The MAC sublayer accepts MSDUs from higher layers in the protocol stack
to send them to the equivalent layer of the protocol stack in another station. The
MAC adds information to the MSDU in the form of headers and trailers to gener-
ate a MAC protocol data unit (MPDU). The MPDU is then passed to the physical
layer to be sent over the wireless medium to other stations. The MAC may frag-
ment MSDUs into several frames to increase the probability of each individual
frame being delivered successfully. The MAC frame contains addressing informa-
tion, information to set the network allocation vector (NAV), and a frame check
sequence to verify the integrity of the frame. The general IEEE 802.11 MAC
frame format is shown in Figure 21.16.
The MAC frame format contains four address fi elds. Any particular frame
type may contain one, two, three, or four address fi elds. The address format in
IEEE 802.11-1997 is a 48-bit address, used to identify the source and destination
of MAC addresses contained in a frame, as IEEE 802.3. In addition to source
address (SA) and destination address (DA), three additional address types are
defi ned: the transmitter address, the receiver address (RA), and the basic service
set identifi er (BSSID). The BSSID is a unique identifi er for a particular basic ser-
vice set of the IEEE 802.11 WLAN. In an infrastructure basic service set, the
BSSID is the MAC address of the AP.
The
transmitter address
is the address of the MAC that transmitted the
frame onto the wireless medium. This address is always an individual address.
The transmitter address is used by stations receiving a frame to identify the station
to which any responses in the MAC frame exchange protocol will be sent.
The
receiver address
(
RA
) is the address of the MAC to which the frame is
sent over the wireless medium. This address may be either an individual or group
address.
The
source address
(
SA
) is the address of the MAC that originated the frame.
This address is always an individual address.This address does not always match
the address in the transmitter address fi eld because of the indirection that is per-
formed by the distribution system of an IEEE 802.1 WLAN. It is the SA fi eld that
should be used to identify the source of a frame when indicating that a frame has
been received to higher layer protocols.
The
destination address
(
DA
) is the address of the fi nal destination to which
the frame is sent. This address may be either an individual or group address.
This address does not always match the address in the RA fi eld because of the
indirection that is performed by the DS.
MAC Header
Frame
Control
Duration/
ID
Address
2
Address
1
Address
3
Sequence
Control
Address
4
Frame
Body
FCS
