The active scanning method is no longer efficient for discovering the AP or WLAN.
Even roaming can be problematic due to scanning.
Probing takes too much time because of the number of channels in the 6 GHz band.
The traditional active scanning method has three drawbacks.
- Firstly, it takes a significant amount of time, which can affect application performance due to off-channel scanning.
- Secondly, it requires many probe requests and response frames on the air, which reduces airtime efficiency.
- Thirdly, it affects the client’s battery life.
A Wi-Fi 6E client may need to scan all 59 possible 20 MHz channels in the band to discover all available APs which is a problem.
There are three potential methods for in-band discovery of 6 GHz APs:
- Passive: Fast Initial Link Setup (FILS) discovery announcement frames
- Passive: Unsolicited probe response frames
- Active: Preferred Scanning Channels (PSC)
There are three methods to discover access points (AP) in the 6 GHz frequency band. The first two methods are passive and operate within the band.
The first passive method uses FILS discovery announcement frames, which only contain essential information such as SSID, BSSID, and channel. If implemented, the AP will broadcast this frame every 20-time units or approximately 20 milliseconds.
The second passive method uses unsolicited probe response frames, which contain the same detailed information as a beacon. If implemented, the AP sends unsolicited probe responses to a broadcast address approximately every 20 milliseconds.
The third method is an active method and the only way Wi-Fi 6E clients can send probe requests. Active scans on a preferred scanning channel (PSC) are the only method used for in-band probing. The PSC designates every fourth channel for preferred scanning. Access points are expected to use these channels for beaconing, and clients will scan them first. With 80 MHz channels expected to dominate in 6 GHz designs, the access point can easily select the preferred scanning channel within its 80 MHz span for the primary 20 MHz channel, where it transmits beacons and responds to probe requests. The complete list of all the 6 GHz PSC channels is 5, 21, 37, 53, 69, 85, 101, 117, 133, 149, 165, 181, 197, 213, 229. The PSC channels also serve as the primary channels when channel bonding is used for 80 MHz channels.
Out of band discovery
Reduced Neighbouring reports (widely used):
When a client is connected to 5hgz, the client can Request for the RNR and can discover the channel where neighboring APs are operating on 6ghz.
The RNR IE is added in the beacon, probe response. Then report includes the field for each BSS transmitting the same ssid along with the channel and beacon offset
There is a target beacon transmission field time. TBTT info in RNR, indicates the beacon offset in time, based on that info, the client will do off-channel scanning in 6ghz, and the default time is 102.4ms