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Block acknowledgements
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Block acknowledgements

  • All 802.11 unicast frames must be followed by an ACK frame for delivery verification purposes.
  • Multicast and broadcast frames are not acknowledged.
  • An A-MSDU contains multiple MSDUs all wrapped in a single frame with one MAC header and one destination.
  • Therefore, only normal acknowledgments are required when using MSDU aggregation.
  • An A-MPDU contains multiple MPDUs, each with its own unique MAC header.
  • Each of the individual MPDUs must be acknowledged; this is accomplished by using a multiple traffic ID block acknowledgment (MTBA) frame.
  • An MTBA is essentially a Block ACK frame for an A-MPDU.
  • Block ACKs were first introduced by the 802.11e amendment as a method of acknowledging multiple individual 802.11 frames during a frame burst.
  • As pictured in Figure 18.17, block acknowledgments in the form of an MTBA frame are also needed to cover the multiple MPDUs that are aggregated inside a single A-MPDU transmission.
  • Using a single acknowledgment for multiple frames or aggregated frames reduces MAC layer overhead and therefore improves throughput efficiency.
Picture
Figure 18.17 Block acknowledgments

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

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