802.11n - data transmission mode, real speed approximately four times higher than 802.11g (54 Mbps). But this means if the device that sends and receives operates in 802.11n mode.
802.11n devices operate in the frequency range 2.4 - 2.5 or 5 GHz. Usually the frequency is indicated in the documentation for the device or on the packaging. Range: 100 meters (may affect speed).
IEEE 802.11n - fast mode Wi-Fi works, only 802.11ac is faster (this is an unrealistically cool standard). Compatibility of 802.11n with older 802.11a/b/g is possible when using the same frequency and channel.
You may think that I’m strange, but I don’t like Wi-Fi - I don’t know why, but somehow it always seems to me that it’s not as stable as wires (twisted pair). Maybe because I only had USB adapters. In the future I want to get myself a Wi-Fi PCI card, I hope that everything is stable there)) I’m already silent about the fact that WiFi USB without an antenna, the speed will decrease due to any walls... But now we have wires lying around in our apartment, and I agree - it’s not very convenient..))
As I understand it, 802.11n is a good standard, since it already includes the characteristics of 802.11a/b/g.
However, it turns out that 802.11n is not compatible with previous standards. And as I understand it, this is the main reason why 802.11n is still not a very popular standard, but it appeared in 2007. It seems that there is still compatibility - I wrote about it below.
Some characteristics of other standards:
There are many standards and some of them are very interesting for their purpose:
Look, 802.11p determines the type of devices that, within a kilometer radius, travel at a speed of no more than 200 km... can you imagine?)) This is technology!!
802.11n and router speed
Look, there may be such a situation - you need to increase the speed in the router. What to do? Your router can easily support the IEEE 802.11n standard. You need to open the settings, and somewhere there find the option to use this standard, that is, for the device to operate in this mode. If you have an ASUS router, then the setting may look something like this:
In fact, the main thing is the letter N. If you have a TP-Link company, then the setting may look like this:
That's all for the router. I understand that there is not enough information - but at least now you know that the router has such a setting, but how to connect to the router... it’s better to look on the Internet, I admit - I’m not good at this. I just know that I need to open the address.. something like 192.168.1.1, something like that..
If you have a laptop, it may also support the IEEE 802.11n standard. And it is useful to install it if, for example, you create an access point from a laptop (yes, this is possible). Open Device Manager by holding down the Win + R buttons and paste this command:
Then find your Wi-Fi adapter (may be called network adapter Broadcom 802.11n) - click right click and select Properties:
Go to the Advanced tab and find the 802.11n Direct Connection Mode item, select enable:
The setting may be called differently - Wireless Mode, Wireless Type, Wi-Fi Mode, Wi-Fi type. In general, you need to specify the data transfer mode. But the effect in terms of speed, as I already wrote, will be provided that both devices use the 802.11n standard.
I found this important information regarding compatibility:
About compatibility, and also a lot important information Read about 802.11 standards here:
There really is a lot of valuable information there, I advise you to take a look.
AdHoc Support 802.11n what is it? Should I turn it on or not?
AdHoc Support 802.11n or AdHoc 11n - support for temporary AdHoc network operation when connection is possible between different devices. Used for online data transfer. I couldn’t find any information about whether it is possible to organize Internet distribution on the AdHoc network (but anything is possible).
Officially, AdHoc limits the speed to the level of the 11g standard - 54 Mbit/s.
I learned an interesting point - the speed of Wi-Fi 802.11g, as I already wrote, is 54 Mbit/s. However, it turns out that 54 is a total figure, that is, it is reception and sending. So, one way speed is 27 Mbit/s. But that’s not all - 27 Mbit/s is a channel speed that is possible under ideal conditions, it is unrealistic to achieve them - 30-40% of the channel is still interference in the form mobile phones, all sorts of radiation, smart TVs with Wi-Fi and so on. As a result, the speed in reality can actually be 18-20 Mbit/s, or even less. I will not say - but it is possible that this also applies to other standards.
So should I turn it on or not? It turns out that if there is no need, there is no need. Also, if I understand correctly, when turned on, a new local network will be created and perhaps it is still possible to organize the Internet in it. In other words, it may be... that using AdHoc you can create a point Wi-Fi access. I just looked it up on the Internet - it seems possible))
I just remember this... once I bought myself a Wi-Fi adapter from D-Link (I think it was the D-Link N150 DWA-123 model) and there was no support for creating an access point. But here’s the chip, it was either Chinese... or something else... in general, I found out that you can install special unofficial drivers on it, semi-curve ones, and with the help of them you can create an access point.. And this point access seemed to work using AdHoc, unfortunately I don’t remember exactly - but it worked more or less tolerably.
Ad Hoc settings in network card properties
Note - QoS is a technology for distributing traffic in terms of priorities. Provides the required high level of packet transmission for important processes/programs. In simple words, QoS allows you to set high priority to programs that require instant data transfer - Online Games, VoIP telephony, streaming, streaming and the like probably also applies to Skype and Viber.
802.11 Preamble Long and Short - what is this setting?
Yes, these settings are a whole science. The part of the frame that is transmitted by the 802.11 module is called the preamble. There can be a long (Long) and a short (Short) preamble, and apparently this is indicated in the 802.11 Preamble (or Preamble Type) setting. The long preamble uses a 128-bit synchronization field, the short one uses a 56-bit.
802.11 devices operating at the 2.4 GHz frequency are required to support long preambles when receiving and transmitting. 802.11g devices must be able to handle long and short preambles. In 802.11b devices, short preambles are optional.
The values in the 802.11 Preamble setting can be Long, Short, Mixed mode, Green field, Legacy mode. I’ll say right away - it’s better not to touch these settings unless necessary and leave the default value or, if available, select Auto (or Default).
We have already found out above what the Long and Short modes mean. Now briefly about other modes:
- Legacy mode. Data exchange mode between stations with one antenna.
- Mixed mode. Data transmission mode between MIMO systems (fast, but slower than Green field), and between conventional stations (slow, as they do not support high speeds). The MIMO system determines the packet depending on the receiver.
- Green field. Transmission is possible between multi-antenna devices. When a MIMO transmission occurs, conventional stations wait for the channel to become free to avoid collisions. In this mode, receiving data from devices operating in the above two modes is possible, but transmitting data to them is not. This is done to eliminate single-antenna devices during data transmission, thereby maintaining high transmission speeds.
MIMO support what is it?
On a note. MIMO (Multiple Input Multiple Output) is a type of data transmission in which the channel is increased using spatial signal coding and data transmission is carried out by several antennas simultaneously.
20.10.2018About the new standard wireless communication IEEE 802.11n has been talked about for several years now. This is understandable, because one of the main disadvantages of the existing IEEE 802.11a/b/g wireless communication standards is the data transfer speed is too low. Indeed, the theoretical throughput of the IEEE 802.11a/g protocols is only 54 Mbit/s, and the actual data transfer rate does not exceed 25 Mbit/s. The new wireless communication standard IEEE 802.11n should provide transmission speeds of up to 300 Mbit/s, which looks very tempting compared to 54 Mbit/s. Of course, the actual data transfer rate in the IEEE 802.11n standard, as test results show, does not exceed 100 Mbit/s, but even in this case, the actual data transfer speed is four times higher than in the IEEE 802.11g standard. The IEEE 802.11n standard has not yet been fully adopted (this should happen before the end of 2007), but almost all wireless equipment manufacturers have already begun producing devices compatible with the Draft version of the IEEE 802.11n standard.
