Beacons and Probe Requests in Residential WiFi

Question

I was just wondering if a Residential WiFi access point and router (utilising WPA2 security) could be configured to be able to utilise passive (beacon) and active (probe) methods (to allow discovery of the WiFi SSID) at the same time?

Is it the case that there can only be one or the other or can both operate at the same time?

Also, what is the relative effect on bandwidth (due to management overhead) when comparing the active and passive approaches?

Thanks in advance for any information on this.

Answer 1

At the risk of oversimplifying, it’s clients that scan to find APs, not the other way around. APs don’t really have any say in how clients scan. APs must transmit beacons at regular intervals as well as respond to probe requests with probe responses, so APs always do support both active and passive scans at the same time.

How a client scans is a client implementation detail that most vendors don’t bother to document. Many clients do “hybrid” scans where channels that can be scanned actively are scanned actively, and “DFS” channels (where you can’t transmit until you’re sure no nearby radar installation is using it) are scanned passively until the client sees a transmission on the channel, which would indicate that it is okay to transmit on that channel, in which case the client upgrades to an active scan on that channel.

Passive scanning has no effect on bandwidth, as beacons must already be sent at regular intervals for non-scan-related reasons. Active scanning is one small probe request per channel, and one small probe response per in-range AP, which is nothing. Even if an ill-behaved client were scanning all channels once per second, it works out to only about one tenth of one percent of the airtime on each channel.