With increasing Wi-Fi PHY data rates, the payload airtime shrinks while the PHY header airtime remains constant. Aggregating multiple frames helps ensure the PHY payload duration exceeds the overhead of the PHY preamble and header.
There are two types of aggregation: AMSDU aggregation and AMPDU aggregation. A transmitter can choose to use only AMSDU aggregation, only AMPDU aggregation, or a combination — AMSDU within AMPDU — which is more common nowadays. Each Wi-Fi implementation may have different preferences for selecting the type of aggregation.
AMSDU is an aggregation of multiple MSDUs. A MAC header and FCS field are added to the AMSDU to create an MPDU (or MAC frame). Padding bytes are included to round the size up to a multiple of 4 bytes (32 bits), aligning with memory read/write boundaries.
AMPDU is an aggregation of multiple MPDUs, each of which is treated as a subframe. A delimiter and padding are added to each MPDU. The full AMPDU becomes the PHY layer’s payload, known as the PSDU.
Please don’t ask trivial questions like what MSDU, MPDU, or PSDU stand for — focusing on acronyms can distract from understanding what they actually do.
Some receivers require additional time for de-aggregation and may need minimal spacing between MPDUs. These receivers advertise a Minimum MPDU Start Spacing parameter (a.k.a. MPDU Density) in microseconds, included in the HT Capabilities IE. To meet this spacing requirement, the transmitter inserts additional zero-length MPDU delimiters between MPDUs.
All AMPDU subframes must be QoS Data frames with the same TID and RA. In essence, WMM (Wi-Fi Multimedia) support is a prerequisite for aggregation.
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