Association Service

Before an STA is allowed to send a data message via an AP, it will first
become associated with the AP. This service is needed either after an STA
powers up or enters into a BSS area.
The STA needs to get synchronization information from the AP (or
from other STAs, when in ad hoc mode) invoking the association service.
For acquiring this synchronization information, the STA scans all channels
by one of the following two strategies (depending on the value of the Scan-
Mode parameter): • Passive scan, in which the STA will scan for beacon frames to collect
synchronization information and to understand if the beacon frame
comes from an infrastructure BSS or from an IBSS.
• Active scan, in which the STA transmits probe frames containing
the desired service set identifier (SS_ID) and waits for a probe
response from the BSSs within its area. The probe response is sent
either from the AP of an infrastructure BSS or from the STA that
generated the last beacon in an IBSS.
In general, beacon and probe frames contain information for joining a
new network. Then STA chooses the BSS, which satisfies the desired SS_ID,
sends an association request message (by setting the corresponding value in
the control field of the MAC frame) to the selected BSS and waits for the
corresponding association response frame. If there is not a BSS that satisfies
its requests, STA may start an IBSS with its own characteristics. Figure 4.11
shows an example of active scanning in an ESS context.
Another two services can also be invoked: reassociation and disassociation.
The reassociation can be invoked if an STA wants to move from one AP
to another or if an STA wants to change association attributes while it
remains associated to the same AP. The disassociation service is invoked
whenever an existing association is to be terminated.