Question

Describe the process of Active and Passive scanning in 802.11 networks?

Answer 1

Answer:

The IEEE 802.11 standard defines two types of scanning techniques:

1. Active scanning

2. Passive scanning

Active Scanning

• In Active scanning the station tries to locate an AP by transmitting probe request frame, and waits for probe response from the AP.
• In active scanning station finds out network rather than waiting for network to announce its availability to all the stations.
• The IEEE 802.11 standards do not specify how long the customer should wait, but 10 ms is a representative period.
• If no transmissions are heard by a time called the minimum channel time, the channel is declared empty and the station restarts the process on a new channel and transmits the probe request frame.
• Active scanning is the default scanning technique, which enables imediate response from an AP.

Passive Scanning

• In Passive scanning the station waits to receive a beacon frame from the AP. The beacon frame is a frame sent out periodically by the AP containing synchronization information.
• Station moves to each channel as per channel list and waits for beacon frames.
• By default, APs send beacons every 100 ms. Subsequently, passive scanning station must wait at least 100ms on every channel to ensure that all beacons are been collected.
• A passive scan generally takes more time han active scan, since the station must listen and wait for a beacon probing to find an AP.