In the 802.11 MAC as it was originally specified in 1997, every frame required a positive acknowledgment. Each transmission was not complete until an acknowledgment was received. Network traffic is generally bursty. For example, a user reading web pages will send out a request, receive a flurry of packets carrying the requested web page, and then the network will generally be idle while the user reads the page.
The original conception of the 802.11 MAC required that each frame sent to the receiver be acknowledged separately, as in Figure 5-7(a). The quality-of-service extensions in 802.11e brought in block acknowledgments (usually abbreviated as BlockACK), which allowed the sender to transmit a stream of frames and have them all acknowledged at once.
Conceptually, the BlockACK extensions work similar to the selective ACK option in TCP. Two forms were defined in 802.11e, and subsequently carried over to 802.11n, and both are illustrated in Figure 5-7(b). With the Immediate Block ACK form, the sender transmits a series of frames and expects an acknowledgment right away, while in the Delayed Block ACK form, the receiver can send its acknowledgment later.
reference text: 802.11n survival guide