If the frame aggregation is carried out prior to MAC layer encapsulation – then the aggregation is termed as Aggregated MAC Service Data Unit (A-MSDU). The A-MSDU frame format is provided below

Fig Courtesy: 802.11-2012^TM Standard

As is seen from the figure, each A-MSDU sub-frame consists of a Destination Address (DA), Source Address (SA), Length of the sub-frame and 0-3 bytes of padding to align the sub-frame to a 4 byte boundary.

The A-MSDU frame aggregation follows the below rules as defined in the standard

• The Destination address and the Source Address would be the same across all the sub-frames in an A-MSDU
• The sub-frame within an A-MSDU cannot be fragmented
• The Access Category/TID priority within different sub-frames within an A-MSDU will not change
• The A-MSDU packet expiry will occur only after all the constituent MSDUs of the A-MSDU have expired
• The MAC variable – dot11HighthroughputOptionImplemented should be true for A-MSDU to be supported on an 802.11 Station
• Support for the reception of an A-MSDU, where the A-MSDU is carried in a QoS data MPDU with ACK Policy equal to Normal ACK and the A-MSDU is not aggregated within an A-MPDU, is mandatory for an HT STA (NOTE: ALL data MPDUs are A-MPDUs in 11ac and beyond).
• The use of an A-MSDU carried in a QoS data MPDU under a Block Ack agreement is determined per the Block ACK ADDBA agreement. A STA shall not transmit an A-MSDU within a QoS data MPDU under a Block Ack agreement unless the recipient indicates support for A-MSDU by setting the A-MSDU Supported field to “1” in its BlockAck Parameter Set field of the ADDBA Response frame.

A-MPDU Aggregation