MSDU is short for MAC service data unit.
MPDU is short for MAC protocol data unit.
In wireless network security, an MSDU is an Ethernet packet. After integrity check MIC, framing, encryption, serial number assignment, CRC checksum, and MAC header are added to an MSDU, the MSDU becomes an MPDU. An MPDU is a data frame encapsulated using 802.11.
The A-MSDU technology aggregates multiple MSDUs into a large payload. Typically, when an AP or a STA receives an MSDU from the protocol stack, it tags the MSDU with an Ethernet header, called the A-MSDU subframe. The A-MSDU technology encapsulates multiple A-MSDU subframes into an MPDU, which is called an A_MPDU subframe, in accordance with the 802.11 protocol, as shown in the following figure.
The A-MPDU technology aggregates several A_MPDU subframes encapsulated in accordance with the 802.11 protocol. Sending several MPDUs at a time reduces the PLCP preamble and header required to send each 802.11 packet, increasing the system throughput, as shown in the following figure.