Channels and channel planning

Wi-Fi Channels are available over two, soon to be three (with Wi-Fi 6E), bands of unlicensed spectrum. A band is a group of adjacent frequencies. The 2.4 GHz band (sometimes call the b/g band because 802.11b and 802.11g used it) has only 3 'non-overlapping channels'—1, 6 and 1—in most of the world. The reason these three channels aren't numbered as 1, 2, and 3 is because they were named before Wi-Fi existed.

Wi-Fi is a spread spectrum technology, which means unlike an FM radio, it sends redundant signals over multiple small channels at low power. For the non-RF engineers, the important point is a Wi-Fi channel must spread over 20 MHz (or more, see below). Three channels are the minimum necessary for a channel plan.

Wi-Fi is a half-duplex technology – only one radio can talk at a time. Exceptions, when the physics works out, are part of Wi-Fi 6, but the primary approach is one AP can talk to one client device at a time. However, if the AP radio cells are on different channels, then they can use both channels, and two APs on two channels can each talk to one client at the same time. Finally, 2 APs can use the same channel if they are far enough apart that they do not hear each other. The illustration below shows why 3 channels are the minimum for any channel plan.

2.4 GHz channel plan

Interference is why channel planning is so important. For our purposes, there are two kinds of interference—noise and other Wi-Fi. Noise is when another RF source is putting energy out on the frequency we want to use—a microwave oven, baby monitor or Bluetooth are common sources. Other Wi-Fi, often called co-channel interference, may interfere on the Wi-Fi protocol level. APs may be forced to wait their turn according to the Wi-Fi protocol. Two APs on the same channel that can clearly 'hear' each other will perform worse than a single AP, because they will contend for the medium and be forced to back off when the other is sending. When APs on the same channel are far enough apart, the co-channel interference just adds to the background noise level, and that can be transmitted over.

Understanding that devices contend for a shared medium might be the single most important concept in good Wi-Fi design. In one case study, a hotel wanted a very redundant Wi-Fi network, so they deployed the equivalent of the above channel plan but with 40 instead of 20 APs 2.4 GHz in the same space. With two APs covering the same spatial area on the same channel, constantly contending for media access, performance was miserable. In many environments, the biggest interference problem is your own network interfering with itself, although in dense urban environments, neighboring networks become significant.

The 5 GHz band has MUCH more capacity, with 25 available channels. The example channel plan above could have every single AP on a different channel in 5 GHz, resulting in NO co-channel or self-interference. However, there are other factors to consider in distributing channel capacity.

5 GHz channel plan

Channel bonding - Because Wi-Fi is a spread spectrum technology, with data encoded into multiple subcarriers, two 20 MHz wide channels can be combined into a 40 MHz channel to deliver twice the throughput to a client. Notice, that's not twice the total throughput. The theoretical throughput total of two 20 MHz channels is the same as the throughput of one 40 MHz channel, but the throughput peak to a single client is higher, and that total throughput can be achieved with fewer radios (APs). 80 MHz and even 160 MHz wide channels are defined in the standard, but 160 is impractical in 5 GHz unless you are deploying a single AP to a building in an isolated location. Wi-Fi 6 FortiAPs support 20, 40 and 80 MHz wide channels in 5 GHz, but only allow 20 MHz channels in 2.4 GHz.

Channel bonding is a way of redistributing the available capacity for fewer and faster devices. The total capacity of a multiple FortiAP LAN is roughly the same if all channels are used, regardless of the bonding plan. As a general rule, use 40 MHz wide channels in 5 GHz. In high density deployments, consider dropping to 20 MHz wide. For example, in a large conference room or auditorium, a single FortiAP can easily cover the room, but it cannot have 1000 devices connected to it. 20 MHz channels allow you to throw more APs at the capacity problem until you run out of channels. On the other hand, an office with 5 FortiAPs and a dozen devices each could consider 80 MHz wide channels.

Dynamic Frequency Selection (DFS) channels are another complication to be aware of. Sixteen of the twenty-five 5 GHz channels are used by military, weather radar, and satellite communications. Such use predate the development of Wi-Fi. Several Wi-Fi generations ago, it was unusual to make use of the DFS channels, but that is no longer the case. FortiAPs will automatically choose channels on boot up, and if a radar event is detected, will change to another channel automatically, as is required by regulation. The main point is that a network administrator should not be afraid to use the DFS channels. These channels represent a great deal of additional capacity and FortiAPs will automatically allocate channels around any radar.

Design for 5 GHz over 2.4 – because 2.4 GHz is the lowest common denominator, there is a tendency to design for 2.4 GHz first. However, it's much better to design with 5 GHz as the primary access, and use 2.4 as a kind of auxiliary band. Virtually all Wi-Fi client devices, even older ones, support 5 GHz, and there is just too much additional capacity to not focus on 5 GHz. 2.4 GHz is effectively becoming the IoT band. Bluetooth uses 2.4 GHz, and even the very small number of 2.4 only Wi-Fi devices are because they are very cost conscious IoT devices. IoT devices usually have low throughput needs so they can operate at lower data rates (see below).

Distributed Automatic Radio Resource Provisioning (DARRP) is a Fortinet Wireless Controller technology that assigns radio channels automatically to APs, and updates regularly to account for changing conditions. DARRP ensures the wireless infrastructure is optimized for maximum performance. Wi-Fi 6 FortiAPs include a third monitoring radio so that this advanced feature has all the necessary data to make intelligent decisions. The third radio can continuously monitor the RF environment for interference, noise, and signals from neighboring APs. With this data, the FortiGate WiFi Controller is able to determine the optimal channel and RF power levels for each AP on the network, without a need for administrator intervention.

Frequency Band Handoff (Band Steering) - Every Wi-Fi device that operates in 5 GHz not only gains for itself, but it also leaves behind capacity for 2.4 GHz only devices. The FortiOS WiFi Controller can tell from a client's Wi-Fi probes if a device is dual band capable, and it will respond on 5 GHz, balancing the use of spectrum if signal strength is high enough.

Client Load Balancing across APs is a related idea supported by FortiAPs. In this case, clients are balanced across different APs, rather than different radios in the same AP. If the client count exceeds the configurable threshold (default of 30), clients are moved to another AP with sufficient signal strength.