802.11n and later WLAN standards introduced the concept of data frame aggregation. Even in earlier 802.11 standards, it was possible to send certain frames aggregated with a data frame (for e.g. CF-END frame with a QoS Frame), however, Data frames were never aggregated till then.
The reason for aggregation of data frames can be depicted in the picture below
FIG COURTESY: http://www.eetimes.com/document.asp?doc_id=1278239
As can be seen from the figure above – each data frame is accompanied by an ACK and two SIFS period before another Data frame can be transmitted. The 802.11e standard further introduced a Block ACK session mechanism which allowed multiple Data frames to be transmitted separated by a SIFS frame which was followed by a Block ACK Request/Block ACK frame. Even then, multiple SIFS transmissions would still account for a significant amount of air time. To further improve air time utilization – aggregation of Data frames was introduced in the 802.11n Standard.
The Block ACK session establishment as defined by 802.11e was continued in the 802.11n and later standards for the establishment of an aggregation session for a particular TID.
The 802.11n standard defined two methods of aggregation – A-MSDU and A-MPDU
The AMSDU frame aggregation was to append multiple frames before MAC layer encapsulation and A-MPDU frame aggregation was to append multiple MAC Protocol Data Units (MPDUs) together