Why Beacon needed when there is probe response

Question

I was trying to understand the use of beacon and probe response in 802.11 network. If advertising the existence of a AP is only the goal, then why beacon frame needed? Passive scanning may be important while the station is in roaming but is it the only usage of beacon frames? Please clarify what other purposes does this frame serve in a typical 802.11 network.

Answer 1

Passive scanning is important not when the station is in roaming. On the contrary, roaming decision is taken as a result of scanning results (passive and active). The main reasons for beacons:

• Keep the networks list updated for roaming
• Beacons are indication to the connected 802.11 station that the AP is alive
• 802.11 stations may use the beacons to facilitate power saving mechanisms based on beacon data and interval
• Beacons contain indications for the attached station of buffered data awaiting to be sent to the station