Hello everyone,
Like wireless broadcasting, radio, and mobile phone communications in daily life, WLAN uses radiofrequency as the carrier. This post will introduce you to the 2.4 GHz frequency band used by WLAN.
Radiofrequency is an electromagnetic wave with a frequency between 3 Hz and about 300 GHz. It can also be called radiofrequency or radio. People have defined the wireless spectrum for this electromagnetic wave, which is divided into extremely low frequency, ultra-low frequency, intermediate frequency, high frequency, ultra-high frequency, etc. according to the frequency range. The radiofrequency range used by WLAN is the 2.4GHz frequency band (2.4GHz～2.4835GHz) and The 5GHz frequency band (the frequency range is 5.150GHz～5.350GHz and 5.725GHz～5.850GHz), which belong to UHF (300MHz～3GHz) and UHF (3GHz～30GHz) respectively. The following figure shows the position of the WLAN radio frequency in the frequency spectrum.
Figure 1 Position of the WLAN radio in the spectrum
Extremely low frequency (3 Hz-30 Hz): submarine communication or direct conversion to sound.
Super low frequency (30 Hz-300 Hz): Direct conversion to sound or AC transmission system (50-60 Hz).
Ultra-low frequency (300 Hz-3 KHz): Mine communications or direct conversion to sound.
Very low frequency (3 KHz-30 KHz): Direct conversion to sound, ultrasound, geophysical research.
Low frequency (30 KHz-300 KHz): International broadcasting.
Medium Frequency (300 KHz-3 MHz): Amplitude Modulation (AM) broadcasting, maritime and aeronautical communications.
High frequency (3 MHz-30 MHz): short-wave, civilian radio.
Very High Frequency (30 MHz-300 MHz): FM radio, television broadcasting, aeronautical communications.
Ultra-high frequency (300 MHz-3 GHz): TV broadcasting, wireless telephone communications, wireless networks, microwave ovens.
Super High frequency (3 GHz-30 GHz): Wireless network, radar, satellite reception.
Extremely High Frequency (30 GHz-300 GHz): Radio and Astronomy, Remote Sensing, Body Scanners.
Above 300 GHz: infrared, visible, ultraviolet, ray, etc.
The 2.4GHz frequency band and the 5GHz frequency band used by WLAN belong to the ISM frequency band. ISM stands for Industrial, Scientific, and Medical. The ISM frequency band is defined by the Federal Communications Commission (FCC) of the United States. It is mainly open to the three institutions of industry, science, and medical treatment. As long as the power of the equipment meets the limit, these frequency bands can be used without applying for a license (Free License), which is greatly convenient The application and promotion of WLAN.
Figure 2 ISM band
The WLAN channel is a radio frequency with a certain frequency bandwidth, just like a highway must have a certain width in order to carry the information to be transmitted. For the 2.4GHz band, the bandwidth of the 2.4GHz band is 2.4835GHz-2.4GHz=0.0835GHz=83.5MHz. Does WLAN use the entire 83.5MHz bandwidth as a channel? Here we use an analogy to help everyone understand the WLAN channel. When we watch radio and television, we all know the channel: channel 1, channel 2. If we want to watch channel 1, we can't watch channel 2. Only one channel can be selected at a time. What happens if channel 1 uses the frequency of channel 2 to transmit? Those two channels are not available to everyone, or the screen is full of snowflakes. High school physics tells us that if a wave encounters a wave with the same frequency, it will cause interference, and it will be superimposed or attenuated according to the phase difference (for example waves with the same frequency and 180° phase difference will cancel each other). Therefore, channel 1 has a fixed frequency, and channel 2 also has its fixed frequency, which does not interfere with each other.
We can understand the WLAN channel as the channel of the TV. If the WLAN uses the entire 2.4GHz frequency band as a channel, when there are two or more APs in the same coverage area, everyone uses the same channel, which will cause serious interference. (Just as channel 1 uses the channel of channel 2), neither of the two APs can effectively provide WLAN services. Therefore, in the WLAN standard protocol, the 2.4GHz frequency band is divided into 13 overlapping channels, and the bandwidth of each channel is 20MHz (802.11g and 802.11n each occupies 20MHz, and 802.11b each occupies 22MHz). Each channel has its own center frequency.
Channel 14 is specifically defined for Japan. The channels open in the 2.4GHz frequency band in various countries are different. North America (the United States, Canada) opens channels 1 to 11, Europe opens channels 1 to 13, and China also opens channels 1 to 13. In general, we will talk more about the 2.4GHz frequency band divided into 13 overlapping channels.
These 13 channels can find 3 independent channels, that is, there are no overlapping channels. The independent channel has no frequency overlapping area, and the use of these 3 independent channels by adjacent APs will not interfere with each other. As shown in the figure below, 1, 6, and 11 are three independent channels that do not overlap each other.
Figure 3 Bandwidth diagram
That is all I want to share with you! Thank you!