In this article we will look at the basic provisions of the new IEEE 802.11n standard and its main differences from the 802.11a/b/g standards.
We have already talked about the 802.11a/b/g wireless communication standards in some detail on the pages of our magazine. Therefore, in this article we will not describe them in detail; however, in order for the main differences between the new standard and its predecessors to be obvious, we will have to make a digest of previously published articles on this topic.
Considering the history of wireless communication standards used to create wireless local area networks (WLAN), it is probably worth recalling the IEEE 802.11 standard, which, although no longer found in its pure form, is the progenitor of all other wireless communication standards for networks WLAN.
IEEE 802.11 standard
The 802.11 standard provides for the use of a frequency range from 2400 to 2483.5 MHz, that is, an 83.5 MHz wide range divided into several frequency subchannels.
The 802.11 standard is based on the technology of spreading the spectrum (Spread Spectrum, SS), which implies that the initially narrow-band (in terms of spectrum width) useful information signal is converted during transmission in such a way that its spectrum is much wider than the spectrum of the original signal. Simultaneously with the broadening of the signal spectrum, a redistribution of the spectral energy density of the signal occurs - the signal energy is also “spread out” across the spectrum.
The 802.11 protocol uses Direct Sequence Spread Spectrum (DSSS) technology. Its essence lies in the fact that to broaden the spectrum of an initially narrow-band signal, a chip sequence, which is a sequence of rectangular pulses, is built into each transmitted information bit. If the duration of one chip pulse is n times less than the duration of the information bit, then the width of the spectrum of the converted signal will be n times the width of the spectrum of the original signal. In this case, the amplitude of the transmitted signal will decrease by n once.
The chip sequences embedded in the information bits are called noise-like codes (PN-sequences), which emphasizes the fact that the resulting signal becomes noise-like and is difficult to distinguish from natural noise.
It’s clear how to broaden the signal spectrum and make it indistinguishable from natural noise. To do this, in principle, you can use an arbitrary (random) chip sequence. However, the question arises of how to receive such a signal. After all, if it becomes noise-like, then isolating a useful information signal from it is not so easy, if not impossible. Nevertheless, this can be done, but for this you need to select the chip sequence accordingly. Chip sequences used to broaden the signal spectrum must satisfy certain autocorrelation requirements. In mathematics, autocorrelation refers to the degree to which a function is similar to itself at different points in time. If you select a chip sequence for which the autocorrelation function will have a pronounced peak for only one point in time, then such an information signal can be distinguished at the noise level. To do this, the received signal is multiplied by the chip sequence in the receiver, that is, the autocorrelation function of the signal is calculated. As a result, the signal again becomes narrow-band, so it is filtered in a narrow frequency band equal to twice the transmission rate. Any interference that falls within the band of the original broadband signal, after multiplication by the chip sequence, on the contrary, becomes broadband and is cut off by filters, and only part of the interference falls into the narrow information band; its power is significantly less than the interference acting at the receiver input.
There are quite a lot of chip sequences that meet the specified autocorrelation requirements, but the so-called Barker codes are of particular interest to us, since they are used in the 802.11 protocol. Barker codes have the best noise-like properties among known pseudo-random sequences, which has led to their widespread use. The 802.11 family of protocols uses Barker code that is 11 chips long.
In order to transmit a signal, the information sequence of bits in the receiver is added modulo 2 (mod 2) with the 11-chip Barker code using an XOR (exclusive OR) gate. Thus, a logical one is transmitted by a direct Barker sequence, and a logical zero by an inverse sequence.
The 802.11 standard provides two speed modes - 1 and 2 Mbit/s.
With an information speed of 1 Mbit/s, the speed of individual Barker sequence chips is 11x106 chips per second, and the spectrum width of such a signal is 22 MHz.
Considering that the width of the frequency range is 83.5 MHz, we find that a total of three non-overlapping frequency channels can fit in this frequency range. All frequency range However, it is customary to divide it into 11 overlapping frequency channels of 22 MHz, spaced 5 MHz apart. For example, the first channel occupies the frequency range from 2400 to 2423 MHz and is centered relative to the frequency of 2412 MHz. The second channel is centered relative to the frequency of 2417 MHz, and the last, 11th channel is centered relative to the frequency of 2462 MHz. When viewed this way, channels 1, 6 and 11 do not overlap with each other and have a 3 MHz gap relative to each other. It is these three channels that can be used independently of each other.
To modulate a sinusoidal carrier signal at a data rate of 1 Mbit/s, relative binary phase modulation (DBPSK) is used.
In this case, information encoding occurs due to a phase shift of the sinusoidal signal relative to the previous signal state. Binary phase modulation provides two possible phase shift values - 0 and p. Then a logical zero can be transmitted by an in-phase signal (the phase shift is 0), and a logical one can be transmitted by a signal that is phase shifted by p.
An information speed of 1 Mbit/s is mandatory in the IEEE 802.11 standard (Basic Access Rate), but a speed of 2 Mbit/s (Enhanced Access Rate) is optionally possible. To transmit data at this speed, the same DSSS technology with 11-chip Barker codes is used, but Differential Quadrature Phase Shift Key is used to modulate the carrier wave.
In conclusion physical level 802.11 protocol, we note that with an information speed of 2 Mbit/s, the speed of individual chips of the Barker sequence remains the same, that is, 11x106 chips per second, and therefore the width of the spectrum of the transmitted signal does not change.
IEEE 802.11b standard
The IEEE 802.11 standard was replaced by the IEEE 802.11b standard, which was adopted in July 1999. This standard is a kind of extension of the basic 802.11 protocol and, in addition to speeds of 1 and 2 Mbit/s, provides speeds of 5.5 and 11 Mbit/s, for which so-called complementary codes (Complementary Code Keying, CCK) are used.
Complementary codes, or CCK sequences, have the property that the sum of their autocorrelation functions for any cyclic shift other than zero is always zero, so they, like Barker codes, can be used to recognize a signal from a background of noise.
The main difference between CCK sequences and the previously discussed Barker codes is that there is not a strictly defined sequence through which either a logical zero or a one can be encoded, but a whole set of sequences. This circumstance makes it possible to encode several information bits in one transmitted symbol and thereby increases the information transmission speed.
The IEEE 802.11b standard deals with complex complementary 8-chip sequences defined on a set of complex elements taking values (1, –1, +j, –j}.
Complex signal representation is a convenient mathematical tool for representing a phase-modulated signal. Thus, a sequence value equal to 1 corresponds to a signal in phase with the generator signal, and a sequence value equal to –1 corresponds to an antiphase signal; sequence value equal j- a signal phase-shifted by p/2, and the value is equal to – j, - signal phase shifted by –p/2.
Each element of the CCK sequence is a complex number, the value of which is determined using a rather complex algorithm. There are a total of 64 sets of possible CCK sequences, with the choice of each determined by the sequence of input bits. To uniquely select one CCK sequence, six input bits are required. Thus, the IEEE 802.11b protocol uses one of 64 possible eight-bit CKK sequences when encoding each character.
At a speed of 5.5 Mbit/s, 4 bits of data are simultaneously encoded in one symbol, and at a speed of 11 Mbit/s - 8 bits of data. In both cases, the symbolic transmission rate is 1.385x106 symbols per second (11/8 = 5.5/4 = 1.385), and taking into account that each character is specified by an 8-chip sequence, we find that in both cases the transmission speed of individual chips is 11x106 chips per second. Accordingly, the signal spectrum width at speeds of both 11 and 5.5 Mbit/s is 22 MHz.
