A Field Guide to Wireless LANs for Administrators and Power Users

Reprise of IEEE 802.11's MMPDU Structure

All of IEEE 802.11's MMPDUs have a common overall structure, which is shown in Figure 5-1 and might be referred to as the "MMPDU skeleton," since it represents the overall format of every MMPDU.

The MMPDU header is expanded in Figure 5-2.

The D subfield in Figure 5-2 represents the MMPDU's Duration, the DA and SA represent the Destination and Source MAC addresses, and the BSSID is the MAC address of the STA in the AP (in infrastructure mode), or the randomly computed BSSID that has been computed for the WLAN (in IBSS mode). The SC subfield is the Sequence Control subfield, which is used to sequence each frame and to support MAC-layer frame fragmentation. Observe that the MMPDU header is always only 24 octets, while the MPDU header can be either 24 or 30 octets in length.

All IEEE 802.11 frames begin with this two-octet FC field, which determines the format of the frame to follow, be it a Control frame, a Management frame (MMPDU), or a Data frame (MPDU). For the reader's convenience, the format of the FC field will be repeated here, in Figure 5-3. The definitions of all the bits in the FC field can be found in the main body of Chapter 4.

For MMPDUs, the FC field's Type and Subtype values help determine the unique structure of each of the possible MMPDU frame types. The main body of Chapter 4 showed the format of each type of MMPDU. The FC field of every IEEE 802.11 frame's header always has the format depicted in Figure 5-3, regardless of whether the frame is a Control frame, a Management frame (MMPDU), or a Data frame (MPDU).