Acknowledgement (ACK) - Control Frame

 For the wireless communication, reliable data transmission is critical. Two key mechanisms that ensure this reliability are Acknowledgement (ACK) frames and Block Acknowledgement (Block ACK) frames. Let's dive deep into how these frames work and why they're essential for the Wi-Fi networks.

The Fundamentals of ACK Frames

ACK frames serve a dual purpose in wireless networks:

  1. Confirmation of Receipt: An ACK frame signals the sender that the frame is received. ACK could be used for data, management, or PS-Poll frame . This eliminates the need for automatic retransmission.
  2. Duration Information: For fragment bursts, the ACK frame transmits duration information to nearby Station(STAs), functioning similarly to Clear to Send (CTS) frames.

ACK frame structure: 

  • Frame Control: Contains control information for the frame (2 bytes)
  • Duration: Specifies timing information for upcoming transmissions (2 bytes)
  • RA (Receiver Address): Identifies the individual MAC address of the STA receiving the ACK (6 bytes)
  • FCS (Frame Check Sequence): Used for error detection (4 bytes)

    • If you noticed, ACK Frame structure is similar to CTS Frame as described in my other Blog (
    RTS/CTS: The Hidden Node Problems in WiFi Networks)

    Wireshark Filter: wlan.fc.type_subtype == 29


    How ACK Frames Prevent Network Congestion ?

    Without ACK frames, wireless networks would be significantly less efficient. When a transmitter doesn't receive an ACK frame, it assumes the original frame was lost due to interference or other issues and initiates retransmission.

    To prevent network congestion, a random backoff timer is used that increases with each retransmission attempt up to a maximum of 1023. This mechanism is crucial because:

    • It prevents a single STA from consuming excessive airtime
    • It encourages data rate adaptation rather than endless retries
    • It's more efficient to send a frame multiple times at a higher data rate (54 Mbps) than to drop to a lower rate (150 Mbps)

    Duration Field in ACK Frames:

    The Duration field value depends on what type of frame is being acknowledged:

    • Zero value: Used when acknowledging a PS-Poll frame or when acknowledging management/data frames where the More Fragments subfield is zero. An example is shown in the image above. 
    • Calculated value: If more fragments are expected, the Duration value is calculated as the time needed to transmit subsequent data frames, ACK frames, and SIFS intervals

    This calculated value is measured in microseconds.


    Reference : CWAP Official Study and Reference Guide

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