IEEE 802.11g standard
The IEEE 802.11g standard, adopted in 2003, is a logical development of the 802.11b standard and involves data transmission in the same frequency range, but at higher speeds. Additionally, 802.11g is fully compatible with 802.11b, meaning any 802.11g device must be able to work with 802.11b devices. The maximum data transfer rate in the 802.11g standard is 54 Mbit/s.
Two competing technologies were considered during the development of the 802.11g standard: the orthogonal frequency division OFDM method, borrowed from the 802.11a standard and proposed by Intersil, and the binary packet convolutional coding method PBCC, proposed by Texas Instruments. As a result, the 802.11g standard contains a compromise solution: OFDM and CCK technologies are used as base technologies, and the optional use of PBCC technology is provided.
The idea of convolutional coding (Packet Binary Convolutional Coding, PBCC) is as follows. The incoming sequence of information bits is converted in a convolutional encoder so that each input bit corresponds to more than one output bit. That is, the convolutional encoder adds certain redundant information to the original sequence. If, for example, each input bit corresponds to two output bits, then we talk about convolutional coding with a speed r= 1/2. If every two input bits correspond to three output bits, then it will be 2/3.
Any convolutional encoder is built on the basis of several sequentially connected memory cells and XOR gates. The number of storage cells determines the number of possible encoder states. If, for example, a convolutional encoder uses six memory cells, then the encoder stores information about six previous states signal, and taking into account the value of the incoming bit, we find that such an encoder uses seven bits of the input sequence. Such a convolutional encoder is called a seven-state encoder ( K = 7).
The output bits generated by the convolutional encoder are determined by XOR operations between the values of the input bit and the bits stored in the storage cells, that is, the value of each output bit generated depends not only on the incoming information bit, but also on several previous bits.
PBCC technology uses seven-state convolutional encoders ( K= 7) with speed r = 1/2.
The main advantage of convolutional encoders is the noise immunity of the sequence they generate. The fact is that with redundant coding, even in the event of reception errors, the original bit sequence can be accurately restored. A Viterbi decoder is used at the receiver side to restore the original bit sequence.
The dibit generated in the convolutional encoder is subsequently used as a transmitted symbol, but it is first subjected to phase modulation. Moreover, depending on the transmission speed, binary, quadrature or even eight-position phase modulation is possible.
Unlike DSSS technologies (Barker codes, SSK sequences), convolutional coding technology does not use spectrum broadening technology through the use of noise-like sequences, however, spectrum broadening to standard 22 MHz is also provided in this case. To do this, variations of possible QPSK and BPSK signal constellations are used.
The considered PBCC coding method is optionally used in the 802.11b protocol at speeds of 5.5 and 11 Mbit/s. Similarly, in the 802.11g protocol for transmission speeds of 5.5 and 11 Mbit/s, this method is also used optionally. In general, due to the compatibility of the 802.11b and 802.11g protocols, the encoding technologies and speeds provided by the 802.11b protocol are also supported in the 802.11g protocol. In this regard, up to a speed of 11 Mbps, the 802.11b and 802.11g protocols are the same, except that the 802.11g protocol provides speeds that the 802.11b protocol does not.
Optionally, in the 802.11g protocol, PBCC technology can be used at transmission rates of 22 and 33 Mbit/s.
For a speed of 22 Mbit/s, compared to the PBCC scheme we have already considered, data transmission has two features. First of all, 8-position phase modulation (8-PSK) is used, that is, the phase of the signal can take on eight different values, which allows three bits to be encoded in one symbol. In addition, a puncture encoder (Puncture) has been added to the circuit, with the exception of the convolutional encoder. The meaning of this solution is quite simple: the redundancy of the convolutional encoder, equal to 2 (for each input bit there are two output bits), is quite high and under certain noise conditions it is unnecessary, so the redundancy can be reduced so that, for example, every two input bits correspond to three output bits . For this, you can, of course, develop an appropriate convolutional encoder, but it is better to add a special puncture encoder to the circuit, which will simply destroy extra bits.
Let's say a puncture encoder removes one bit from every four input bits. Then every four incoming bits will correspond to three outgoing ones. The speed of such an encoder is 4/3. If such an encoder is used in conjunction with a convolutional encoder with a speed of 1/2, then the total encoding speed will be 2/3, that is, for every two input bits there will be three output bits.
As already noted, PBCC technology is optional in the IEEE 802.11g standard, and OFDM technology is mandatory. In order to understand the essence of OFDM technology, let's take a closer look at the multipath interference that occurs when signals propagate in an open environment.
The effect of multipath signal interference is that, as a result of multiple reflections from natural obstacles, the same signal can reach the receiver in different ways. But different propagation paths differ from each other in length, and therefore the signal attenuation will not be the same for them. Consequently, at the receiving point, the resulting signal represents the interference of many signals having different amplitudes and shifted relative to each other in time, which is equivalent to the addition of signals with different phases.
The consequence of multipath interference is distortion of the received signal. Multipath interference is inherent in any type of signal, but it has a particularly negative effect on wideband signals, since when using a broadband signal, as a result of interference, certain frequencies add up in phase, which leads to an increase in the signal, and some, on the contrary, out of phase, causing a weakening of the signal at a given frequency.
Speaking about multipath interference that occurs during signal transmission, two extreme cases are noted. In the first of them, the maximum delay between signals does not exceed the duration of one symbol and interference occurs within one transmitted symbol. In the second, the maximum delay between signals is greater than the duration of one symbol, so as a result of interference, signals representing different symbols are added, and so-called inter-symbol interference (ISI) occurs.
It is intersymbol interference that has the most negative effect on signal distortion. Since a symbol is a discrete signal state characterized by the values of carrier frequency, amplitude and phase, the amplitude and phase of the signal change for different symbols, and therefore it is extremely difficult to restore the original signal.
For this reason, at high data rates, a data encoding method called Orthogonal Frequency Division Multiplexing (OFDM) is used. Its essence lies in the fact that the stream of transmitted data is distributed over many frequency subchannels and transmission is carried out in parallel on all such subchannels. In this case, a high transmission speed is achieved precisely due to the simultaneous transmission of data over all channels, while the transmission speed in a separate subchannel may be low.
Due to the fact that the data transmission rate in each of the frequency subchannels can be made not too high, the prerequisites are created for effective suppression of intersymbol interference.
Frequency division of channels requires that an individual channel be narrow enough to minimize signal distortion, but at the same time wide enough to provide the required transmission speed. In addition, to economically use the entire bandwidth of a channel divided into subchannels, it is desirable to arrange the frequency subchannels as close to each other as possible, but at the same time avoid interchannel interference to ensure their complete independence. Frequency channels that satisfy the above requirements are called orthogonal. The carrier signals of all frequency subchannels are orthogonal to each other. It is important that the orthogonality of the carrier signals guarantees the frequency independence of the channels from each other, and therefore the absence of inter-channel interference.
This method of dividing a wideband channel into orthogonal frequency subchannels is called orthogonal frequency division multiplexing (OFDM). To implement it in transmitting devices, an inverse fast Fourier transform (IFFT) is used, which transforms the previously multiplexed n-channels signal from time O th representation into frequency.
One of the key advantages of the OFDM method is the combination of high transmission speed with effective resistance to multipath propagation. Of course, OFDM technology itself does not eliminate multipath propagation, but it creates the prerequisites for eliminating the effect of intersymbol interference. The fact is that an integral part of OFDM technology is the Guard Interval (GI) - a cyclic repetition of the end of the symbol, attached at the beginning of the symbol.
