liwen01 2024.09.01
preamble
In the last decade or so, communication technology has been developing rapidly, and communication standards have been updated frequently. Some devices are still using the 802.11/b/g/n protocol, and some have been supported to WiFi6 and WiFi7.
However, there are few books or data about wireless WiFi in China, and even if you can find them, most of them are quite old. This article tries to use the latest data to introduce some basic knowledge about WiFi.
Here are a few questions to see how much you know about the evolution of WiFi technology:
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What WiFi protocol standards are generally compatible and supported by home routers?
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What exactly does 802.11 b/g/n/ac/ax mean? How does it differ from WiFi 4/5/6/7?
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Why is there a huge difference in maximum rates between protocols? What is the principle behind their implementation?
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What key technology developments in WiFi development have led to significant increases in WiFi rates?
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Why are all the actual speeds much lower than the theoretical rate?
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What are the differences between wired Ethernet and wireless WiFi in the OSI Layer 7 model?
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802.11 a/b/g/n, etc. At what level of the OSI are these standards located?
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How many channels are available for WiFi at 2.4Ghz/5GHz and are there any other restrictions?
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How many non-overlapping channels does WiFi have at 2.4Ghz/5GHz each?
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Why do we see so few devices using WiFi5?
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Do two routers using different protocols in the same space interfere or affect each other?
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What frequency bands does the latest WiFi7 work on?
This article mainly introduces WiFi channels, wireless network OSI model, and some key technologies of 802.11b/g/n standard protocols. Due to space limitation, WiFi5, WiFi6, and WiFi7 will be introduced in the next article.
(i) IEEE Association and 802.11 standards
- IEEE Institute of Electrical and Electronics Engineers (IEEE), an international professional society organization, is the world's largest technical professional organization.
- IEEE 802 It is a standards series project that includes multiple technical standards for Ethernet, LANs, and MANs.
- IEEE 802.11 is a working group within the IEEE 802 family of standards that focuses on wireless local area network (WLAN) technology.
- 802.11a/b/g/n... is a standard developed by a task force under the IEEE 802.11 Working Group.
In the earlier years, we see more WiFi classified by 802.11b/g/n letters to distinguish, but with the continuous development of WiFi protocols, the WiFi Alliance has assigned new names to the different WiFi standards, that is, WiFi4, WiFi5, WiFi6, and WiFi7 by numerical designations; the main purpose is to facilitate everyone to remember and distinguish.
802.11 be, also known as WiFi7, is expected to be officially released this year (2024), and the WiFi7 devices that you can buy online now should be pre-certified devices that have some of the features of WiFi7, but there may be some differences from the officially released standard.。
Before introducing the various WiFi protocol standards, let's first understand the concepts related to WiFi channels.
(ii) WiFi channels
Currently in the security IPC equipment, the use of more or 802.11b/g/n three standards, but there are many manufacturers began to switch to 802.11ax (WiFi6) protocol up.
In fact most products are switching directly from 802.11n (WiFi4) to 802.11ax (WiFi6).
Why didn't you use WiFi5 instead of leapfrogging directly from WiFi4 to WiFi6?
Since WiFi5 only supports the 5GHz band, it is not compatible with devices that used to use 2.4GHz.
(1) 2.4Ghz band channel
- 802.11b uses a channel bandwidth of 22MHz, all other standards currently in use have a 20Mhz channel bandwidth.
- Center frequency of each adjacent channel, with a difference of 5 MHz (except for channel 14)
- Traditionally recognized, there are 3 non-overlapping channels (1, 6, 11)
Since 802.11b (using DSSS modulation technology with a bandwidth of 22 MHz) has faded out of WLAN networks, regardless of compatibility issues, channels 1, 5, 9, and 13 can usually be considered non-overlapping channels as well.
For channel 12~14, different countries have different requirements and specifications, the actual product design needs to be adapted according to the country code.
(2) 5GHz band signals
- The 5 GHz band is typically divided into four UNII (Unlicensed National Information Infrastructure) sub-bands.
