A simple guide to Wi-Fi, a must-read when buying a router

In this article, we will talk about the wireless access function of wireless routers, which is the history, frequency bands, and key technologies related to Wi-Fi that we are familiar with.

Wireless router

The wireless access function of a wireless router is the wireless local area network (WLAN) mentioned earlier. Currently, there is only one mainstream WLAN technology, Wi-Fi, so the two can be considered equivalent.

Wi-Fi is technically certified and trademarked by the Wi-Fi Alliance. In actual applications, Wi-Fi is often written as WiFi or Wifi, but these two spellings are not recognized by the alliance.

Wi-Fi Alliance (full name: International Wi-Fi Alliance Organization, English: Wi-Fi Alliance, referred to as WFA), is a business alliance that owns the Wi-Fi trademark. It is responsible for Wi-Fi certification and trademark authorization, and its headquarters is located in Austin, Texas, USA.

Wi-Fi, a catchy name widely believed to be an abbreviation for Wireless Fidelity, is actually a misreading. It is just a simple name without any actual meaning, and of course there is no full name.

The technical standard behind Wi-Fi is the 802.11 series of protocols developed by the Institute of Electrical and Electronics Engineers (IEEE) of the United States.

IEEE full name: Institute of Electrical and Electronics Engines

1. Development of Wi-Fi Protocol

Since the first version in 1997, the 802.11 series of protocols have continued to evolve, going through multiple versions such as 802.11a/b/g/n/ac, and the supported Internet speeds have continued to increase. The latest protocol version is 802.11ax, which is the rapidly developing Wi-Fi 6 in recent years.

The development history of the IEEE 802.11 series of standards, from the first generation to the sixth generation

In the first many years, although Wi-Fi developed from generation to generation, there was no such thing as Wi-Fi generations in the world. The protocol number was simply 802.11 followed by a few letters, which was very unfriendly to ordinary users.

It was not until 2018 that the Wi-Fi Alliance decided to promote the next-generation technology standard 802.11ax as the more understandable Wi-Fi 6, and the previous generation 802.11ac and 802.11n naturally became Wi-Fi 5 and Wi-Fi 4. As for earlier technologies, no one paid attention to them anyway, so there was no need to use a pseudonym.

After the birth of Wi-Fi 6, the name Wi-Fi 5 came into being

On September 16, 2019, the Wi-Fi Alliance announced the launch of the Wi-Fi 6 certification program. Since then, the name of Wi-Fi 6 has spread all over the world, and most newly released devices now support Wi-Fi 6.

2. Wi-Fi channels and frequency bands used

Wi-Fi mainly operates in the 2.4GHz and 5GHz frequency bands. These two frequency bands are called ISM (Industrial Scientific Medical) bands. As long as the transmission power meets the national standard requirements, they can be used directly without authorization.

The ISM frequency bands vary from country to country.

2.4GHz is the world's first ISM frequency band, with a spectrum range of 2.40GHz to 2.4835GHz, with a total bandwidth of 83.5M.

The commonly used Bluetooth, ZigBee, and wireless USB also work in the 2.4GHz band. In addition, microwave ovens and cordless phones also use the 2.4GHz band. Even the internal chip of the wired USB interface will emit useless 2.4GHz signals when working, causing interference.

It can be seen that there are many devices working at the same time on 2.4GHz, the frequency band is crowded and the interference is serious. When thousands of lights are on and you and your neighbors upstairs and downstairs are happily surfing the Internet with Wi-Fi, the router is silently selecting channels and coordinating interference in the background.

Wi-Fi divides the 83.5M bandwidth on 2.4G into 13 channels, each of which is 20M. Note that these channels overlap. Originally, only 3 channels could be used, but now 13 channels have been squeezed in. Interference between them is inevitable, and we can only try to reduce it. At worst, everyone can use a slower speed and take turns to use it.

