The primary goal of both 802.11n and 802.11ac was to provide high throughput and bigger data rates. However, frame aggregation is one key aspect of 802.11n/ac that did enhance airtime efficiency. Frame aggregation is a method of combining multiple frames into a single frame transmission. Fixed MAC layer overhead and medium contention overhead are reduced, which results in less airtime consumption.
Two terms that everyone should understand are MSDU and MPDU. An 802.11 MAC Service Data Unit (MSDU) is the layer 3–7 payload of an 802.11 data frame. An 802.11 MAC Protocol Data Unit (MPDU) is essentially a technical term for wireless frame. An MPDU consists of a frame header, body and trailer with the MSDU payload encapsulated in the frame body.
802.11n defined two methods of frame aggregation. The first method of frame aggregation known as Aggregate MAC Service Data Unit (A-MSDU) aggregates multiple MSDUs into a single frame transmission. As shown in Figure 1, the second method known as Aggregate MAC Protocol Data Unit (A-MPDU) aggregates multiple frames into a single transmission followed by a Block Acknowledgement.
A-MPDU quickly became the most commonly used method of frame aggregation because there is less retransmission overhead when Block Acknowledgements are utilized. For this reason, the 802.11ac amendment only defined the use of A-MPDU aggregation. All 802.11ac frames are transmitted using the Aggregate MAC Protocol Data Unit (A-MPDU) format. Although A-MPDU is required for 802.11ac transmission, it should be noted that A-MSDU and A-MPDU can be used together. The payload of an A-MPDU transmission can be multiple A-MSDUs.
802.11ax defines multiple enhancements at the Physical Layer, such as OFDMA to improve efficiency. A lesser-known efficiency enhancement actually resides in the MAC sublayer.
Prior to 802.11ax, the individual MPDUs must all be of the same 802.11e QoS access category when A-MPDU frame aggregation is used. Voice MPDUs cannot be mixed with Best Effort or Video MPDUs within the same aggregated frame. 80211ax introduces Multi-Traffic Identifier Aggregated MAC Protocol Data Unit (Multi-TID AMPDU), which allows the aggregation of frames from multiple traffic identifiers (TIDs), from the same or different QoS access categories. The ability to mix MPDUs of different QoS traffic classes allows 802.11ax radios to aggregate more efficiently, reducing overhead and thus increasing throughput and therefore overall network efficiency.