- DFS (Dynamic Frequency Selection) Channels are designed to avoid interference with critical radar systems that require WiFi devices to monitor radar signals and automatically switch channels when detected.
- 20 MHz channel: is the most commonly used channel bandwidth and is suitable for use in environments with more equipment to avoid interference
- 40 MHz channel: Provides higher throughput by aggregating two adjacent 20 MHz channels, but is more susceptible to interference
- 80 MHz and 160 MHz channels: Suitable for applications with very high throughput requirements (e.g., 4K streaming, HD videoconferencing), but less so in practice because they take up more spectrum resources
- All channels of UNII-2 Extended (5470-5725 MHz) cannot be used in China.
- In China.Only the 5 channels of UNII-3 can be used in all scenarios.
- In China, there are 13 non-overlapping 5GHz bands that can be used
The data in the table above is from a Huawei open-source document, and we can see that the low-frequency and medium-frequency are limited to indoor use. However, we have looked at a lot of other materials and found that they do not match Huawei's data, for example, the following chart, which shows that they do not limit some of our channels in UNII-1 and UNII-2.
By checking the latest version of the "Regulations of the People's * on the Division of Radio Frequencies", we can see that Huawei's data is correct, and in 2023 we have regulations that state that UNII-1 and UNII-2 channels can only be used indoors.
So.For channels that can be used directly at 5GHz in China, information up to 2023 will include channels inside UNII-1 and UNII-2, but after 2023, UNII-1 and UNII-2 will be labeled for indoor use only.
(3) 6GHz band channels
Some 6GHz channels will be used in WiFi6 and WiFi7, but the use of 6GHz channels is not yet open in our country.
The 6GHz band extends from 5925MHz to 7125MHz, totaling 1200MHz of spectrum. It can be channel-bound into 3 channels of 320MHz, 7 channels of 160MHz, 14 channels of 80MHz or 29 channels of 40MHz. If used directly without binding, it provides 59 20MHz channels.
Compared to 2.4GHz and 5GHz, the 6GHz band has more spectrum resources than the former two combined.
With the gradual popularization of WiFi6 and WiFi7, China should open up some 6GHz WiFi channels in the future.
(iii) OSI model in wireless networks
The 7-layer model is commonly referenced for network layering in computer courses:Physical layer, data link layer, network layer, transport layer, session layer, representation layer, application layer。
The model above is actually very generalized, in reality it is much more complex, and we can't see any difference between Ethernet and wireless from this diagram.
The main differences between Ethernet and wireless networks in terms of the OSI model are in the first and second layers, the physical and data link layers。
(1) Physical Layer (Physical Layer)
The physical layer is primarily responsible for transferring raw bit streams (0s and 1s) between network devices. It involves physical connections, such as cables, optical fibers, and radio waves, as well as the electrical and mechanical characteristics of data transmission. Common physical layer devices include network cards, hubs, and cables.
Ethernet: Ethernet uses wired connections, such as twisted-pair cables or fiber optics, to transmit data. The physical layer defines the characteristics of the electrical signals, voltages and pulses that are transmitted.
Wireless (Wi-Fi): Wi-Fi transmits data over the air via radio waves. The physical layer involves the selection of the radio frequency, the configuration of the antenna, and how the signal is modulated and demodulated
(2) The data link layer (Data Link Layer)
The data link layer is responsible for establishing reliable communication links between neighboring nodes. It sends data frames from one node to the next and handles frame transmission errors. The data link layer also includes the MAC (medium access control) sublayer and the LLC (logical link control) sublayer. Common devices are switches and bridges.
Ethernet: At the data link layer, Ethernet typically uses Ethernet frames for data encapsulation.MAC addresses are used to identify network devices and control access to the medium (CSMA/CD, carrier listening multiple access/conflict detection mechanism).
Wireless (Wi-Fi). Wireless networks also use frames for data encapsulation at the data link layer, but Wi-Fi frames are formatted differently than Ethernet frames.CSMA/CA (Carrier Listening Multiple Access/Conflict Avoidance Mechanism) to manage media access, and added encryption (e.g., WPA/WPA2 ) and authentication (e.g., 802.1X) features to enhance security.
