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[From Beginner to Expert - WLAN Fundamentals] Section 2 WLAN Radio Frequencies a

Latest reply: Jan 11, 2021 12:14:22 5588 6 3 0 3

In a sense, wireless communication has existed since people began shouting at each other.

Does that seem unbelievable? We do not often think about it, but speech itself is a method of wireless communication. Indeed, some of the mankind's earliest methods of communication were wireless for example, beating drums or lighting beacon fires.

Our physics lessons tell us that sound is a vibration that propagates as a typically audible mechanical wave of pressure and displacement, through a medium such as air or water. Beating a drum transfers information using sound, and beacon fires and flag semaphore convey information by means of visual signals. In these two cases, the information carriers are the media through which sound or signals are propagated air and visible light, respectively.

WLAN is a modern method of wireless communications. Although fundamentally different from primitive methods, there is one thing that they all have in common: the need for information carriers.

Like wireless broadcasting, wireless television, and mobile communications, WLANs also use radio frequencies to carry information. Specifically, WLANs use the 2.4 GHz band (2.4 GHz to 2.4835 GHz) and 5 GHz band (5.150 GHz to 5.350 GHz and 5.725 GHz to 5.850 GHz), which are parts of the ultra high frequency (UHF) band and super high frequency (SHF) band, respectively. Radio frequencies are any of the electromagnetic wave frequencies that lie in the range extending from around 3 Hz to 300 GHz. The following figure shows the position of the WLAN frequency bands in the radio spectrum.

[From Beginner to Expert - WLAN Fundamentals] Section 2 WLAN Radio Frequencies a-1841147-1 

 

Available frequency ranges vary from country to country. In China, for example, the 5.150 GHz to 5.350 GHz and 5.725 GHz to 5.850 GHz frequency ranges are used. You can check which 5 GHz frequency ranges are available in different countries by consulting the Country Code & Channel Compliance Table available at the end of this post.

The 2.4 GHz and 5 GHz bands used by WLANs are part of the industrial, scientific and medical (ISM) radio bands, which were originally reserved internationally for industrial, scientific and medical purposes. The ISM bands do not require a license, and low-power devices are generally able to operate normally in them, which has resulted in the common use of WLANs. 

 [From Beginner to Expert - WLAN Fundamentals] Section 2 WLAN Radio Frequencies a-1841147-2

We have now learned a bit about radio frequencies. Now let's find out how they transfer information.

Information is impressed on carrier frequencies through a process called modulation. There are multiple modulation methods, including amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM). Using these methods, the sender transfers information by changing the amplitude, frequency, or phase of carrier waves. After receiving modulated carrier waves, the receiver demodulates them to extract the original information. FM and AM radio are two common types of broadcasting that transfer information through modulation and demodulation.

WLANs also use AM, FM, or PM to transfer information, but these are usually used to transfer ***og signals. Digital communications requires more complex modulation methods, such as amplitude-shift keying (ASK), frequency-shift keying (FSK), phase-shift keying (PSK), and quadrature phase-shift keying (QPSK). QPSK modulates the amplitude and phase of carrier waves to transfer information. The following figure shows the modulated carrier waves.

[From Beginner to Expert - WLAN Fundamentals] Section 2 WLAN Radio Frequencies a-1841147-3

It might seem as though there are no big differences between RF communications and wired communications. In the wired communications with which we are more familiar, signals are modulated to electrical or optical pulses before being transmitted over electrical or optical cables. However, RF communications presents more challenges, such as reflection and diffraction. Both the RF communication process and wired communication process can be simplified into the "information source -> channel -> destination" model. The information source (message sender) sends a message, which is conveyed over a channel to the final destination (message receiver). And what is a channel? In WLAN communications, channels are radio frequencies, and these are like the wires in wired communications.

A WLAN channel contains multiple RFs and has a specific width, called bandwidth. The bandwidth of the entire 2.4 GHz band is 2.4835 GHz 2.4 GHz = 0.0835 GHz = 83.5 MHz. Does a WLAN use the whole 83.5 MHz of bandwidth for a single channel?

Let's think about it like this: when we watch television, we have to choose a television channel, for example, China Central Television (CCTV) 1. We cannot watch two channels, like CCTV 1 and CCTV 5, at the time. What would happen if CCTV 1 and CCTV 5 used the same frequency to transfer information? We would not be able to receive information from both of them, or static would be displayed.

We know from physics that when two waves with the same frequency are combined, constructive interference or destructive interference occurs depending on the phase difference. For this reason, CCTV 1 and CCTV 5 use specified RFs to transfer information and avoid interference.

[From Beginner to Expert - WLAN Fundamentals] Section 2 WLAN Radio Frequencies a-1841147-4 

WLAN channels are similar to TV channels. If a WLAN used the 2.4 GHz band as a channel, severe interference would occur when two or more APs used the same channel in an area, similar to what would happen if CCTV 5 used the same channel as CCTV 1). These two APs would be unable to provide usable WLAN services. To prevent this situation, WLAN protocols divide the 2.4 GHz band into 13 overlapping channels of a specific bandwidth (20 MHz in 802.11g/n and 22 MHz in 802.11 b), and each channel has a center frequency (like the 200 MHz center frequency for CCTV 1).

When discussing the 2.4 GHz band, we generally mention these 13 overlapping channels. However, different countries allow different 2.4 GHz channels for open use namely, channels 1 to 11 in the United States and Canada, and channels 1 to 13 in Europe and China and channel 14 is valid only in Japan.  