The guard interval creates pauses between individual symbols, and if its duration exceeds the maximum signal delay time due to multipath propagation, then intersymbol interference does not occur.
When using OFDM technology, the duration of the guard interval is one-fourth of the duration of the symbol itself. In this case, the symbol has a duration of 3.2 μs, and the guard interval is 0.8 μs. Thus, the duration of the symbol together with the guard interval is 4 μs.
Speaking about the OFDM frequency division technology used at various speeds in the 802.11g protocol, we have not yet touched upon the issue of the carrier signal modulation method.
The 802.11g protocol uses binary and quadrature phase modulation BPSK and QPSK at low bit rates. When using BPSK modulation, only one information bit is encoded in one symbol, and when using QPSK modulation, two information bits are encoded. BPSK modulation is used to transmit data at speeds of 6 and 9 Mbit/s, and QPSK modulation is used at speeds of 12 and 18 Mbit/s.
For transmission at higher speeds, quadrature amplitude modulation QAM (Quadrature Amplitude Modulation) is used, in which information is encoded by changing the phase and amplitude of the signal. The 802.11g protocol uses 16-QAM and 64-QAM modulation. The first modulation involves 16 different signal states, which allows 4 bits to be encoded in one symbol; the second - 64 possible signal states, which makes it possible to encode a sequence of 6 bits in one symbol. 16-QAM modulation is used at 24 and 36 Mbps, and 64-QAM modulation is used at 48 and 54 Mbps.
In addition to the use of CCK, OFDM and PBCC coding, the IEEE 802.11g standard also optionally provides various hybrid coding options.
In order to understand the essence of this term, remember that any transmitted data packet contains a header (preamble) with service information and a data field. When referring to a packet in CCK format, it means that the header and data of the frame are transmitted in CCK format. Similarly, with OFDM technology, the frame header and data are transmitted using OFDM encoding. Hybrid coding means that different coding technologies can be used for the frame header and data fields. For example, when using CCK-OFDM technology, the frame header is encoded using CCK codes, but the frame data itself is transmitted using multi-frequency OFDM encoding. Thus, CCK-OFDM technology is a kind of hybrid of CCK and OFDM. However, this is not the only hybrid technology - when using PBCC packet coding, the frame header is transmitted using CCK codes, and the frame data is encoded using PBCC.
IEEE 802.11a standard
The IEEE 802.11b and IEEE 802.11g standards discussed above refer to the 2.4 GHz frequency range (from 2.4 to 2.4835 GHz), and the IEEE 802.11a standard, adopted in 1999, involves the use of a higher frequency range (from 5 .15 to 5.350 GHz and 5.725 to 5.825 GHz). In the USA, this range is called the Unlicensed National Information Infrastructure (UNII) range.
In accordance with FCC rules, the UNII frequency range is divided into three 100-MHz sub-bands, differing in their restrictions on maximum power radiation. The low band (5.15 to 5.25 GHz) provides only 50 mW of power, the middle (5.25 to 5.35 GHz) 250 mW, and the high (5.725 to 5.825 GHz) 1 W. The use of three frequency subbands with a total width of 300 MHz makes the IEEE 802.11a standard the most broadband of the 802.11 family of standards and allows the entire frequency range to be divided into 12 channels, each of which has a width of 20 MHz, with eight of them lying in the 200 MHz range from 5 .15 to 5.35 GHz, and the remaining four channels are in the 100 MHz range from 5.725 to 5.825 GHz (Fig. 1). At the same time, four upper frequency channels, providing highest power transmissions are used primarily for transmitting signals outdoors.
Rice. 1. Division of the UNII range into 12 frequency subbands
The IEEE 802.11a standard is based on the Orthogonal Frequency Division Multiplexing (OFDM) technique. To separate channels, an inverse Fourier transform is used with a window of 64 frequency subchannels. Since each of the 12 channels defined in the 802.11a standard is 20 MHz wide, each orthogonal frequency subchannel (subcarrier) is 312.5 kHz wide. However, out of 64 orthogonal subchannels, only 52 are used, with 48 of them used for data transmission (Data Tones), and the rest for transmission of service information (Pilot Tones).
In terms of modulation technology, the 802.11a protocol is not much different from 802.11g. At low bit rates, binary and quadrature phase modulation BPSK and QPSK are used to modulate subcarrier frequencies. When using BPSK modulation, only one information bit is encoded in one symbol. Accordingly, when using QPSK modulation, that is, when the signal phase can take four different values, two information bits are encoded in one symbol. BPSK modulation is used to transmit data at 6 and 9 Mbps, and QPSK modulation is used at 12 and 18 Mbps.
To transmit at higher speeds, the IEEE 802.11a standard uses 16-QAM and 64-QAM quadrature amplitude modulation. In the first case there are 16 different signal states, which allows you to encode 4 bits in one symbol, and in the second there are already 64 possible signal states, which allows you to encode a sequence of 6 bits in one symbol. 16-QAM modulation is used at 24 and 36 Mbps, and 64-QAM modulation is used at 48 and 54 Mbps.
The information capacity of an OFDM symbol is determined by the type of modulation and the number of subcarriers. Since 48 subcarriers are used for data transmission, the capacity of an OFDM symbol is 48 x Nb, where Nb is the binary logarithm of the number of modulation positions, or, more simply, the number of bits that are encoded in one symbol in one subchannel. Accordingly, the OFDM symbol capacity ranges from 48 to 288 bits.
The sequence of processing input data (bits) in the IEEE 802.11a standard is as follows. Initially, the input data stream is subjected to standard operation scrambling. After this, the data stream is fed to the convolutional encoder. The convolutional coding rate (in combination with puncture coding) can be 1/2, 2/3 or 3/4.
Since the convolutional coding rate can be different, when using the same type of modulation, the data transmission rate is different.
Consider, for example, BPSK modulation, where the data rate is 6 or 9 Mbit/s. The duration of one symbol together with the guard interval is 4 μs, which means that the pulse repetition rate will be 250 kHz. Considering that one bit is encoded in each subchannel, and there are 48 such subchannels in total, we obtain that the total data transfer rate will be 250 kHz x 48 channels = 12 MHz. If the convolutional coding speed is 1/2 (one service bit is added for each information bit), the information speed will be half the full speed, that is, 6 Mbit/s. At a convolutional coding rate of 3/4, for every three information bits one service bit is added, so in this case the useful (information) speed is 3/4 of the full speed, that is, 9 Mbit/s.
Similarly, each modulation type corresponds to two different transmission rates (Table 1).
Table 1. Relationship between transmission rates
and modulation type in the 802.11a standard
Transfer rate, Mbit/s |
Modulation type |
Convolutional coding rate |
Number of bits |
Total number of bits in a symbol |
Number of information bits in a symbol |
After convolutional encoding, the bit stream is subjected to interleaving, or interleaving. Its essence is to change the order of bits within one OFDM symbol. To do this, the sequence of input bits is divided into blocks whose length is equal to the number of bits in the OFDM symbol (NCBPS). Next, according to a certain algorithm, a two-stage rearrangement of bits in each block is performed. In the first stage, the bits are rearranged so that adjacent bits are transmitted on non-adjacent subcarriers when transmitting an OFDM symbol. The bit swapping algorithm at this stage is equivalent to the following procedure. Initially, a block of bits of length NCBPS is written row by row into a matrix containing 16 rows and NCBPS/16 rows. Next, the bits are read from this matrix, but in rows (or in the same way as they were written, but from a transposed matrix). As a result of this operation, initially adjacent bits will be transmitted on non-adjacent subcarriers.