(3) Wireless network data frame encapsulation
To further divide the physical and data link layers of a wireless network, we can see that the physical layer has: PLCP and PMD layers, and the data link layer has: MAC layer and LLC layer.
Here we briefly introduce a basic function of each layer, and detailed analysis of WiFi data frames will be introduced in later chapters.
- LLC sublayer：(Logical Link Control) The logical link control sublayer, which provides a unified interface to upper-layer network protocols and manages the control of logical links and data transmission.
- MAC sublayer：(Medium Access Control) The media access control sublayer, which manages the access of devices to the shared communication medium and the transmission of data frames.
- PLCP sublayer：(Physical Layer Convergence Procedure) The Physical Layer Convergence Process sublayer, which is responsible for converting the data frame format between the MAC layer and the PMD sublayer.
- PMD sublayer：(Physical Medium Dependent) The physical medium related sublayer, which deals directly with the transmission and reception of physical signals.
The protocol standards we often refer to as 802.11 b/g/n are actually located in the physical layer.
(4) Physical layer spread spectrum technology
Spread spectrum technology is the technology used for data transmission in wireless LANs. Spread spectrum technology was originally used by the military to prevent eavesdropping or signal interference.
WiFi (Wireless Local Area Network) uses Spread Spectrum technology to improve the reliability of communication and anti-jamming ability, the application of Spread Spectrum technology in WiFi is mainly realized through the following ways:
(a) Direct Sequence Spread Spectrum (DSSS)
DSSS Data is spread over a wider frequency spectrum by performing an out-of-parallel operation with a pseudo-random noise code (PN code). This has the advantage of making the signal less energy dense in the spectrum, thus increasing the signal's resistance to noise and interference.
(b) frequency-hopping spread spectrum (Frequency Hopping Spread Spectrum, FHSS)
FHSS Avoiding interference by quickly hopping between multiple frequencies, which requires an agreement in advance on the frequency hopping pattern between the transmitter and receiver, is less commonly used in WiFi.
(c) Orthogonal frequency division multiplexing (OFDM).Frequency-Division Multiplexing, OFDM)
OFDM uses multiple orthogonal subcarriers, each transmitting a portion of the data, which greatly reduces the effects of multipath effects and improves spectral efficiency.
It doesn't matter if you can't understand the three spread spectrums above, they are described in slightly more detail below.
(iv) 802.11b
802.11b is a standard released in 1999.Why is its maximum theoretical figure only 11Mps?
This is related to the coding method and modulation used in the 802.11b physical layer:
(a) BPSK with QPSK modulation
BPSK: (Binary Phase Shift Keying) Each symbol represents 1 bit, i.e. only 1 bit can be transmitted each time a symbol is modulated.
QPSK: (Quadrature Phase Shift Keying) Each symbol represents 2 bits and is more efficient than BPSK because it can distinguish between four phases.
(b) Barker and CCK coding
Barker codingThe Barker code is an 11-bit sequence (e.g., 10110111000), which has the advantage of reducing interference in wireless transmissions, although it reduces efficiency.
Each bit is encoded as an 11-bit Barker code, and the resulting data object forms a chip.11 times more information is actually transmitted than is efficiently transmitted。
CCK Code. (Complementary Code Keying) Complementary Code Keying employs complex mathematical conversion functions to encode 4 or 8 bits in each code word using several 8-bit sequences.
Complementary keying encoding method can effectively prevent noise and multipath interference, the disadvantage is that complementary keying in order to combat multipath interference, the technology is complex, difficult to implement。
(c) 802.11b rate calculations
About 802.11b Calculations for each rate:
1Mbps (Barker + BPSK)
modulation method: BPSK, 1 bit per symbol.
coding method: Barker encoding, where each symbol is encoded to 11 bits.
in the end: Since the symbol rate is 1 MSym/s and BPSK modulation is 1 bit per symbol, the maximum theoretical rate is 1 Mbps.
