Overall MAC Frame Format

 
Overall MAC Frame Format
The IEEE 802.11 standard specifies an overall MAC frame format, as shown in Figure 4.7.
This frame structure is found in all frames that stations transmit, regardless of frame type.
After forming the applicable frame, the MAC coordination passes the frame’s bits to the
Physical Layer Convergence Protocol (PLCP), starting with the first bit of the Frame Control
field and ending with the last bit of the frame check sequence (FCS). 
• Frame Control This field carries control information being sent from station to station.
Figure 4.8 illustrates specific subfields within the Frame Control field.
• Duration/ID In most frames, this field contains a duration value, depending on the
type of frame sent. (See the section “MAC Frame Types,” later in this chapter, for
possible values.) In general, each frame contains information that identifies the duration
of the next frame transmission. As an example, the Duration/ID field in data and acknowledgment (ACK) frames specifies the total duration of the next fragment and
acknowledgment. Stations on the network monitor this field and hold off transmissions
based on the duration information.
In Power Save–Poll control frames only, the Duration/ID field carries the 14 least significant
bits of the association identity of the sending station. The two remaining bits for this
field are set to 1. Possible values for this identification are currently in the decimal range
1–2007.
• Address 1, 2, 3, and 4 The address fields contain different types of addresses, depending
on the type of frame being sent. These address types may include the basic service
set identification (BSSID), source address, destination address, transmitting station
address, and receiving station address. IEEE standard 802-1990 defines the structure of
the addresses, which are all 48 bits in length.
The addresses can be either individual or group addresses. There are two types of group
addresses: multicast, which associate with a group of logically related stations, and
broadcast addresses, which refer to all stations on a given LAN. A broadcast address
consists of all ones.
• Sequence Control The leftmost four bits of this field consist of a Fragment Number
subfield, indicating the fragment number of a particular MSDU. This number starts with
0 for the first fragment, then increments by 1 for each successive transmission. The next
12 bits of this frame are the Sequence Number subfield, starting at 0 and incrementing
by 1 for each subsequent MSDU transmission. Each fragment of a specific MSDU will
have the same sequence number.
A station shall have one or more outstanding MDDUs concurrently. On reception of a
frame, a station can filter duplicate frames by monitoring the sequence and fragment
numbers. The station knows the frame is a duplicate if the sequence number and fragment
number are equal to the frame immediately preceding, or the Retry bit is set to 1.
Duplicate frames can occur when a station receives a frame without errors and sends an
ACK frame back to the sending station, then transmission errors destroy the ACK frame
en route. After not receiving the ACK over a specific time period, the sending station
retransmits a duplicate frame. The destination station performs an acknowledgement of
the retransmitted frame even if the frame is discarded due to duplicate filtering.
• Frame Body This field has a variable length payload and carries information that pertains
to the specific frame being sent. In the case of a data frame, this field may contain a
LLC data unit (also called an MSDU). MAC management and control frames may
include specific parameters in the Frame Body that pertain to the particular service the
frame is implementing. If the frame has no need to carry information, then this field has
length of zero. The receiving station will determine the frame length from a field within
the applicable Physical layer headers (see Chapter 5, “IEEE 802.11 Physical (PHY)
Layer”).
• Frame Check Sequence (FCS) The MAC layer at the sending station calculates a 32-
bit frame check sequence (FCS) using a cyclic redundancy code (CRC) and places the
result in this field. The MAC layer uses the following generator polynomial over all
fields of the MAC header and Frame Body to calculate the FCS:
G(x)=X32+X26+X23+X22+X16+X12+X11+X10+X8+X7+X5+X4+X2+X+1.
The result’s highest-order coefficient is placed in the field at the leftmost bit. The
receiver implements a CRC to check for transmission errors in the frame.