In the last post, we introduced 802.11 MPDU consists "MAC Header", "Frame Body", and "Frame Check Sequence". Follow posts, we will focus on MAC header which has frame control, duration, addressing and so on.
Above the picture from CWAP
Note that if all the field are used, the maximum size of an 802.11 MAC header is 32 bytes. But, the 802.11n adds a new field to the 802.11 MAC header, called the HT Control field which has 4 byte long. If the HT Control field is used, the maximum size of 802.11 MAC header would be 36 bytes. The size of an 802.11 MAC hear is not always the same for two reasons. First, the QoS Control field is used only in QoS Data frames. Second, no every fame use all four fields. For example the follow MAC header has 24 bytes which has only three addresses (Transmitter address and Source address are the same) and no QoS Control Field.
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