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.
Figure 18.17 Block acknowledgments
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
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