FortiAP devices are thin wireless access points (AP) supporting the latest Wi-Fi technologies (multi-user MIMO 802.11ac Wave 1 and Wave 2, 4x4), as well as 802.11n, 802.11AX , and the demand for plug and play deployment.
This article describes how to encourage WiFi clients to connect to 5 GHz band instead of 2.4 GHz band – for better WiFi experience.

2.4 GHz (802.11 b/g/n) is considered legacy and crowded band, therefore WiFi clients connected on 2.4 GHz band may exhibit WiFi performance issues (low speed, high latency, packet-loss) .

Typically WiFi clients are inclined to connect to strongest WiFi signal and this is WiFi client's decision. However, from the FortiGate & FAP side we can encourage clients to connect to 5 GHz band by incorporating following steps (config screenshots attached below):

Note: Before making new changes on FortiGate, download FortiGate backup config file.

1) Disable spectral-scan or WIDS on Radio-2 (5 GHz) in the FortiAP profile - if Spectrum Analysis background scanning on 5 GHz radio is not used in the setup.
2) On FortiAP profile - enable Frequency-handoff and AP Handoff
3) On FortiAP profile try below:

- Change 'TX power control' to 'Auto' on both Radio-1 and Radio-2

- Set 'Tx Power-control' to new Transmit Power ranges on 2.4 GHz and 5 GHz bands:
2.4 GHz -> 6 to 12 dB
5 GHz -> 12 to 18 dB
Note: These values are used when the FAPs are 30-50 ft. apart, you can change these values according to your environment. Ensure to keep at least 6 dB lower value on 2.4 GHz than 5 GHz band to make 5 GHz signal stronger than 2.4 GHz

- Change Radio 2 Band to "802.11ac/n" and set "Channel width" to 40 MHz and enable "Short Guard Interval"

Thereafter, forget SSID on WiFi clients and then test 5 GHz clients’ behavior

4) Alternatively, if your setup has all latest WiFi clients (802.11 ax/ac/n) that support 5 GHz band then they can be forced to connect only on 5 GHz band by creating a new dedicated SSID only for 5 GHz capable-clients and broadcast this SSID only on Radio-2 (5 GHz band) in the FortiAP profile

Create another SSID for other legacy WiFi clients (802.11 b/g/n) that are 2.4 GHz only-capable client types for WiFi service
5) DARRP (Radio resource provision) can be enabled on both the radios (2.4 GHz and 5 GHz)

Through DARRP, each FortiAP unit autonomously and periodically determines the channel that is best suited for wireless communications. FortiAP units to select their channel so that they do not interfere with each other in large-scale deployments where multiple access points have overlapping radio ranges.

Note: Disable DARRP & WIDS in the FortiAP profile if you’re running FortiGate on v6.4-2 because there’s a known Bug 656804 on FortiOS v6.4-2

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Fix: Upgrade FortiGate to new v6.4-3 image with hotfix. For details refer Customer Support Bulletin CSB-201027-1: CAPWAP traffic dropped for WiFi tunneled SSIDs after upgrading to FortiOS 6.4.3

For v6.4-3 hotfix image, contact Fortinet support

6) Upgrade FortiAP to latest available firmware if possible.

If the setup has FortiAP-W2 models (use v6.4.x latest), check FortiAP release notes for FortiAP and FortiOS compatible versions.

Sample FortiAP profile Radio-1 and Radio-2 config GUI screenshots:

Related documents.