802.11 Protocol Architecture
FIGURE 2.5a
Passive Scanning
Once the station detects beacons from one or more access points, the
station will decide which access point with which to associate based on
a vendor-proprietary algorithm. The station will negotiate a connection
on the applicable channel by proceeding with authentication and
association processes. An advantage of passive scanning is that it does
not require the transmission of any additional frames, which reduces
overhead traffic on the wireless medium and improves overall network
throughput.
Active Scanning
Active scanning requires that a station broadcast probe request frames
indicating the SSID of the network that the station is configured to join.
The station that sends the probe request frames will receive probe
response frames from access points within range and having the
specified SSID. This process, like that of passive scanning, provides
information that the station can use to determine the access point with
which to associate. Alternately, a station can send probes containing a
broadcast SSID (a null value) that causes all access points within reach
to respond. The active scanning process is illustrated in Figure 2.5b.