This is followed by a second bit permutation step, the purpose of which is to ensure that adjacent bits do not simultaneously appear in the least significant bits of the groups defining the modulation symbol in the signal constellation. That is, after the second stage of permutation, adjacent bits appear alternately in the high and low digits of the groups. This is done in order to improve the noise immunity of the transmitted signal.
After interleaving, the bit sequence is divided into groups according to the number of positions of the selected modulation type and OFDM symbols are formed.
The generated OFDM symbols are subjected to fast Fourier transform, resulting in the formation of output in-phase and quadrature signals, which are then subjected to standard processing - modulation.
IEEE 802.11n standard
Development of the IEEE 802.11n standard officially began on September 11, 2002, that is, one year before the final adoption of the IEEE 802.11g standard. In the second half of 2003, the IEEE 802.11n Task Group (802.11 TGn) was created, whose task was to develop a new wireless communication standard at speeds above 100 Mbit/s. Another task group, 802.15.3a, also dealt with the same task. By 2005, the processes of developing a single solution in each of the groups had reached a dead end. There was a confrontation in the 802.15.3a group Motorola and all other members of the group, and the members of the IEEE 802.11n group split into two approximately identical camps: WWiSE (World Wide Spectrum Efficiency) and TGn Sync. The WWiSE group was led by Aigro Networks, and the TGn Sync group was led by Intel company. In each of the groups for a long time neither of alternative options could not get the 75% of votes necessary for his approval.
After almost three years of unsuccessful opposition and attempts to work out a compromise solution that would suit everyone, the 802.15.3a group members voted almost unanimously to eliminate the 802.15.3a project. Members of the IEEE 802.11n project turned out to be more flexible - they managed to agree and create a unified proposal that would suit everyone. As a result, on January 19, 2006, at a regular conference held in Kona, Hawaii, a draft specification of the IEEE 802.11n standard was approved. Of the 188 members of the working group, 184 were in favor of adopting the standard, and four abstained. The main provisions of the approved document will form the basis for the final specification of the new standard.
The IEEE 802.11n standard is based on OFDM-MIMO technology. Many of the technical details implemented in it are borrowed from the 802.11a standard, but the IEEE 802.11n standard provides for the use of both the frequency range adopted for the IEEE 802.11a standard and the frequency range adopted for the IEEE 802.11b/g standards. That is, devices that support the IEEE 802.11n standard can operate in either the 5 or 2.4 GHz frequency range, with the specific implementation depending on the country. For Russia, IEEE 802.11n devices will support the 2.4 GHz frequency range.
The increase in transmission speed in the IEEE 802.11n standard is achieved, firstly, by doubling the channel width from 20 to 40 MHz, and secondly, by implementing MIMO technology.
MIMO (Multiple Input Multiple Output) technology involves the use of multiple transmitting and receiving antennas. By analogy, traditional systems, that is, systems with one transmitting and one receiving antenna, are called SISO (Single Input Single Output).
Theoretically, a MIMO system with n transmitting and n receiving antennas can provide peak throughput in n times larger than SISO systems. This is achieved by the transmitter breaking the data stream into independent bit sequences and transmitting them simultaneously using an array of antennas. This transmission technique is called spatial multiplexing. Note that all antennas transmit data independently of each other in the same frequency range.
Consider, for example, a MIMO system consisting of n transmitting and m receiving antennas (Fig. 2).
Rice. 2. Implementation principle of MIMO technology
The transmitter in such a system sends n independent signals using n antennas On the receiving side, each m antenna receives signals that are a superposition n signals from all transmitting antennas. So the signal R1, received by the first antenna, can be represented as:
Writing similar equations for each receiving antenna, we get the following system:
Or, rewriting this expression in matrix form:
Where [ H] - transfer matrix describing the MIMO communication channel.
In order for the decoder on the receiving side to be able to correctly reconstruct all signals, it must first determine the coefficients hij, characterizing each of m x n transmission channels. To determine the coefficients hij MIMO technology uses a packet preamble.
Having determined the coefficients of the transfer matrix, you can easily restore the transmitted signal:
Where [ H]–1 - matrix inverse to the transfer matrix [ H].
It is important to note that in MIMO technology, the use of multiple transmitting and receiving antennas makes it possible to increase the throughput of a communication channel by implementing several spatially separated subchannels, while data is transmitted in the same frequency range.
MIMO technology does not affect the data encoding method in any way and, in principle, can be used in combination with any methods of physical and logical data encoding.
MIMO technology was first described in the IEEE 802.16 standard. This standard allows the use of MISO technology, that is, several transmitting antennas and one receiving antenna. The IEEE 802.11n standard allows up to four antennas at the access point and wireless adapter. Mandatory mode implies support for two antennas at the access point and one antenna and wireless adapter.
The IEEE 802.11n standard provides both standard 20 MHz and double-width channels. However, the use of 40 MHz channels is an optional feature of the standard, since the use of such channels may contravene the laws of some countries.
The 802.11n standard provides two transmission modes: standard transmission mode (L) and high throughput (HT) mode. In traditional transmission modes, 52 frequency OFDM subchannels (frequency subcarriers) are used, of which 48 are used for data transmission, and the rest for transmission of service information.
In modes with increased throughput with a channel width of 20 MHz, 56 frequency subchannels are used, of which 52 are used for data transmission, and four channels are pilot. Thus, even when using a 20 MHz channel, increasing the frequency subchannels from 48 to 52 increases the transmission speed by 8%.
When using a double-width channel, that is, a 40 MHz channel, in standard transmission mode the broadcast is actually carried out on a double channel. Accordingly, the number of frequency subcarriers doubles (104 subchannels, of which 96 are information). Thanks to this, the transfer speed increases by 100%.
When using a 40-MHz channel and high-bandwidth mode, 114 frequency subchannels are used, of which 108 are information subchannels and six are pilot ones. Accordingly, this allows you to increase the transmission speed by 125%.
Table 2. Relationship between transmission rates and modulation type
and convolutional coding speed in the 802.11n standard
(20 MHz channel width, HT mode (52 frequency subchannels))
Modulation type |
Convolutional coding rate |
Number of bits in one symbol in one subchannel |
Total number of bits in an OFDM symbol |
Number of information bits per symbol |
Data transfer rate |
|
Two more circumstances due to which the IEEE 802.11n standard increases the transmission speed are a reduction in the duration of the GI guard interval in OGDM symbols from 0.8 to 0.4 μs and an increase in the speed of convolutional coding. Recall that in the IEEE 802.11a protocol, the maximum convolutional coding rate is 3/4, that is, for every three input bits one more is added. In the IEEE 802.11n protocol, the maximum convolutional coding rate is 5/6, that is, every five input bits in the convolutional encoder are converted into six output bits. The relationship between transmission rates, modulation type and convolutional coding rate for a standard 20 MHz wide channel is given in Table. 2.
In the nearly two decades since the first 802.11 wireless standards were introduced, five universal standards have emerged: 802.11a, 802.11b, 802.11g, 802.11n, and 802.11ac. With each new standard, Wi-Fi network speeds have only increased.
It turned out that this is not the limit: they are being replaced by new Wi-Fi standard– 802.11 ax (or 11AX), which is aimed at improving Wi-Fi performance in environments with large amounts of data traffic and frequent network congestion.
