GAS may be used by stations prior to authentication, or association to a wireless Access Point (AP) in a Basic Service Set (BSS).
GAS supports higher-layer protocols that employ a query/response mechanism.
In a BSS infrastructure, the purpose of Generic Advertisement Service (GAS) is to enable a station to identify the availability of information related to network services.
While the specification of network services information is out of scope of IEEE 802.11, there is a need for stations to query for information on network services provided by SSPNs or other external networks beyond an AP (Access Point) before they associate to the wireless LAN.
There are a number of reasons why providing information pre-association is beneficial.