Block acknowledgement (BA) was initially defined in IEEE 802.11e as an optional scheme to improve the MAC efficiency. Recently ratified amendment 802.11n enhances this BA mechanism then made it as mandatory to support by all 802.11n-capable devices (formally known as HT - High Throughput devices).
Instead of transmitting an individual ACK for every MPDU (i.e., frame), multiple MPDUs can be acknowledged together using a single BA frame. Block Ack (BA) contains bitmap size of 64*16 bits. These 16 bits accounts the fragment number of the MPDUs to be acknowledged. Each bit of this bitmap represent the status (success/failure) of a MPDU.
Block acknowledgement consist of a setup and tear-down phases. In the setup phase, capability information such as buffer size and BA policy are negotiated with the receiver. Once the setup phase completed, the transmitter can send frames without waiting for ACK frame. Finally the BA agreement is torn down with a so-called DELBA frame.
Read more about Block Acknowledgement: Block Ack Policies, BA Variants
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