Frame Transmission and Association and Authentication States

Allowed frame types vary with the association and authentication states. Stations are either authenticated or unauthenticated and can be associated or unassociated. These two variables can be combined into three allowed states, resulting in the 802.11 Hierarchy of Network Development:

1. Initial state; not authenticated and not associated
2. Authenticated but not yet associated
3. Authenticated and associated

Each state is a successively higher point in the development of an 802.11 connection. All mobile stations start in State 1, and data can be transmitted through a distribution system only in State 3. (IBSSs do not have access points or associations and thus only reach Stage 2.) Figure 4-60 is the overall state diagram for frame transmission in 802.11.

Frame Classes

Frames are also divided into different classes. Class 1 frames can be transmitted in State 1; Class 1 and 2 frames in State 2; and Class 1, 2, and 3 frames in State 3.

Class 1 frames

Class 1 frames may be transmitted in any state and are used to provide the basic operations used by 802.11 stations. Control frames are received and processed to provide basic respect for the CSMA/CA "rules of the road" and to transmit frames in an IBSS. Class 1 frames also allow stations to find an infrastructure network and authenticate to it. Table 4-11 shows a list of the frames that belong to the Class 1 group.

Figure 4-60. Overall 802.11 state diagram

Table 4-11. Class 1 frames

Control	Management	Data
Request to Send (RTS)	Probe Request	Any frame with ToDS and FromDS false (0)
Clear to Send (CTS)	Probe Response
Acknowledgment (ACK)	Beacon
CF-End	Authentication
CF-End+CF-Ack	Deauthentication
	Announcement Traffic Indication Message (ATIM)

 

Class 2 frames

Class 2 frames can be transmitted only after a station has successfully authenticated to the network, and they can be used only in States 2 and 3. Class 2 frames manage associations. Successful association or reassociation requests move a station to State 3; unsuccessful association attempts cause the station to stay in State 2. When a station receives a Class 2 frame from a nonauthenticated peer, it responds with a Deauthentication frame, dropping the peer back to State 1.[*] Table 4-12 shows the Class 2 frames.

[*] This rejection action takes place only for frames that are not filtered. Filtering prevents frames from a different BSS from triggering a rejection.

Table 4-12. Class 2 frames

Control	Management	Data
None	Association Request/Response	None
	Reassociation Request/Response
	Disassociation

 

Class 3 frames

Class 3 frames are used when a station has been successfully authenticated and associated with an access point. Once a station has reached State 3, it is allowed to use distribution system services and reach destinations beyond its access point. Stations may also use the power-saving services provided by access points in State 3 by using the PS-Poll frame. Table 4-13 lists the different types of Class 3 frames.

Table 4-13. Class 3 frames

Control	Management	Data
PS-Poll	Deauthentication	Any frames, including those with either the ToDS or FromDS bits set

If an access point receives frames from a mobile station that is authenticated but not associated, the access point responds with a Disassociation frame to bump the mobile station back to State 2. If the mobile station is not even authenticated, the access point responds with a Deauthentication frame to force the mobile station back into State 1.