3 RF basics you should know to better design your WLAN
If you want to design an effective WLAN deployment, you first need to understand 3 very useful radio frequency concepts.
In order to design and implement a successful Wi-Fi network, you need to be familiar with the following 3 RF fundamentals.
1. WIRELESS CHANNELS
The first thing to know is that all Wi-Fi devices communicate through a channel. Each channel is characterised by a number, which corresponds to a precise radio frequency. The 2 main frequency bands used among WLAN access points are 2.4 GHz and 5 GHz. Worldwide there are 14 channels using the 2.4 GHz band, whose availability varies from country to country (for instance, channel 14 is only available in Japan). It is good to know that the only 3 channels where signals don’t overlap are the number 1, 6 and 11. Nevertheless, especially in highly congested spaces such as working environments, all channels are used. As a result, signals invade other channels’ bandwidth and end up creating interference. A solution to this is to opt for channels operating on the 5 GHz band. In fact, 5 GHz is less congested, as the majority of Wi-Fi devices such as computers, Bluetooth devices, cellular and cordless phones operate on the 2.4 GHz band. Furthermore, 5 GHz provides a higher number of frequency channels, of which 23 do not overlap: all access point operating on the 5Ghz band usually support channels 36, 40, 44, 48, 54, 56, 60 and 64. Lastly, it is useful to point out that, in the 2.4 GHz band, active Wi-Fi signals are 20-22 MHz wide, whereas in the 5 GHz band signals can be 20, 40, 80, up to 160 MHz wide. In general, 20 Mhz channel width is recommended for high-dense enterprise deployments, while 40 MHz works well in areas with medium-low crowded Wi-Fi networks.
2. RF PROPAGATION
Radio frequency propagates through space with different behaviours: reflection, absorption, refraction, diffraction and scattering. Being aware of how RF moves around a particular space is relevant to understand why, when positioning an access point in a location, radio frequency waves can or not reach certain places within a room and neighbor areas.
- Under reflection, RF signals bounce to another direction when they hit reflecting materials that are larger than the wave, i.e. metals; reflection highly occurs in indoor WLAN deployments.
- Absorption occurs when RF signals are converted to heat and absorbed by certain materials, such as concrete or water.
- In the case of refraction, RF signals change direction when they pass through a material with a different density; refraction mainly occurs in outdoor WLAN deployments, which are affected by changes in atmospheric conditions and air temperatures.
- With diffraction, RF waves change direction when they move around an object of a certain size, shape or material.
- Scattering can be intended as “many reflections of the RF wave”, and occurs when the wavelength of the RF signal is larger that the one of the medium/material/object the signal is passing through.
Knowing how radio frequencies react depending on the material they come into contact with, helps you avoid possible interferences caused by physical obstacles, i.e. water, bricks, trees, microwaves, etc. In this way, you will know better where to design your WLAN and where to place your access points.
3. RF MEASUREMENTS
In order to understand how strong the RF signal of your WLAN is, you need to know how to measure it. We measure RF power levels through milliwatts (mW) and decibel-milliwatt (dBm): a mW is an absolute unit corresponding to 1/1000th of a Watt, and a dBm is a decibel relative to a milliwatt. In general, it is more convenient to use dBm than mW: as we use very low output power levels, generally comprised between 0 and 1 mW, it is easier to say that your access point transmits X dBm rather than saying it transmits 0.0000X mW. The table below shows the relation between mW and dBm.
1 mW = 0 dBm
10 mW = 10 dBm
100 mW = 20 dBm
1 W = 1000 mW = 30 dBm
Keeping in mind this table, you can now measure RF power levels with the rule of 10s and 3s, according to which:
- For every gain of 3 dB, the power in mW is doubled
- For every loss of 3 dB, the power in mW is halved
- For every gain of 10 dB, the power in mW is multiplied by 10
- For every loss of 10 dB, the power in mW is divided by 10