||This article may be too technical for most readers to understand. (December 2009)|
Every frame transmitted by an 802.11 device has a significant amount of overhead, including radio level headers, Media Access Control (MAC) frame fields, interframe spacing, and acknowledgment of transmitted frames. At the highest data rates, this overhead can consume more bandwidth than the payload data frame. To address this issue, the 802.11n standard defines two types of frame aggregation: MAC Service Data Unit (MSDU) aggregation and MAC Protocol Data Unit (MPDU) aggregation. Both types group several data frames into one large frame. Because management information needs to be specified only once per frame, the ratio of payload data to the total volume of data is higher, allowing higher throughput.
MSDU aggregation relies on the fact that most mobile access points and most mobile client protocol stacks use Ethernet as their "native" frame format. It collects Ethernet frames to be transmitted to one or multiple destinations and wraps them in a single 802.11n frame. This is efficient because Ethernet headers are much shorter than 802.11 headers. An A-MSDU contains only MSDUs whose DA and SA parameter values map to the same receiver address (RA) and transmitter address (TA) values, i.e., all the MSDUs are intended to be received by a single receiver, and necessarily they are all transmitted by the same transmitter. NOTE—It is possible to have different DA and SA parameter values in A-MSDU subframe headers of the same A-MSDU as long as they all map to the same Address 1 and Address 2 parameter values. (reference: IEEE std 802_11-2012)
MPDU aggregation also collects Ethernet frames to be transmitted to a single destination, but it wraps each frame in an 802.11n MAC header. Normally this is less efficient than MSDU aggregation, but it may be more efficient in environments with high error rates, because of a mechanism called block acknowledgement. This mechanism allows each of the aggregated data frames to be individually acknowledged or retransmitted if affected by an error.
- "802.11n: The Next Generation of Wireless Performance" (PDF). Cisco. Retrieved 2009-04-27.