2.4G spectrum and channels (Channel 14 is not allowed to be used in the country)

To what extent do the channels overlap? The following figure shows that among these channels, only 1, 6, 11 or 2, 7, 12, or 3, 8, 13 are completely non-overlapping, which shows the degree of congestion in the 2.4GHz band. It is like a very narrow road with many cars passing through it, and frequent traffic jams will inevitably cause a decrease in the speed of travel.

2.4G non-overlapping channel distribution

With 802.11n, users can use 40M channels, but the total bandwidth of the 2.4GHz band is still only 83.5M, which can only accommodate two channels. Therefore, only when the network is idle at night can a single user use a 40M channel. In addition, due to interference from the neighbor's house, the high speed of 802.11n is difficult to achieve to a large extent.

2.4G 40M bandwidth channel

If the 2.4GHz band is a narrow path, the 5GHz band is undoubtedly a broad road.

The available range of the 5GHz frequency band is 4.910GHz to 5.875GHz, with a bandwidth of more than 900M, which is more than 10 times that of 2.4G! This spectrum is too wide, and different countries have defined the range in which Wi-Fi can be used based on their own conditions.

For example, in China's 5GHz spectrum, there are 13 20M channels available for Wi-Fi, and continuous 20M channels can be combined into 40M, 80M, or even 160M channels.

China 5G channel distribution map

5GHz has a large bandwidth and runs on fewer devices, so it is naturally faster and has less interference. Therefore, if you want your home network to achieve a good speed experience, you can consider using 5GHz to cover the entire house.

However, every inch has its own strengths and weaknesses. Although 5GHz has a large bandwidth and low interference, its signal propagation attenuates quickly, is easily blocked, and has a weak ability to penetrate walls.

Penetration loss of 2.4G and 5G Wi-Fi signals

Therefore, compared with 2.4GHz, 5GHz signals are usually much weaker. As for how many meters they can cover, it is difficult to give a specific number because it is related to the router's antenna gain, receiving sensitivity, the distribution of walls and obstacles in the home, and the Internet speed that individuals expect to achieve.

If you only consider the networking of various smart home devices at home, the coverage and capacity of 2.4GHz are usually sufficient. However, if you need high-speed Internet access and maximize the value of home broadband, you must rely on 5GHz.

Therefore, it is recommended that Wi-Fi coverage not consider 2.4GHz, and directly use 5GHz full-house coverage as the design goal. Generally speaking, it is difficult for a single router to achieve full coverage in a complex home environment, and it is necessary to consider the networking and roaming issues between multiple routers, which will be discussed later.

3. Key Wi-Fi Technologies

Why is Wi-Fi getting faster and faster? In fact, the IEEE's 802.11 series of protocols have been learning from 3GPP's 4G and 5G, and the underlying technologies used are universal.

OFDM/OFDMA

OFDM stands for Orthogonal Frequency Division Multiplexing. The system divides the carrier bandwidth into multiple mutually orthogonal subcarriers in the frequency domain, which is equivalent to dividing a road into multiple parallel lanes, which greatly improves traffic efficiency.

In Wi-Fi 5 and earlier (802.11a/b/g/n/ac), the subcarrier width is 312.5KHz. In Wi-Fi 6 (802.11ax), the subcarrier width is reduced to 78.125KHz, which is equivalent to dividing a road of the same width into more lanes.

Wi-Fi 6 has more subcarriers

Under OFDM, each user must occupy all subcarriers in the full bandwidth at the same time. If a user does not have much data to send and the frequency resources are not fully used, other users will not be able to use them flexibly and can only wait in line, which is not efficient in using spectrum resources.

To solve this problem, Wi-Fi 6 introduces OFDMA technology, with an additional letter A at the end, and its full name becomes Orthogonal Frequency Division Multiple Access. Multiple access means multiplexing of multiple users.

OFDM vs. OFDMA

OFDMA can support multiple users to share all subcarriers at the same time, which is equivalent to a transportation company packaging the data of multiple users and loading them together, making full use of the capacity of the carriage, thus speeding up the delivery of goods and improving the spectrum efficiency.

