is passed down to the MAC layer for transmission.
  2. Structure:
    • The MSDU consists of the user data that needs to be transmitted over the network. It does not include the MAC layer headers and control information added during the encapsulation process.
  3. Higher-Layer Data:
    • The MSDU carries the payload from higher-layer protocols, such as the Network layer (Layer 3) in the OSI model. This payload may include IP packets, TCP segments, or other protocol data.
  4. Encapsulation:
    • Before transmission, the MSDU undergoes encapsulation within the MAC layer, where the necessary headers, control information, and FCS are added to form the complete MPDU.
  5. Usage:
    • MSDUs are passed from higher-layer protocols to the MAC layer for transmission over the Wi-Fi network. The MAC layer is responsible for organizing and formatting the MSDU into an MPDU suitable for wireless transmission.
  6. Decapsulation:
    • Upon receiving an MPDU, the destination device’s MAC layer extracts the MSDU by removing the MAC layer headers and control information. The resulting MSDU is then delivered to higher-layer protocols for further processing.

In summary, MPDU (MAC Protocol Data Unit) and MSDU (MAC Service Data Unit) are integral components of the Wi-Fi communication process. The MSDU carries the higher-layer data payload, while the MPDU encapsulates this payload with MAC layer headers for transmission over the wireless medium. Understanding the distinction between these units is essential for comprehending the data transmission process in Wi-Fi networks.

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