IEEE 802.11 defines three types of stations based on their mobility in a wireless LAN: no-transition, BSS•transition, and ESS-transition mobility. A station with no-transition mobility is either stationary (not moving) or moving only inside a BSS. A station with BSS-transition mobility can move from one BSS to another, but the movement is confined inside one ESS. A station with ESS-transition mobility can move from one ESS to another. However, IEEE 802.11 does not guarantee that communication is continuous during the move. MAC Sublayer IEEE 802.11 defines two MAC sublayers: the distributed coordination function (DCF) and point coordination function (PCF). Figure shows the relationship between the two MAC sublayers, the LLC sublayer, and the physical layer.

Distributed Coordination Function

One of the two protocols defined by IEEE at the MAC sublayer is called the distributed coordination function (DCF). DCF uses CSMAICA as the access method. Wireless LANs cannot implement CSMA/CD for three reasons:

1. For collision detection a station must be able to send data and receive collision signals at the same time. This can mean costly stations and increased bandwidth requirements.
2. Collision may not be detected because of the hidden station problem.
3. The distance between stations can be great. Signal fading could prevent a station at one end from hearing a collision at the other end.

• D. In all frame types except one, this field defines the duration of the transmission that is used to set the value of NAY. In one control frame, this field defines the ID of the frame.
• Addresses. There are four address fields, each 6 bytes long. The meaning of each address field depends on the value of the To DS and From DS subfields and will be discussed later.
• Sequence control. This field defines the sequence number of the frame to be used in flow control.
• Frame body. This field, which can be between 0 and 2312 bytes, contains information based on the type and the subtype defined in the FC field.
• FCS. The FCS field is 4 bytes long and contains a CRC-32 error detection sequence.

Frame Types A wireless LAN defined by IEEE 802.11 has three categories of frames: management frames, control frames, and data frames. Management Frames Management frames are used for the initial communication between stations and access points.

Control Frames Control frames are used for accessing the channel and acknowledging frames. Figure shows the format.