MIMO/Beamforming

The number of antennas on routers is increasing, from no antennas to one, two, three, four, six, eight... Now, no matter what the price of the router, it looks like a crab, with its claws and fangs bared, which is very intimidating.

Why do we need so many antennas? It is to better implement MIMO (Multiple Input Multiple Output) technology. Simply put, when transmitting signals, multiple antennas are used to send multiple different data at the same time, and the speed is naturally doubled; when receiving, multiple antennas receive signals from the mobile phone at the same time, just like wearing hearing aids, and the receiving sensitivity is also enhanced.

Single-User MIMO (SU-MIMO)

If all antennas serve only one user at the same time, it is called single-user MIMO (SU-MIMO). Going further, the router has four-way transmission and the mobile phone has four-way reception, which can also be more precisely called 4x4 MIMO.

Sometimes, the router has many antennas and is very powerful, but when you look around, you find that all the mobile phones are weak. The router can send 4 signals, but the mobile phone can only receive 2 at most, so in the end the router has to cooperate and send only 2. Isn't this a waste?

Multi-User MIMO (MU-MIMO)

There is a solution. If a mobile phone has few receiving antennas, wouldn't multiple mobile phones have more? Therefore, the router considers multiple mobile phones together and regards them as a powerful virtual mobile phone, so that high-order MIMO can be achieved. This kind of MIMO with multiple mobile phones participating is called multi-user MIMO (MU-MIMO), also known as virtual MIMO.

In addition, multiple antennas can also form directional narrow beams through beamforming technology to accurately cover users. Since the energy of narrow beams is concentrated, they can cover farther and penetrate walls better.

Beamforming

It seems that the more antennas a router has, the better. Do you have to buy a router with more antennas? This may be a trap. No matter how many antennas a router has, it is just piling up some visible hardware. It looks awesome, but whether the internal design can support so many antennas is still unknown.

More importantly, both MIMO and beamforming require support from software algorithms, which are much more complex than hardware. Different manufacturers have different algorithm optimization capabilities, which may lead to large performance differences.

Therefore, it is recommended that when purchasing a router, you do not need to pay too much attention to how many antennas you can see on the outside, but instead look at their product promotion. Does it support beamforming, 4x4MIMO, or MU-MIMO? If the manufacturer is very aggressive in promoting this aspect, it at least shows that they are confident in these features and use them as selling points.

Modulation Coding Strategy (MCS)

Modulation and coding are divided into two parts: modulation and coding. Together, they determine the number of bits that can be sent simultaneously per unit time. The modulation and coding strategy generally combines the two parts into multiple levels. The higher the level, the faster the data transmission rate.

The role of modulation is to map the encoded data (a random combination of 0s and 1s) to the smallest unit of the frame structure mentioned above: OFDM symbol. The modulated signal can finally be transmitted.

Constellation diagrams for BPSK, QPSK, 16QAM, 64QAM and 256QAM

Common modulation methods include BPSK, QPSK, 16QAM, 64QAM and 256QAM, and the number of bits that can be sent simultaneously is 1, 2, 4, 6 and 8. Wi-Fi 6 can support 1024QAM and can send 10 bits of data at the same time, which naturally greatly improves the rate.

Comparison chart between 256QAM and 1024QAM

However, when the original data is encoded, a lot of redundant bits are added for error correction, and the real useful data only accounts for a part. When we consider the Internet speed, we are only talking about the sending and receiving speed of useful data, and the redundant bits are discarded during decoding.

This requires the introduction of the concept of bit rate, which is the ratio of useful data to the total amount of data after encoding. If the bit rate is 3/4, it means that 3/4 of the encoded data is useful data and 1/4 is redundant bits added later.

Different modulation methods, plus different code rates, constitute the modulation and coding strategy (MCS). The following table is the MCS table in Wi-Fi 6. It can be seen that the highest-order MCS is 11, which corresponds to 1024QAM plus a code rate of 5/6.

Wi-Fi 6 MCS table

It is through the continuous evolution of these technologies that Wi-Fi standards advance from generation to generation, with increasingly higher speeds, allowing us to surf the Internet more smoothly.