The introduction of Wi-Fi 6E and the 6 GHz frequency band brings new methods of communication between clients and APs.
Traditionally, wireless devices communicate with access points in a specific exchange of information. Client devices use an active “hunt-and-seek” approach to scan for existing APs. This active scanning approach involves sending probe request frames along the 2.4 GHz and 5 GHz frequency spectrum. An AP would respond with a probe response frame containing all the necessary basic service set (BSS) information to connect to the network. This information would consist of SSID, BSSID, channel width, and security information among other things.
The probe responses sent by APs would also broadcast their signal strength in relation to the client device. The client would select what AP to connect to based on the strongest broadcasted signal strength. These probe messages are often sent off-channel in order to assist a client device with roaming from AP to AP. So if a client device is already associated with a 5 GHz AP on, say, channel 48, it will still send probe requests to nearby channels (36, 64, etc.) to identify a list of other potential connections nearby. This is often referred to as “neighbor reporting” and was introduced by the IEEE in the 802.11k amendment.
This active “hunt-and-seek'' approach to network connectivity is no longer necessary and is actually discouraged in Wi-Fi 6E on the 6 GHz band. It’s now inefficient for a client device to send so many probe requests across channels because the 6 GHz band introduces so many more channels. There are completely new AP discovery methods in Wi-Fi 6E, both in-band and off-band.
There are three methods of in-band discovery of 6 GHz APs. Fast Initial Link Setup (FILS) and unsolicited probe response (UPR) frames are two passive methods of in-band discovery. Preferred Scanning Channels (PSC) is an active method of in-band discovery.
FILS uses “discovery announcement frames” which are essentially condensed beacon frames. Only crucial information is sent in a FILS frame: SSID, BSSID, and channel. If FILS is configured, the 6 GHz AP will broadcast an announcement discovery frame approximately every 20 milliseconds.
An unsolicited probe response (UPR) frame contains all the same information sent in a beacon. If used, the 6 GHz AP will broadcast a full probe response frame every 20 milliseconds.
The third discovery method in Wi-Fi 6E, which is active, is Preferred Channel Scanning (PSC). This is actually the only method by which Wi-Fi 6E client devices are allowed to send probe requests. With PSC, client devices are limited to sending probe requests on every fourth 20 MHz channel. The full list of 6 GHz PSC channels is 5, 21, 37, 53, 69, 85, 101, 117, 133, 149, 165, 181, 197, 213, and 229.
Remember that these are in-band discovery methods, meaning that this only applies to Wi-Fi 6E clients connecting to wireless networks on the 6 GHz band.
In practice, most Wi-Fi 6E-capable devices will continue to support both the 2.4 and 5 GHz frequency bands. Vendors will create “tri-band” APs to support 2.4, 5 and 6 GHz. An out-of-band discovery method must exist in order for devices to navigate between all 3 bands. This method, introduced in 802.11v is known as reduced neighbor reporting (RNR). Essentially, when a Wi-Fi 6E-capable AP sends a probe response frame it includes, (along with basic service set (BSS) information for the 2.4 or 5 GHz band) RNR information about its 6 GHz radio. This RNR will serve as enough information for the client device to roam between 6 GHz and 2.4 or 5 GHz networks.
While this is a high-level and fairly simplified explanation, the objective of 6 GHz out-of-band discovery is to send as few probe requests as possible with only necessary information.