MSDU Aggregation (A-MSDU)ΒΆ
MSDU Aggregation (A-MSDU) is the process by which a station may combine multiple MSDUs (created by the Ethernet encapsulation) into a single outgoing 802.11 transmission in order to mitigate the effect of MAC overhead. The process is described in the 802.11-2012 standard in section 9.11. To be eligible for aggregation, Ethernet frames must:
- Share receiver addresses (i.e.
addr1) after the Ethernet encapsulation process. - Share transmitter addresses (i.e.
addr2) after the Ethernet encapsulation process. - Be received by an HT-capable station.
- Not exceed a total length communicated by each station in the HT Capabilities management tag after aggregation.
Fig. 4.19 MSDU Aggregation (A-MSDU)
The exact makeup of any given A-MSDU can only be determined at the time of transmission since it depends on whether there are other eligible MSDUs enqueued for the intended recipient. The High MAC Framework provides a function for dequeing multiple enqueued MSDUs provided they all meet the above requirements. The High MAC Framework then provides a function for combining the multiple MSDUs into a single MPDU to be passed off to the MAC application running in CPU Low.
From the perspective of the CPU Low MAC application, an MPDU containing an A-MSDU is indistinguishable from non-aggregate MPDUs. The entire set of aggregated packets is protected with a single frame check sequence (FCS).