Joining a Network

Joining a Network
Once a station is turned on, it needs to first determine whether another station or access point
is present to join before authenticating and associating with an applicable station or access
point. The station accomplishes this discovery phase by operating in a passive or active scanning
mode. After joining with a BSS or an ESS, the station accepts the SSID (service set identifier),
TSF (timing synchronization function) timer value, and PHY setup parameters from the
access point.
With passive scanning, a station listens to each channel for a specific period of time, as defined
by the ChannelTime parameter. The station just waits for the transmission of beacon frames
having the SSID that the station wants to join. Once the station detects the beacon, the station
can negotiate a connection by proceeding with authentication and association processes.  Active scanning involves the transmission of a Probe frame indicating the SSID of the network
that the station wants to join. The station that sent the probe will wait for a Probe Response
frame that identifies the presence of the desired network.
Some vendors enable you to set up each radio card so that it associates with a preferred access
point even if the signal from that particular access point is lower than the signals from other
access points. This may be useful if there’s a need to regulate the flow of traffic through particular
access points. In most cases, the station will reassociate with another access point, though,
if it doesn’t receive beacons from the preferred access point.
A station can also send probes using a broadcast SSID that causes all networks within reach to
respond. An access point will reply to all probe requests in the case of infrastructure-based networks.
With independent BSS networks (that is, those with no access point), the station that
generated the last beacon frame will respond to probe requests. The Probe Response frame
indicates the presence of the desired networks, and the station can complete its connection by
proceeding with the authentication and association processes.