Now we have two new 802.11 friends called A-MSDU and A-MPDU. Their job: to make the protocol analyst's job more difficult.
A-MSDU = Aggregate MSDU
A-MPDU = Aggregate MPDU
Both can be used together, with restrictions (of course, because it's not as much fun unless you have extra rules to follow).
You can string MSDUs together - called subframes when aggregated - in the payload of a single QoS MPDU. An A-MSDU subframe structure consists of:
DA (6), SA (6), Length (2), MSDU (0-2304), Padding (0-3) -- all of the numbers are in octets