If Multiple Mac Protocol Data units are aggregated to form one frame – then the aggregation is termed as A-MPDU aggregation. The A-MPDU aggregation is depicted pictorially below

Fig Courtesy: 802.11-2012^TM Standard

As defined in the standard – the MPDU delimiter is used as below

The purpose of the MPDU delimiter is to locate the MPDUs within the A-MPDU so that the structure of the A-MPDU can usually be recovered when one or more MPDU delimiters are received with errors. A delimiter with MPDU length of 0 is valid. This value is used to meet the minimum MPDU start spacing requirement.

The MPDU delimiter CRC field is an 8-bit CRC value. It is used as a frame check sequence (FCS) to protect the Reserved and MPDU Length fields. The CRC field is the ones complement of the remainder generated by the modulo 2 division of the protected bits by the polynomial  x⁸ + x² + x¹ +1, where the shift-register state is preset to all ones.

The A-MPDU frame aggregation follows the below rules as specified in the standard

• All the MPDUs within an A-MPDU are addressed to the same Receiver Address
• All QoS data frames within an A-MPDU that have a TID for which an HT-immediate Block Ack agreement exists have the same value for the Ack Policy subfield of the QoS Control field
• All protected MPDUs within an A-MPDU have the same Key ID.
• The Duration/ID fields in the MAC headers of all MPDUs in an A-MPDU carry the same value.
• Unless Multi-TID aggregation is specified – The priority of all frames in the A-MPDU are the same Access Category or TID

A-MPDU Parameters