WLAN Passive vs. Active Scanning: Understanding the Difference

As we know, before joining any network, a client, station, or mobile station needs to find it first. In the wired world, simply plugging in the cable or jack will establish the network connection. In the wireless world, this is more challenging and requires the identification of a compatible network before the joining process can begin. This identification process of the network is referred to as scanning.

Several parameters are needed in the scanning process. These parameters are specified by the user. Some of them are set as default in the WLAN client driver software. The parameters are BSSType, BSSID, SSID, ScanType, ChannelList, ProbeDelay, MinChannelTime, and MaxChannelTime.

WLAN Passive Scanning

In passive scanning, the WLAN station moves to each channel as per the channel list and waits for beacon frames. These frames are buffered and are used to decode and extract information about the BSSs (Basic Service Sets). This passive scanning will save battery power as it does not need to transmit.

As shown in the figure above, the WLAN client receives beacon frames from three access points and, therefore, it will declare that it has found only three BSSs.

WLAN Active Scanning

The station plays an active role in active scanning. On each of the channels, probe request frames are used to obtain responses from the network of choice.

In active scanning, the station finds out the network rather than waiting for the network to announce its availability to all the stations.