Wi-Fi 802.11 ax – increased speed and capacity
If you've ever tried to connect to Wi-Fi at a concert or at an airport, of course you know how many limitations networks have in such a dense environment. Too many users trying to receive a wireless signal puts too much strain on the network, reducing performance and signal stability. Standard 11AX solves this problem by offering better system routing data where it is needed.
The main goal of previous wireless networking standards was achieving maximum theoretical speed. And only the latest standard - 802.11 ac - expanded the possibilities for connecting multiple antennas.
Wi-Fi 11AX still divides the frequency band into multiple channels using OFDMA (Orthogonal Frequency Division Multiple Access) technology. But, at the same time, 11AX can significantly increase the speed of a wireless network and better manage its throughput, especially in high traffic volumes and overlapping networks.
What is the speed on Wi-Fi 11AX
The maximum speed of a single 802.11ac stream is about 866 Mbps, while a single 802.11ax stream reaches 1.2 Gb/s. This means the ability to stream Ultra-HD 4K video with zero latency, download entire software packages in the blink of an eye, and integrate with your entire family of smart devices.
The speeds you can get depend, of course, on the network and the equipment it uses. A large professional network that already has a strong signal will obviously have significantly higher speeds than networks in smaller companies. Either way, it is possible to achieve a fourfold increase in the current signal, which means a significant increase in the overall network capacity.
Lower speed limit? In addition to improving performance and range, 11AX is designed to increase the capacity of the 2.4 GHz and 5 GHz frequency bands in different environments– from home to school, business, airport, stadium, etc. It doesn’t matter in the slightest where you use the Wi-Fi network, you can achieve 4 times the current speed.
Efficiency of Wi-Fi 11AX standard
Speed is not the only important factor. 11AX also aims to implement mechanisms that provide a consistent and reliable flow of data to a larger number of users. This means improved performance and continued connectivity even in the face of high volumes of network traffic.
The 11AX standard operates on both 2.4 and 5 GHz frequencies, preserving existing channel capacities while increasing network capacity and expanding the way data can be transferred to multiple devices.
Standard 11AX also supports Orthogonal Frequency Division Multiple Access (OFDMA), a technology designed to improve throughput mobile networks LTE.
In its current application, every time a router transmits data to a device, it uses the entire bandwidth in the channel, regardless of the type of data or amount of information that is actively downloaded. With OFDMA, these channels can be separated, increasing the amount of data that can be sent and received simultaneously.
Besides, new 802.11ax standard allows you to schedule a “wake-up” time when communication is allowed (which reduces load). The 11AX not only supports 1024QAM encoding to carry more units of information per symbol, but also long OFDM symbols for higher channel capacity and less interference.
Features and Benefits of Wi-Fi 11AX
Most Wi-Fi users understand that connecting multiple devices reduces network throughput, resulting in slowdowns, unnecessary caching, and connection dropouts.
The new standard, also called High-Efficiency Wireless (HEW), provides another level of Wi-Fi control.
The standard includes the following main functions:
- Backwards compatible with previous Wi-Fi standards (802.11 a/b/g/n/ac)
- Ability to operate on the 5 GHz and 2.4 GHz bands simultaneously (and not one or the other, as in previous standards).
- Channel width 2/5/10 MHz for bands wider than 20 MHz.
- Increased throughput and performance:
- 1.5 times faster than 802.11ac
- 3.8 times faster than 2.4GHz 802.11n
- High capacity in high-density environments (such as stadiums)
- Up to 8 times faster than non-MU-MIMO devices using MU-MIMO upper and lower level (DL/UL) links
- 20% more airtime from the router, meaning more data can be transferred
- Improved power management for longer battery life
- Color BSS - in other words, each network will receive its own color, making them easy to distinguish
When is the 11AX standard launching?
Due to Wi-Fi 11AX improves average data transfer speeds On a per-user basis, this standard is best suited for high-density environments such as hotels, apartment buildings, and campuses.
When many users' devices are connected to the same network, they have to compete for available resources and transfer data sequentially, one at a time. With 11AX, multiple devices can simultaneously transmit data using the same frequency and the same network.
That is Wi-Fi in 11AX standard– this is not only an increase in network speed. This standard improves performance and eliminates problems caused by Wi-Fi network congestion and congestion.
Hi all! Today we will talk again about routers, wireless networks, technologies...
I decided to prepare an article in which I would talk about what kind of strange letters b/g/n are these that can be found when setting up a Wi-Fi router, or when purchasing a device (Wi-Fi characteristics, for example 802.11 b/g). And what is the difference between these standards.
I have already noticed several times that with a variety of problems with connecting phones or tablets to Wi-Fi, changing the Wi-Fi operating mode helps.
If you want to see what modes your device supports, then look at its specifications. Typically supported modes are listed next to “Wi-Fi 802.11”.
On the package (or on the Internet), you can also see in what modes your router can operate.
Here is an example of the supported standards that are indicated on the adapter box:
How to change the b/g/n operating mode in the Wi-Fi router settings?
I'll show you how to do this using the example of two routers, from ASUS And TP-Link. But if you have a different router, then look for changing the wireless network mode settings (Mode) on the tab Wi-Fi settings, where you set the name for the network, etc.
On a TP-Link router
Go to the router settings. How to enter them? I'm already tired of writing about this in almost every article :)..
Once you are in the settings, go to the tab on the left Wireless – Wireless Settings.
And opposite the point Mode You can select the wireless network operating standard. There are many options there. I recommend installing 11bgn mixed. This item allows you to connect devices that operate in at least one of three modes.
But if you still have problems connecting certain devices, then try the 11bg mixed, or 11g only. And to achieve a good data transfer speed, you can set 11n only. Just make sure that all devices support the standard n.
Using the example of an ASUS router
It's the same here. Go to settings and go to the tab "Wireless network".
Opposite the point “Wireless Network Mode” you can choose one of the standards. Or install Mixed, or Auto (which is what I recommend doing). For more details on standards, see just above. By the way, ASUS displays help on the right where you can read useful and interesting information on these settings.
To save, click the button “Apply”.
That's all, friends. I'm waiting for your questions, advice and suggestions in the comments. Bye everyone!
The IEEE 802 Standards Committee formed the 802.11 Wireless LAN Standards Working Group in 1990. This group began developing a universal standard for radio equipment and networks operating at 2.4 GHz, with access speeds of 1 and 2 Mbps (Megabits-per-second). Work on creating the standard was completed after 7 years, and the first 802.11 specification was ratified in June 1997. The IEEE 802.11 standard was the first standard for WLAN products from the independent international organization that develops most standards for wired networks. However, by that time, the originally designed data transfer speed in the wireless network no longer satisfied the needs of users. In order to make Wireless LAN technology popular, cheap, and most importantly, satisfying today's stringent requirements of business applications, developers were forced to create a new standard.
In September 1999, IEEE ratified an extension of the previous standard. Called IEEE 802.11b (also known as 802.11 High rate), it defines a standard for wireless networking products that operate at speeds of 11 Mbps (similar to Ethernet), allowing these devices to be successfully deployed in large organizations. Product Compatibility various manufacturers is guaranteed by an independent organization called the Wireless Ethernet Compatibility Alliance (WECA). This organization was founded by wireless industry leaders in 1999. Currently, WECA members are more than 80 companies, including: famous manufacturers, like , , etc. Products that meet Wi-Fi requirements (WECA term for IEEE 802.11b) can be found on the website.
