802.11 Protocol Architecture
LAN. It is possible to interface with this MIB through Simple Network
Management Protocol (SNMP) over the IP network.
The remainder of this chapter will describe these functions and
elements in greater detail.
Scanning
Each 802.11 station periodically scans each RF channel in order to find
a BSS to join. The process of scanning is critical when a station is first
activated. After powering up, the station will initiate scanning to find
an initial BSS to join. As RF conditions change, the station will
periodically scan and possibly reassociate with another BSS.
Before a station switches to a different channel than the channel on
which it is associated, the station may (with some vendors) send a Null
Function data frame with the power management bit in the frame
control field of the MAC header set to “1” to inform the access point to
buffer frames intended for the station. Once the station has retuned to
its associated channel, the station will send another Null Function data
frame with the power management bit set to “0” to inform the access
point to begin forwarding frames again. This behavior prevents missed
frames during scanning.
There are two forms of scanning: passive scanning and active scanning.
Passive Scanning
Passive scanning is the process through which a station listens to each
channel (or set of channels) for a specific period of time. The station
waits for the transmission of beacon management frames (a.k.a.
beacons) having the SSID of the network that the station is configured
to join. Beacons contain fixed fields and information elements that
hold information about the BSS which are used by stations to
determine whether or not the station may associate. Some vendors
allow configuration of access points to remove the SSID value from the
beacon so that the access point is not “announcing” its SSID to nearby
stations. The passive scanning process is illustrated in Figure 2.5a.
