802.11 Protocol Architecture
Synchronization
Some of the 802.11 functions require all stations to have synchronized
clocks. Each access point (or master station in an Ad Hoc network)
periodically broadcasts beacon frames at specific points in time based
on a configurable interval. Stations update their clocks with a time
value found in each beacon.
Frame transmission
A station must follow specific medium access procedures prior to
frame transmission with the goal of only transmitting a frame when the
medium is idle. Special timing elements enhance this process. A
station receiving a unicast data or management frame must respond
immediately with a positive acknowledgement frame if no errors are
found.
Authentication
When a BSS is selected, the station goes through an authentication
process to prove that it is a member of a set of stations authorized to
join this BSS.
Association
If authentication is successful, the station can complete the association
process. The association process involves exchange of capability
information between the station and the BSS and establishes an access
point/station mapping. In the case of an ESS, the access point allocates
memory and assigns an association ID (AID) to the station requesting
association. After association, the station can send and receive 802.11
data frames through the access point (a.k.a. invoking the DSS) unless
an additional security mechanism is in place.
Reassociation
As users roam about a facility, they often travel out of range of the
access point to which they are associated. In this case, the station
reassociates with another access point. Generally, the decision to
reassociate depends on received signal strengths of the corresponding
