6GHz represents the largest allocation of unlicensed bandwidth in history – but its sheer size presents some unique challenges. New rules vary by country. Under FCC rules in the U.S. and in many other countries, 1200 MHz can be divided into 59 channels of 20 MHz, almost three times the channels in the existing 5 GHz band. In the UK and European Union, less bandwidth has been allocated but this still doubles the amount of capacity available. This additional bandwidth is a boon for Wi-Fi 6E performance with up to seven 160 MHz channels and less interference but presents a challenge for AP discovery in 6GHz.
What is AP Discovery?
Discovery is the process where a client device, on powering-up or entering a building, finds a suitable access point to connect to. The simplest way to perform discovery, used by most client devices today, is to scan channels in turn by transmitting one or more probe requests, listening for responses from access points in the area, examining the probe responses to see if any of the SSIDs match profiles in the client, then stepping to the next channel. This has three drawbacks: it takes significant time, which may affect application performance while the radio is away from its serving channel; it requires many probe request and response frames on the air, which reduces airtime efficiency; and it affects client battery life.
The time – in the order of 20 msec per channel – is already an issue in the 5 GHz band. It becomes more significant when we realize that a Wi-Fi 6E client may have to scan every one of the 59 possible 20 MHz channels in the band to discover all available access points.
AP Discovery in 6GHz: Active Scanning
The new standard addresses this challenge with several new features. First, it 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 preferred scanning channel arrangement will help when a client device has no prior connection with the network and must discover access points with probe requests. But with multi-band access points, it may be that the device already has a connection to the same physical access point in another band, 2.4 or 5 GHz. For this case, the new standard includes methods where the access point can advertise on the lower bands and include information there about its operating channels in the 6 GHz band. Scanning or connecting the 2.4 or 5 GHz band as a stepping stone to a connection at 6 GHz may prove to be the fastest way to make a high-bandwidth connection, as a client can move immediately to the correct channel at 6 GHz and commence its association. Frames on the air are reduced, and the client does not deplete its battery by transmitting or receiving discovery frames.
AP Discovery in 6GHz: Passive Scanning
As an alternative to active scanning, with probe requests and responses, the Wi-Fi 6E standard offers a path to passive scanning directly in 6 GHz. Two mechanisms – Fast Initial Link Setup (FILS) announcements and unsolicited probe responses – allow the access point to broadcast what is essentially a mini-beacon every 20 msec. This allows a client device to tune its radio to a channel, knowing that it will hear an advertisement from the access point within the 20 msec interval, without transmitting probe requests. This reduces frames on the air and extends client battery life.
Next Steps
Gain a deeper understanding of AP discovery in Wi-Fi 6E and how to deploy in greenfield and existing buildings in the Technical Guide to Wi-Fi 6E and the 6 GHz Band.