The need for wireless access to local networks is growing as the number of mobile devices, such as laptops and PDAs, as well as with the growing desire of users to be connected to the network without having to “plug” a network cable into their computer. It is predicted that by 2003 there will be more than a billion mobile devices in the world, and the market value of WLAN products by 2002 is projected to be more than $2 billion.
IEEE 802.11 standard and its extension 802.11b
Like all IEEE 802 standards, 802.11 operates at the bottom two layers of the ISO/OSI model, the physical layer and the data link layer (Figure 1). Any network application, network operating system, or protocol (such as TCP/IP), will work just as well on an 802.11 network as on an Ethernet network.
Rice. 1. ISO/OSI model levels and their compliance with the 802.11 standard.
The basic architecture, features, and services of 802.11b are defined in the original 802.11 standard. The 802.11b specification only addresses the physical layer, adding only higher access speeds.
802.11 operating modes
802.11 defines two types of equipment - a client, which is usually a computer equipped with a wireless Network Interface Card (NIC), and an access point (AP), which acts as a bridge between the wireless and wired networks. An access point usually contains a transceiver, a wired network interface (802.3), and software, engaged in data processing. An ISA, PCI or PC Card network card in the 802.11 standard, or built-in solutions, for example, an 802.11 telephone headset, can act as a wireless station.
The IEEE 802.11 standard defines two modes of network operation: Ad-hoc mode and client/server mode (or infrastructure mode). In client/server mode (Fig. 2), a wireless network consists of at least one access point connected to a wired network and a certain set of wireless end stations. This configuration is called the Basic Service Set (BSS). Two or more BSSs forming a single subnet form an Extended Service Set (ESS). Since most wireless stations need to access file servers, printers, Internet, accessible on a wired local network, they will work in client/server mode.
Rice. 2. Client/server network architecture.
Ad-hoc mode (also called point-to-point or independent basic set of services, IBSS) is simple network, in which communication between numerous stations is established directly, without the use of a special access point (Fig. 3). This mode is useful if the wireless network infrastructure has not been created (for example, a hotel, exhibition hall, airport), or for some reason cannot be created.
Rice. 3. Ad-hoc network architecture.
802.11 Physical Layer
At the physical layer, two broadband radio frequency transmission methods and one in the infrared range are defined. RF methods operate in the 2.4 GHz ISM band and typically use the 83 MHz band from 2.400 GHz to 2.483 GHz. Broadband signal technologies used in RF methods increase reliability, throughput, and allow many unrelated devices to share the same frequency band with minimal interference to each other.
The 802.11 standard uses Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). These methods are fundamentally different and incompatible with each other.
FHSS uses Frequency Shift Keying (FSK) technology to modulate the signal. When operating at a speed of 1 Mbps, FSK Gaussian modulation of the second level is used, and when operating at a speed of 2 Mbps, the fourth level is used.
The DSSS method uses Phase Shift Keying (PSK) modulation technology. In this case, at a speed of 1 Mbps, differential binary PSK is used, and at a speed of 2 Mbps, differential quadratic PSK modulation is used.
Physical layer headers are always transmitted at 1 Mbps, while data can be transmitted at 1 and 2 Mbps.
Infrared (IR) transmission method
The implementation of this method in the 802.11 standard is based on the emission of a non-directional (diffuse IR) signal by the IR transmitter. Instead of directional transmission, requiring appropriate orientation of the emitter and receiver, the transmitted IR signal is emitted into the ceiling. Then the signal is reflected and received. This method has obvious advantages over the use of directional emitters, but there are also significant disadvantages - a ceiling is required that reflects IR radiation in a given wavelength range (850 - 950 nm); The range of the entire system is limited to 10 meters. In addition, IR rays are sensitive to weather conditions, so the method is recommended for use only indoors.
Two data transfer rates are supported - 1 and 2 Mbps. At a speed of 1 Mbps, the data stream is divided into quartets, each of which is then encoded into one of 16 pulses during modulation. At 2 Mbps, the modulation method is slightly different - the data stream is divided into bit pairs, each of which is modulated into one of four pulses. The peak power of the transmitted signal is 2 W.
FHSS method
Using the frequency hopping method, the 2.4 GHz band is divided into 79 1 MHz channels. The sender and receiver agree on a channel switching scheme (there are 22 such schemes to choose from) and data is sent sequentially over different channels using this scheme. Every data transfer on an 802.11 network occurs via different schemes switches, and the circuits themselves are designed to minimize the chances of two senders using the same channel at the same time.
The FHSS method allows for very simple diagram transceiver, however, is limited to a maximum speed of 2 Mbps. This limitation is due to the fact that exactly 1 MHz is allocated for one channel, which forces FHSS systems to use the entire 2.4 GHz band. This means that frequent channel switching must occur (for example, in the US the minimum speed is 2.5 switches per second), which in turn leads to increased overhead.
DSSS method
The DSSS method divides the 2.4 GHz band into 14 partially overlapping channels (only 11 channels are available in the US). In order for multiple channels to be used simultaneously in the same location, they must be spaced 25 MHz apart (not overlap) to avoid mutual interference. Thus, a maximum of 3 channels can be used simultaneously in one location. Data is sent using one of these channels without switching to other channels. To compensate extraneous noise, an 11-bit Barker sequence is used, where each bit of user data is converted into 11 bits of transmitted data. Such high redundancy for each bit can significantly increase transmission reliability, while significantly reducing the power of the transmitted signal. Even if part of the signal is lost, in most cases it will still be restored. This minimizes the number of repeated data transmissions.
Changes made by 802.11b
The main addition made by 802.11b to the main standard is support for two new data transfer rates - 5.5 and 11 Mbps. The DSSS method was chosen to achieve these speeds because the frequency hopping method cannot support higher speeds due to FCC restrictions. This means that 802.11b systems will be compatible with 802.11 DSSS systems, but will not work with 802.11 FHSS systems.
To support very noisy environments, as well as operation over long distances, 802.11b networks use dynamic rate shifting, which allows the data rate to automatically change depending on the properties of the radio channel. For example, a user can connect at a maximum speed of 11 Mbps, but if the level of interference increases or the user moves away a long distance, the mobile device will begin transmitting at a lower speed - 5.5, 2 or 1 Mbps. In the event that stable operation at a higher speed is possible, the mobile device will automatically begin transmitting at a higher speed. high speed. Rate shifting is a physical layer mechanism and is transparent to higher layers and the user.
Data Link level 802.11
The 802.11 link layer consists of two sublayers: Logical Link Control (LLC) and Media Access Control (MAC). 802.11 uses the same LLC and 48-bit addressing as other 802 networks, allowing wireless and wired networks to be easily combined, but the MAC layer is fundamentally different.
The MAC layer of 802.11 is very similar to that implemented in 802.3, where it supports multiple users on a shared media where the user verifies the media before accessing it. 802.3 Ethernet networks use the Carrier Sence Multiple Access with Collision Detection (CSMA/CD) protocol, which defines how Ethernet stations access the wired line and how they detect and handle collisions that occur when multiple devices attempt to connect simultaneously. network communication. To detect a collision, a station must be able to both receive and transmit simultaneously. The 802.11 standard requires the use of half-duplex transceivers, so in 802.11 wireless networks, a station cannot detect a collision during transmission.
To accommodate this difference, 802.11 uses a modified protocol known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), or Distributed Coordination Function (DCF). CSMA/CA attempts to avoid collisions by using an explicit packet acknowledgment (ACK), which means that the receiving station sends an ACK packet to confirm that the packet was received intact.
