FortiAP Placement Guidelines and Channel Planning

Fortinet recommends a site survey for all Wi-Fi deployments. Wi-Fi is well established as the primary access technology, and most Wi-Fi deployments are a network refresh. This encourages a tendency to just swap out old access points for new FortiAPs without running site surveys. This should be avoided because environments change and AP capabilities also change with Wi-Fi generations.

Some environmental changes can come from surprising places. For example, newly applied paint on walls can have surprising RF effects. With the widespread adoption of Bluetooth, the 2.4 GHz band has become increasingly congested. This, coupled with the reflectivity of chrome-flecked paint, can cause interference with Wi-Fi signals.

In general, coverage areas will be less for a 5 GHz radio, let alone a 6 GHz (6E) radio vs a 2.4 GHz radio due to reduced wall penetration of signal.

A walk-through examination of the site with a spot-check of representative areas may be a good idea on a refresh. New sites always benefit from getting a clear idea of wall properties and corresponding dB loss. Glass walls in office can range from very transparent to Wi-Fi, to very opaque—wire supported and leaded glass have surprised more than a few Wi-Fi designers.

Capacity and coverage estimations

As a general rule, for a pre-survey estimate, an indoor area requires about 1 FortiAP per 2000 Sq-Ft, with around 30 devices per AP radio. All the Wi-Fi 6 FortiAPs have 1 radio for monitoring and 2 service radios. Users are usually assumed to have 3 devices each, so 20 users and 60 devices can be serviced per physical AP. UTP series FortiAPs can have both radios on 5 GHz, so set both radios to 40 MHz wide channels for increased bandwidth and capacity.

The above estimates are conservative estimates, and there are some factors that can move the average up or down. For coverage, walls are the key concern. A FortiAP can cover a very large space with an open floor plan, while floor plans with many small offices will require more APs. At the same time, the client count per AP may be a more important factor; capacity more than coverage is also a major design consideration. Designing for capacity rather than coverage is helped by designing for 5 GHz bands.

It is best to avoid requiring client-to-FortiAP radio connections to pass through more than one wall. Although one cannot see Wi-Fi signals, line of site connectivity to client devices is best, so a designer can visualize coverage by thinking about how they would deploy light fixtures. Placing APs in closets, behind ductwork and other obstructions will not maximize performance.

Channel Planning - Design for 5 GHz

Wi-Fi in general has evolved quite a bit in a relatively short time, and so have other wireless technologies such as Bluetooth and its variations. The default assumption for Wi-Fi in the past was to design for 2.4 GHz and treat 5 GHz as secondary. Now that Wi-Fi 6 is available, Fortinet recommends designing for 5 GHz as the primary band.

The large number of 5 GHz channels make for much more forgiving channel plans. WLAN self-interference is massively reduced. Furthermore, 2.4 GHz continues to become more and more crowded with wider adoption of Bluetooth.

Very few client devices are so old that they do not fully support 5 GHz. In addition, DFS (Dynamic Frequency Selection) regulations apply to the APs, not the clients. A good reference for client 5 GHz support can be found here: http://clients.mikealbano.com.

FortiAP Profiles

UTM series FortiAPs include a band selectable radio and a good channel plan that prioritizes 5 GHz but provides high support for 2.4 GHz to alternate the selectable radio between 5 GHz and 2.4 in the FortiAP Profiles. FortiAP profiles are per AP model, but APs can also be grouped if there is a need for different profiles on the same model on different parts of the campus.

Radio Resource Provision should be enabled in FortiAP profiles and transmit power set to automatic in order for the campus network to take full advantage of FortiOS DARRP (Distributed Automatic Radio Resource Provisioning). DARRP will optimizes channel selection and AP Tx power periodically. The default is to adjust every 24 hours at 2 am. The timing can be adjusted in the FortiGate CLI.

Designing for High Density environments

High density areas such as auditoriums and cafeterias are a good illustration of the power of multiple 5 GHz channels over only three 2.4 GHz channels. In a single large room, the maximum number of devices, at 30 per radio, that can be on the 3 channels in 2.4 GHz is 90. More client devices can be accommodated with more FortiAPs, but then all FortiAPs on the same channel will have to contend for airtime. Meanwhile, 5 GHz can potentially have 600 devices when using 20 MHz wide channels with no FortiAPs needing to contend with each other—which is the best way to design for such a high-density deployment.

Wi-Fi 6 is specifically designed to maximize performance in high density situations. The FAP-831F supports 8x8 MIMO and is an excellent choice for such a situation.

See the Fortinet WiFi concepts document for details on Wi-Fi 6 for high density.

FortiAP 831F - 8x8:8 MU-MIMO Indoor/High Density This high throughput enterprise class 802.11ax indoor AP provides three radios and 8 spatial streams. This top-of-the line access point supports OFDMA, a 5.0 Gigabit Ethernet port, plus an additional 1 Gbps Ethernet port for PoE diversity. The AP can provide 24/7 scanning across both bands while still providing access on both the 2.4 GHz and 5 GHz bands. The integrated BLE radio can be used for beacons and locationing applications.