Signal

Any wireless communication system depends on a modulated signal. A transmitted signal is spread over a large volume of space, and is always much stronger than the signal actually received at the client radio. The receiver can only get 1 millionth of the original signal and consider it excellent. With Wi-Fi, the clearer the signal, the faster the data transmission rate because smaller changes in signal strength and phase (modulations) can be used to encode more data.

MCS rate (Modulation and Coding Scheme) is the list of available data rates for Wi-Fi transmissions. There are fixed modulation types—that is, 6.5 Mbps and 13 Mbps are possible at the low end, but not 12 Mbps. Signal strength must be high enough relative to background RF-noise to achieve high throughput rates.

You can refer to Modulation and Coding Scheme (MCS) index tables such as https://mcsindex.com. Wi-Fi performance is dependent on signal strength, noise, number of streams, and channel width.

SNR (signal-to-noise ratio) is the difference between received signal and the background RF. Even a very clean environment has some undifferentiated signal. RF noise is analogous to the noise in an environment when two people are talking. Compare conversing in a library vs a restaurant during the lunch rush. In the first case, whispers are easily understood, in the latter, people have to speak up. The equivalent RF background could mean the it takes a higher signal to maintain the same MCS rate.

RSSI (Received Signal Strength Indicator) is just what the name implies, and used by Wi-Fi radios, along with the SNR, to determine what MCS they can negotiate. However, it is a relative number (1-252) and every Wi-Fi vendor has their own implementation of it.

dBm (decibel milliwatts) is used for the absolute value of a Wi-Fi signal. Decibels are a logarithmic scale (actually 10 times the log base 10), so 10 dBm is 10 milliwatt, but 20 dBm is 100 milliwatts. Another way of thinking about that is 10 is one zero, 20 is two zeros, 30 is three zeros, etc. Because signal strength changes so drastically, decibels are considered easier to work with than absolute milliwatts. Furthermore, our concern is usually with dropping signal so it is more important to focus on negative values: -10dBm is 0.1mW, -30dBm is 0.001mW, etc. Usually for Wi-Fi, because we are using negative dB values, the bigger the absolute value of a dB measurement, the weaker the signal.

You do not have to be skilled at dB math for the overwhelming majority of Wi-Fi concerns, you just have to know a couple general rules:

• + 26dBm is 400 mW and the top signal emitted by most FortiAPs in the US

• + 23dBm or 200 mW is a more typical FortiAP emitted signal (FortiAPs adjust for environment)

• -67dBm or higher is the usual received signal strength design goal—this should allow quality voice over Wi-Fi calls

• -50 dBm, is much stronger than -67 dBm, and -30dBm is an outstanding received signal

• A dB difference of 3 is double or half. -70 dB is half of -67 dB and -64dB is twice -67 dB

• Background noise is typically around -87 dBm, but very location dependent

A Spectrum Analyzer is necessary to measure how much noise is on a channel. FortiAP radios can be put in Spectrum Analyzer mode and the results viewed in the FortiGate UI. Another advantage of the Wi-Fi 6 APs over some earlier models is the 3^rd monitor radio is always available for troubleshooting without having to use the client service radios.

Signal Loss is the drop in signal strength for various reasons. Signal drops with the square of the distance (1/r²), but in most indoor environments the most important consideration is wall penetration. Wall material matters with Wi-Fi. Drywall blocks less signal than brick, at a typically -8dB. Cinder block is pretty transparent, but solid concrete and rebar are very opaque. One common problem is unexpected metal such as leaded glass, or plaster over chicken wire. These will obstruct Wi-Fi penetration. Another factor is that 5 GHz signals generally penetrate less than 2.4 GHz.

Antennas shape and direct signal. The integrated antennas in FortiAPs are omni-directional (360 degree) and ideally suited for ceiling mount in the middle of most spaces. FortiAPs with external antenna connectors can accept a number of directional options. You can compare omni antennas to direction antennas as similar to standard light bulbs compared to spotlights. An indoor space is usually best served by an omni in the middle. If for some reason APs can only be hung on the sides of a space—which is very common when covering an outdoor area—the directional option will work better. Antennas do not increase total signal, but direct it. A directional antenna with a 60 beamwidth will have roughly twice an omni's signal within the 'spotlight' area, but much weaker signal behind it.

Fortinet offers many different antenna options that can be used with our FortiAP line of products. These antennas can help to optimize coverage and overall wireless performance in a range of installation settings. Antennas come in a variety of patterns and with various numbers of antenna elements to help installers find the best match for their equipment.