In addition to the TXOP mechanism (you can refer TXOP mechanism here)- to further enhance MAC efficiency, the 802.11e standard introduced clubbing all the ACKs for the different data packets sent in a TXOP in one go via a single packet. This packet was termed as a Block ACK (BACK) frame.
The Block-ACK frame can acknowledge data packets sent out by the 802.11 Station in the recent past (up-to previous 64 packets). The Transmitting Station requests the starting frame number in the Block ACK Request from which it desires an ACK from the responder station
Two methods of Block ACK were proposed
- Immediate Block ACK – The Block ACK is sent in the same Transmit Opportunity (TXOP)
- Delayed Block ACK – The Block ACK is sent in a later Transmit Opportunity (TXOP). The Transmitting station has to set the appropriate starting frame number from which it expects an ACK in the Block ACK Request. Delayed block ACK is not implemented by most vendors as the scheme entails maintaining the block ACK bitmap and the data frames across multiple Transmit Opportunities (TXOPs).
The Immediate Block ACK mechanism and the Delayed Block ACK mechanism are shown below
The above mechanism further optimized the transmission by removing individual ACK transmissions and one SIFS duration for each ACK transmission. The Frame burst mechanism described above is termed as contention free burst (CFB) as there is no contention between adjacent frame transmissions