2Mbps (Barker + QPSK)
modulation method: QPSK, 2 bits per symbol.
coding method: Barker Code.
in the end: Symbol rate 1 MSym/s, 2 bits are transmitted per symbol, so the maximum theoretical rate is 2 Mbps.
5.5Mbps (4-bits CCK + QPSK)
modulation method: QPSK, 2 bits per symbol.
coding method: 4-bits CCK coding, which utilizes a complex coding method to improve the bit-per-symbol transmission efficiency.
in the end: Although each symbol represents 2 bits, CCK coding allows each symbol to ultimately carry 4 bits. The maximum theoretical rate is therefore 5.5 Mbps.
11Mbps (8-bits CCK + QPSK)
Modulation type. QPSK, 2 bits per symbol.
coding method: 8-bits CCK encoding with 8 bits per symbol.
in the end: On the basis of QPSK, the optimization of CCK coding makes it possible to transmit 8 bits per symbol, so the maximum theoretical rate is 11 Mbps。
Attention:Above Sym/s is the unit of symbol rate/code element rate used to express the number of symbols transmitted per second in a communication system。
- In BPSK (Binary Phase Shift Keying) modulation, one symbol represents one bit.
- In QPSK (Quadrature Phase Shift Keying) modulation, one symbol represents 2 bits.
(v) 802.11g
802.11g can increase the maximum speed from 11Mbps in 802.11b to 54Mbps, the core of which is the use of OFDM modulated carrier technology.
(1)Orthogonal Frequency Division Multiplexing (OFDM)
Orthogonal Frequency Division Multiplexing (OFDM) is a digital multicarrier modulation scheme that extends the concept of single subcarrier modulation by using multiple subcarriers within the same single channel.
Instead of using a single subcarrier to transmit a high-rate data stream, OFDM uses a large number of closely spaced orthogonal subcarriers transmitted in parallel. Each subcarrier uses a conventional digital modulation scheme. The combination of many subcarriers can achieve data rates similar to conventional single-carrier modulation schemes within an equivalent bandwidth.
OFDM is based on frequency division multiplexing (FDM) technology, in which different information streams are mapped onto separate parallel channels, and each FDM channel is separated from the others by a frequency guard band to minimize interference between neighboring channels.
OFDM schemes differ from traditional FDM in the following relevant ways:
- Multiple carriers (called subcarriers) carry the information flow.
- The subcarriers are orthogonal to each other and a guard interval is added to each symbol to minimize channel delay spread and inter-symbol interference
The above figure illustrates the main concepts of OFDM signaling and the interrelationship between the frequency and time domains.
In the frequency domain, multiple neighboring subcarriers are independently modulated with complex data. An inverse FFT transform is performed on the frequency domain subcarriers to produce OFDM symbols in the time domain.
In the time domain, a guard interval is inserted between each symbol to prevent intersymbol interference at the receiver due to multipath delay extension in the radio channel. Multiple symbols can be connected to create the final OFDM burst signal.
At the receiver, an FFT is performed on the OFDM symbols to recover the original data bits.The FFT here is the Fourier transform in high math.
In 802.11g, there are 48 subcarriers for data transmission and 4 subcarriers for phase reference。
Why can the 802.11g rate reach 54Mbps?
802.11g uses 64-QAM coding in addition to OFDM modulated carrier technology.
(2) 64-QAM encoding method
QAM (Quadrature Amplitude Modulation) In QAM (Quadrature Amplitude Modulation), the data signal is represented by the change in amplitude of two carriers that are orthogonal to each other. Phase modulation of analog signals and PSK (Phase Shift Keying) of digital signals can be considered as special quadrature amplitude modulation in which the amplitude is constant and only the phase changes.
Each symbol in 64-QAM is a constellation of 6-bit states, and each symbol is one of 64 possible combinations of different states from 000 000 to 111 111. Since this modulation scheme uses binary data, the total number of possible combinations is 64 using 6 bits to the power of 2 to the 6th power.
