There are three major 802.11 frame types: management, control, and data frames. Only 802.11 data frames carry an MSDU payload in the frame body. The frame body is of variable size. As you could see the following picture, the maximum frame body size is determined by the maximum MSDU size (2,304 octets) plus any overhead from encryption. The 802.11n-2009 HT amendment defi nes a frame aggregation method called Aggregate MAC Service Data Unit (A-MSDU). An 802.11n station using this method of aggregation can have a frame body with a maximum A-MSDU size (3839 or 7935 octets, depending upon the STA’s capability), plus any overhead from encryption.
The frame check sequence (FCS) also known as the FCS field contains a 32-bit cyclic-redundancy
check (CRC) that is used to validate the integrity of received frames. As you could see the following picture, the FCS is calculated over all the fields of the MAC header and the Frame Body field.
above the picture from CWAP
If any portion of a unicast frame is corrupted, the CRC will fail, and the receiving 802.11 radio will not send an ACK frame to the transmitting 802.11 radio. If an ACK frame is not received by the original transmitting radio, the unicast frame is not acknowledged and will have to be retransmitted.
above the picture from CWAP
check (CRC) that is used to validate the integrity of received frames. As you could see the following picture, the FCS is calculated over all the fields of the MAC header and the Frame Body field.
above the picture from CWAP
If any portion of a unicast frame is corrupted, the CRC will fail, and the receiving 802.11 radio will not send an ACK frame to the transmitting 802.11 radio. If an ACK frame is not received by the original transmitting radio, the unicast frame is not acknowledged and will have to be retransmitted.
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