The MAC frame format

The MAC frame format
The MAC Protocol Data Units (MPDUs) is a sequence of fields that is transmitted
down to the PHY layer, as shown in Figure 13.15 and it forms our MAC Frame. The
MAC frame format comprises the Frame Control field, a Duration/ID field, Address 1,
Address 2 and Address 3 fields, a Sequence Control field, Address 4 field, a Frame Body
field and a Frame Check Sequence (FCS) field.
The frame control field comprises a further set of fields, to include a Protocol
Version field, Type and Sub-type fields, to- and from-DS fields, More Fragments field, a
Retry field, a More Data field, a Protected field, which now replaces the WEP Protected
field and an Order field, as shown in Figure 13.16. The protocol version is 2-bits in
length and retains the current version of the standard in use. The type and sub-type
fields are 2-bits and 4-bits in length respectively, and are used to determine the purpose
of the frame, that is: control, data or management. The to- and from-DS fields are
both 1-bit in length. The to-DS field is set to one to indicate frames that are destined
for the DS whereas, the from-DS field is set to one to indicate frames leaving the DS.
In the more fragment field, which is a 1-bit entity the bit is set to one to indicate that
data and management type frames (as determined by the type and sub-type fields)
have additional data to follow. The retry field is a 1-bit unit that is set to one to indicate
that it is a retransmission of a previous frame. The power management field is also
a 1-bit field and is used to indicate the STA mode in operation; a value of one denotes
power-save mode, whereas a zero indicates that the STA will be in active mode. The
more data field is used to indicate when an STA is in power-save mode that more data
will follow. The protected frame field has been modified from the original specification,
which just accommodated WEP operation. In the 802.11i specification the field  has been reflected to accommodate new additions to the standard, namely the TKIP
and CCMP mechanisms, which we touched upon earlier. The 1-bit field, when set to
one, is used to signify that data within the frame has been processed by a cryptographic
encapsulation algorithm. Finally, the 1-bit order field is used to denote that
MSDUs are being sent in a strict order.
In returning to our general MAC frame in Figure 13.15, the next field is the
duration/ID field which is a 16-bit field and performs a dual task. The first task
involves carrying an Association Identifier (AID) of the STA that transmitted a control
type frame with power-save. In the second task the duration/ID carries a duration
value for each frame type (control, data and management