Accordingly in WiFi there are 16-QAM and 256-QAM coding methods used, 16-QAM transmits 4 bits, 64-QAM transmits 6 bits and 256-QAM transmits 8 bits.
In 802.11g 64-QAM is used and it has a coding rate of 3/4。
(3) 802.11g rate calculation
The data rate is the product of the symbol rate, the number of bits carried per symbol, and the channel coding rate.
modulation method: 64-QAM, each symbol represents 6 bits.
coding rate: 3/4 (coding rate used in forward error correction coding).
symbol rate: 250 ksps。
Each OFDM symbol on all subcarriersTotal transmission time is 4 microseconds (μs), including 3.2 μs data transmission time and 0.8 μs Guard Interval.。
The Symbol Period is therefore 4 microseconds.
Since each symbol period is 4 microseconds, the symbol rate is:
- Number of bits that can be transmitted per symbol = 6bit (because of 64-QAM).
- The carrier coding rate is 3/4, so the actual effective number of bits = 6bit * 3/4 = 4.5 bit.
- There are 48 data subcarriers, so the number of bits that can be transmitted per OFDM symbol = 48 * 4.5bit = 216bit.
- The symbol rate is 250 ksps, so the total data rate = 216 bits/symbol * 250 ksps = 54 Mbps.
From the above calculations, we know that the maximum supported rate for 802.11g is 54Mbps.
At the same time, 802.11g is backward compatible and can be matched to different rates under different modulation methods and coding rates.
| modulation method | coding rate | data rate |
|---|---|---|
| BPSK | 1/2 | 6 Mbps |
| BPSK | 3/4 | 9 Mbps |
| QPSK | 1/2 | 12 Mbps |
| QPSK | 3/4 | 18 Mbps |
| 16-QAM | 1/2 | 24 Mbps |
| 16-QAM | 3/4 | 36 Mbps |
| 64-QAM | 2/3 | 48 Mbps |
| 64-QAM | 3/4 | 54 Mbps |
(vi) 802.11n (WiFi4)
As of today, many devices still use the 802.11n protocol, especially in the security IPC industry.
So, from 802.11g in 2003 to 802.11n (WiFi4) in 2009, what are some of the key technologies that have been implemented to give WiFi4 a quantum leap in speed?
The technologies at the core of WiFi4 are OFDM, FEC, MIMO, 40Mhz, Short Gi.
(1) OFDM for 802.11n
The OFDM Orthogonal Frequency Division Multiplexing technique used here is the same as that used in 802.11g. The difference is:
- 802.11g has a total of 52 subcarriers, 802.11n has 56 subcarriers
- 802.11g has 48 data subcarriers, 802.11n has 52 data subcarriers
Number of data subcarriers x number of bits transmitted per symbol x carrier coding rate x symbol rate = maximum theoretical rate
52 * 6bit * 3/4 * 250 ksps = 58.5Mbps
The number of data subcarriers has been increased by 4, so the rate has increased from 54Mbps in 802.11g to 58.5Mbps.
(2) FEC for 802.11n
Forward Error Correction (FEC) technology encodes the original block of data at the transmitting end and adds redundant information to form a coded block. The receiver parses this redundant information to detect and correct errors during transmission.
This approach eliminates the need for feedback and retransmission, and therefore can significantly improve the efficiency of data transmission, especially in wireless environments with high noise or severe signal fading.
With FEC forward error correction coding, the carrier coding rate is improved from 3/4 to 5/6 of 802.11g.
Number of data subcarriers x number of bits transmitted per symbol x carrier coding rate x symbol rate = maximum theoretical rate
52 * 6bit * 5/6 * 250 ksps = 65Mbps
With FEC encoding, the rate is increased to 65Mbps.
(3) Short Gi for 802.11n
Guard Interval (GI) is a protection time inserted between each OFDM (Orthogonal Frequency Division Multiplexing) symbol to prevent inter-symbol interference (ISI, Inter-Symbol Interference). This type of interference is usually caused by multipath propagation, i.e., the signal undergoes multiple reflections, refractions, and scattering during propagation, which results in the signal reaching the receiving end at different times.
