The High-Throughput (HT) Immediate Block Ack was also introduced in 802.11n along-with aggregation of data packets. The combination of Immediate Block Ack and aggregation was a significant improvement on the Contention Free burst and block ack request/block ack mechanism.

The earlier legacy block ack mechanism required the WLAN device to send a block ack request frame to solicit a block ack from the receiver of the block ack request frame.

The contention free burst and block ack request /block ack frame exchange is shown below

In the case of aggregation, all the data frames are aggregated to form one A-MPDU packet. Also, each QoS data packet that is aggregated has a QoS control field which informs what kind of response is desired by the  transmitter station for the data packet that is transmitted

The QoS field is shown below

Fig Courtesy: 802.11 standard

The description of the ack policy bits is given below

The change that was brought in 802.11n was that if any frame in the aggregate A-MPDU contained the ack policy as normal ack, then the block ack request frame was not required to be sent and the block ack for the aggregated A-MPDU was implicitly implied.

The below image shows the aggregation + HT immediate ack method

With the above modifications, it is seen that the air medium is used in a more efficient manner. Multiple A-MPDUs might be sent as a contention free burst within a TXOP.

HT Delayed Block Ack