Any typical new generation enterprise grade wireless access point (AP) can do both the 2.4GHz and 5GHz at the same time, the wireless client such as a laptop or smartphone can either connect to one band or the other, but not to both at the same time. In other words, in a new setup, one can associate the client with the 2.4GHz band SSIS with its corresponding WPA-PSK (Pre-shared Key). The client will remember this association so that the next time this 2.4 GHz SSID is in range, the association will automatically re-establish. One does not need to put in the password again.
At another time if we ask the client to associate to the 5GHz band for the same SSID and go through the same routine. Now, client remembers and caches the login credentials for both SSIDs for future encounters.
Your client device will see both SSIDs. If you name the two signals with the same SSID, this SSID will appear twice on the detected list. This "duplication" only serves to confuse you. Therefore for dual band APs, you should give each band a unique name that will tell you that one is 2.4GHz and the other is 5GHz. Examples being WLAN24 and WLAN50.
If one wants to switch over from one band to the other, we have to do this manually. New generation Access points support Dual band operation with Band Steering by detecting clients capable of 5 GHz operation and steers them to that frequency which leaves the more crowded 2.4 GHz band available for legacy clients. This helps improve end-user experience by reducing channel utilization, especially in high-density environments. Dual band operation with Band Steering is only available for AP’s with dual-band Access Points (those with at least 2 radios) and is configured on a per-SSID basis.
Basically, the AP will associate with the client at 2.4GHz first but when it detects too much congestion at this band, the AP will automatically tell the client to switch over to that of 5GHz. As you recall, the client already has the passwords cached for both bands. You do not need to do the switch manually and certainly not having to enter the password every time. Therefore band steering is a convenient way to get the best of both worlds.
The band steering feature considers several metrics before it determines if a client should be steered to the 5GHz band, including client RSSI. For example, this feature will only steer a client to the 5GHz band if that client detects an acceptable RSSI value from a 5GHz AP radio, and the signal from the 5 GHz radio is not significantly weaker than the RSSI from the 2.4GHz radio.
This feature also takes into account the current load on each radio of a dual-band AP. The band steering feature will NOT steer more clients to 5G on that AP if there are many clients associated to the AP, and significantly more 802.11a clients than 80211g clients.
How Band Steering works
Wireless devices use two kinds of scanning techniques: passive and active. In a passive scan, the wireless client quickly sweeps each channel listening for beacon frames sent by APs advertising their wireless networks. In an active scan, the wireless client sends probe requests on each channel to solicit a probe response from APs advertising their wireless network. The end user then sees the list of available wireless network connections.
With Band Steering enabled the AP's beacon frames do not advertise the wireless network. Client devices performing a passive scan will qualify the SSID as hidden.
If the AP hears a probe request from the same wireless client on both 2.4GHz and 5GHz within a short period of time, the AP will only send a probe response advertising the wireless network in response to the 5GHz probe. This steers the client to the 5GHz band. If the AP hears probes from a wireless client on 2.4GHz only, it will respond with a probe request advertising the network.
Cisco uses Band select technology for achieving this functionality.