In legacy 802.11 systems, theThe standard length of the GI is 800 nanoseconds (ns)., this time interval is long enough to eliminate most of the intersymbol interference. However, a long GI also means wasting some of the time that could be used for data transmission.
Since the protection time is reduced by 400 ns, each symbol period is 4 microseconds - 0.4 microseconds = 3.6 microseconds
Symbol rate is: 277.778ksps
Number of data subcarriers x number of bits transmitted per symbol x carrier coding rate x symbol rate = maximum theoretical rate
52 * 6bit * 5/6 * 277.778 ksps = 72.2222Mbps
With Short GI technology, the rate is increased to 72.2222Mbps.
(4) 802.11n channel bundling
802.11n allows the use of channel bundling, where two adjacent 20 MHz channels are bundled together to form a single 40 MHz channel. This allows data to be transmitted over a wider spectrum.
By increasing the channel width, more subcarriers can be carried, thus increasing the data rate.
- A standard channel is 20Mhz in width and contains 52 subcarriers.
- The two neighboring channels are bundled into a 40Mhz bandwidth with 108 (52*2+4=108) subcarriers.
Why is the number of subcarriers 4 more when the above two channels are bundled together?
Because there is a gap between channels, when two channels are bound, the frequency band in between the two channels can also be used.
On 2.4G mode there can be up to one 40M channel, on 5G mode the number of 40M channels varies from country to country with a theoretical maximum of 11 40M channels.
Number of data subcarriers x number of bits transmitted per symbol x carrier coding rate x symbol rate = maximum theoretical rate
108 * 6bit * 5/6 * 277.778 ksps = 150Mbps
2.4Ghz band channel bundling considerations:In the 2.4 GHz band, due to the small number of available channels and the narrow channel spacing, channel bundling configurations typically used include:
- Channels 1 and 5: These channels can be bundled together to form a 40 MHz wide channel.
- Channels 6 and 10: These channels can also be bundled together to form a 40 MHz wide channel.
- Channels 11 and 7: These channels can also be bundled together to form a 40 MHz wide channel.
Due to the small channel bandwidth in the 2.4 GHz band, the channel spacing during bundling may result in higher channel overlap and interference, so special attention to interference management is required when using channel bundling in this band.
(5) 802.11n MIMO
MIMO (Multiple Input Multiple Output) Concepts
Multiple inputs and multiple outputs: MIMO technology utilizes multiple transmit and receive antennas for data transmission in wireless communications. By transmitting multiple streams of data simultaneously, MIMO technology can significantly increase the throughput and coverage of a wireless network.
spatial reuse: MIMO technology allows for the simultaneous transmission of multiple data streams over the same spectrum resource, increasing the efficiency of spectrum utilization. This technique is based on the principle of spatial multiplexing, whereby higher data rates are achieved by spatially separated data streams in the same frequency band.
Number of data subcarriers x number of bits transmitted per symbol x carrier coding rate x symbol rate x MIMO = maximum theoretical rate
108 * 6bit * 5/6 * 277.778 ksps *4 = 600Mbps
So the maximum rate for 802.11n is 600Mbps
Let's go back to the very beginning of the WiFi Standards and WiFi Generations chart.We can see that 802.11n (WiFi4) maxes out at 450Mbps at 2.4GHz and 600Mbps at 5Ghz, why?
What I've read online is that 802.11n maxes out at 3 streams at 2.4GHz and 4 streams at 5GHz.
In addition to the key technologies introduced above such as OFDM, FEC, MIMO, 40Mhz, Short Gi, 802.11n also has Frame Aggregation, Block Acknowledgement, and a more efficient MAC layer to greatly improve the overall performance of WiFi.
wind up
Above introduced the WiFi channel, the OSI model of wireless WiFi, and some key technologies of the 802.11b/g/n standard protocols, the next article will introduce some contents related to WiFi5, WiFi6, WiFi7, and the matters that need to be paid attention to when using these standards.
The above content, if there is any error, welcome to criticize and point out in the comment section, thank you.