CSMA/CA works as follows. A station wanting to transmit tests the channel, and if no activity is detected, the station waits for some random amount of time and then transmits if the data medium is still clear. If the packet arrives intact, the receiving station sends an ACK packet, upon receipt of which the sender completes the transmission process. If the transmitting station did not receive the ACK packet, due to the fact that the data packet was not received, or a corrupted ACK arrived, the assumption is made that a collision has occurred, and the data packet is transmitted again after a random period of time.
To determine whether a channel is clear, a Channel Clearance Algorithm (CCA) is used. Its essence is to measure the signal energy at the antenna and determine the received signal strength (RSSI). If the received signal strength is below a certain threshold, then the channel is declared free and the MAC level receives CTS status. If the power is above the threshold, data transmission is delayed according to protocol rules. The standard provides another channel idle detection capability that can be used either alone or in conjunction with RSSI measurement—the carrier probe method. This method is more selective because it tests for the same carrier type as the 802.11 specification. Best Method to use depends on the level of interference in the work area.
Thus, CSMA/CA provides a method for separating access over a radio channel. The explicit acknowledgment mechanism effectively solves interference problems. However, it adds some additional overhead that 802.3 does not have, so 802.11 networks will always be slower than their equivalent Ethernet LANs.
Rice. 4. Illustration of the "hidden point" problem.
Another specific MAC layer problem is the "hidden point" problem, where two stations can both "hear" the access point, but cannot "hear" each other, due to long distance or obstacles (Fig. 4). To solve this problem in 802.11 MAC level added optional Request to Send/Clear to Send (RTS/CTS) protocol. When this protocol is used, the sending station transmits an RTS and waits for the access point to respond with a CTS. Since all stations on the network can "hear" the access point, the CTS signal causes them to delay their transmissions, which allows the transmitting station to transmit data and receive the ACK packet without the possibility of collisions. Because RTS/CTS adds additional network overhead by temporarily reserving media, it is typically used only for very large packets for which retransmission would be too costly.
Finally, the 802.11 MAC layer provides the ability to calculate CRC and fragment packets. Each packet has its own CRC checksum, which is calculated and attached to the packet. There is a difference here from Ethernet networks, in which error handling is handled by more advanced protocols. high level(eg TCP). Packet fragmentation allows large packets to be broken into smaller ones when transmitted over the air, which is useful in very crowded environments or where there is significant interference, as smaller packets are less likely to be damaged. This method reduces the need for retransmission in most cases and thus increases the performance of the entire wireless network. The MAC layer is responsible for reassembling the received fragments, making this process transparent to higher-level protocols.
Network connection
The 802.11 MAC layer is responsible for how the client connects to the access point. When an 802.11 client comes within range of one or more access points, it selects one of them based on signal strength and observed error rates and connects to it. Once the client receives confirmation that it has been accepted by the access point, it tunes to the radio channel on which it operates. From time to time it checks all 802.11 channels to see if another access point provides better quality services. If such an access point is found, then the station connects to it, retuning to its frequency (Fig. 5).
Rice. 5. Connecting to the network and illustrating the correct channel assignment for access points.
Reconnection usually occurs when the station has been physically moved away from the access point, causing the signal to weaken. In other cases, reconnection occurs due to a change in the building's RF characteristics, or simply due to high network traffic through the original access point. In the latter case, this protocol feature is known as “load balancing”, since its main purpose is to distribute the total load on the wireless network as efficiently as possible across the entire available network infrastructure.
The dynamic connection and reconnection process allows network administrators to set wireless network with very wide coverage, creating partially overlapping "honeycombs". The ideal option is one in which neighboring overlapping access points will use different DSSS channels so as not to interfere with each other (Fig. 5).
Streaming support
Streaming data, such as video or voice, is supported in the 802.11 specification at the MAC layer through the Point Coordination Function (PCF). In contrast to Distributed Coordination Function (DCF), where control is distributed among all stations, in PCF mode only the access point controls access to the channel. If a BSS with PCF enabled is installed, the time is evenly split between PCF mode and CSMA/CA mode. During periods when the system is in PCF mode, the access point polls all stations for data. Each station is allocated a fixed period of time, after which the next station is polled. No station can transmit at this time except the one being polled. Since PCF allows each station to transmit in certain time, then maximum latency is guaranteed. The disadvantage of this design is that the access point must poll all stations, which becomes extremely inefficient in large networks.
Power management
In addition to media access control, the 802.11 MAC layer supports power-saving modes to extend the battery life of mobile devices. The standard supports two energy consumption modes, called "continuous operation mode" and "saving mode". In the first case, the radio is always on, while in the second case, the radio is periodically turned on at certain intervals to receive the "beacon" signals that the access point constantly sends. These signals include information regarding which station should receive the data. Thus, the client can receive the beacon, receive the data, and then go back to sleep mode.
Safety
802.11b provides access control at the MAC layer (the second layer in the ISO/OSI model), and encryption mechanisms known as Wired Equivalent Privacy (WEP), which aim to provide wireless networks with security equivalent to those of wired networks. When WEP is enabled, it only protects the data packet, but does not protect the physical layer headers so that other stations on the network can view the data needed to manage the network. To control access, a so-called ESSID (or WLAN Service Area ID) is placed in each access point, without knowledge of which the mobile station will not be able to connect to the access point. Additionally, the access point can maintain a list of allowed MAC addresses, called an Access Control List (ACL), allowing access only to those clients whose MAC addresses are on the list.
For data encryption, the standard provides encryption capabilities using the RC4 algorithm with a 40-bit shared key. Once the station connects to the access point, all transmitted data can be encrypted using this key. When encryption is used, the access point will send an encrypted packet to any station trying to connect to it. The client must use its key to encrypt the correct response in order to authenticate itself and gain access to the network. Above the second layer, 802.11b networks support the same standards for access control and encryption (such as IPSec) as other 802 networks.
Health safety
Because mobile stations and access points are microwave devices, many have questions about the safety of using Wave LAN components. It is known that the higher the frequency of radio emission, the more dangerous it is for humans. In particular, it is known that if you look inside a rectangular waveguide transmitting a signal with a frequency of 10 or more GHz, with a power of about 2 W, then damage to the retina will inevitably occur, even if the duration of exposure is less than a second. The antennas of mobile devices and access points are sources of high-frequency radiation, and although the power of the emitted signal is very low, you should not be in close proximity to a working antenna. As a rule, the safe distance is a distance of the order of tens of centimeters from the receiving and transmitting parts. A more precise value can be found in the manual for the specific device.
Further development
Two competing standards for next-generation wireless networks are currently being developed—IEEE 802.11a and European standard HIPERLAN-2. Both standards operate in the second ISM band, which uses a frequency band around 5 GHz. The declared data transfer speed in new generation networks is 54 Mbps.
802.11b Device Manufacturers
Today, the most famous and popular manufacturers in the WaveLAN solutions market are Lucent (ORiNOCO series) and Cisco (Aironet series). In addition to them, there are quite a large number of companies producing 802.11b compatible equipment. These include companies such as 3Com (3Com AirConnect series), Samsung, Compaq, Symbol, Zoom Telephonics, etc. In the next part of the article, we will look at the characteristics of the ORiNOCO series from Lucent and Aironet from Cisco, and then we will test both series.
Links
- — Working Group 802.11
- — WaveLAN in Ukraine
- — Reviews, WaveLAN testing, legal information
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