Of these 13 channels, there are three that do not overlap: channels 1, 6, and 11. Since they do not overlap, adjacent APs using these three channels will not experience interference. The following figure shows the distribution of channels in the 2.4 GHz band.

[From Beginner to Expert - WLAN Fundamentals] Section 2 WLAN Radio Frequencies a-1841147-5

An example of using channels that do not overlap is shown in the following figure:

[From Beginner to Expert - WLAN Fundamentals] Section 2 WLAN Radio Frequencies a-1841147-6 

When deploying WLANs, we adopt the following cellular channel deployment mode to avoid co-channel interference on adjacent APs. In this deployment mode, adjacent APs use non-overlapping channels.

[From Beginner to Expert - WLAN Fundamentals] Section 2 WLAN Radio Frequencies a-1841147-7 

In the 2.4 GHz band, Huawei APs use channel 1 by default. If customers do not configure channels when deploying WLANs, co-channel interference may occur in overlapped coverage areas. This causes STAs to fail to go online. However, configuring channels for a large number of APs is a difficult task. To alleviate this problem, Huawei products offer automatic channel configuration. After APs go online, ACs can automatically configure channels for them based on the radio environments around the APs.

Huawei products also support radio calibration. Channel deployment and transmit power are fine-tuned based on radio environments so that wireless networks can provide optimal performance. It is recommended that you perform radio calibration after the initial deployment of a WLAN, because the RFs used by the newly deployed WLAN may partially overlap with those of surrounding WLANs. With radio calibration, the WLAN fine-tunes channel deployment and transmit power based on the radio environment and avoids RF overlapping. Moreover, radio environments change constantly, necessitating regular radio calibration in off-peak hours.

The 2.4 GHz band is allowed for open use throughout the world, and an increasing number of wireless devices (such as Bluetooth devices) operate in it. This has resulted in crowding and severe channel interference on the 2.4 GHZ band, and WLAN services may be affected.

Since V200R003C00, Huawei products support spectrum analysis. This function can detect devices causing interference around APs, such as baby monitors, microwave ovens, and Bluetooth devices.

The 5 GHz band is also allowed for WLANs. This band features higher frequencies and greater channel bandwidth. Data rates in this band are higher and channel interference is lower. WLAN protocols divide the 5 GHz band into 24 non-overlapping 20 MHz-wide channels, providing sufficient channel resources for WLANs. More non-overlapping channels make channel bonding, which is the combination of two channels into a wider channel, more valuable. For example, when two 20 MHz non-overlapping channels are bound, the throughput on the bound channel is twice that of a single 20 MHz channel.

In 802.11n, two adjacent 20 MHz channels can be bound into a 40 MHz channel to improve data rates. The 802.11n protocol also defines channel bonding in the 2.4 GHz band. However, congestion in the 2.4 GHz band reduces the practicability of channel bonding. Channel bonding in the 2.4 GHz band is generally not recommended.

The following figure shows channel allocation in the 5 GHz band.

[From Beginner to Expert - WLAN Fundamentals] Section 2 WLAN Radio Frequencies a-1841147-8 

Black semicircles in the figure represent non-overlapping channels, and red semicircles represent channel bonding as defined in the protocol. The UNII-2e sub-band, newly defined in the 5 GHz band, is not allowed for open use in China. At present, the open 5 GHz channels in China are channels 36, 40, 44, 48, 52, 56, 60, 64, 149, 153, 157, 161, and 165. The Country Code & Channel Compliance Table lists the open 5 GHz channels in various countries.

Military radar devices also operate in the 5 GHz band in many countries, and 5 GHz WLAN devices used by civilians may cause severe interference on radar devices. For security purposes, WLAN products in some countries are required to employ dynamic frequency selection (DFS) and transmit power control (TPC). TPC aims to avoid interference with military radar, which is caused by the high transmission power of WLAN products. DFS enables WLAN products to detect RFs used by military radar so that they can choose other RFs if overlap occurs. These two capabilities are required by law in some countries, and products that do not have them cannot be marketed in those countries.

We have now learned a bit about WLAN radio frequencies and channels. Before ending this section, let's discuss another two commonly used concepts in WLANs: dBm and dB.

dBm is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). To convert from absolute power "P" to dBm, use the formula dBm = 10 x log(P/1 mW), namely, 100 mW = 10(log102) = 20 dBm.

dB expresses the ratio between two power levels. To be exact, dB = 10 x log(P1/P2).

A power level of 0 dBm corresponds to a power of 1 mW. A 3 dB increase in level is approximately equivalent to doubling the power, which means that a level of 3 dBm corresponds roughly to a power of 2 mW. For each 3 dB decrease in level, the power is reduced by about one half, making -3 dBm correspond to a power of about 0.5 mW. (Wikipedia)

Question:

In China, channels 1 to 13 in the 2.4 GHz band are allowed for open use. When we deploy WLANs, is it possible to use only the non-overlapping channels 1, 6, and 11?

The post is synchronized to: From Beginner to Expert-WLAN Fundamentals

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user_235153
Created Jun 24, 2016 12:35:41

顶一下。。
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wissal
MVE Created Apr 30, 2018 05:26:34

clear and useful document, thanks
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titusmahwe
Created Aug 19, 2020 03:12:35

awesome
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VinceD
Created Jan 9, 2021 16:17:52

thanks for sharing.
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carlosdua
Created Jan 9, 2021 23:30:37

very useful
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karladu
Created Jan 11, 2021 12:14:22

Thanks for the information
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