The IEEE (Institute of Electrical and Electronic Engineers) is developing WiFi 802.11 standards.
IEEE 802.11 is the base standard for Wi-Fi networks, which defines a set of protocols for the lowest transfer rates.
IEEE 802.11b- describes b O
higher transmission speeds and introduces more technological restrictions. This standard was widely promoted by WECA ( Wireless Ethernet Compatibility Alliance ) and was originally called WiFi .
Frequency channels in the 2.4GHz spectrum are used ().
Ratified in 1999.
RF technology used: DSSS.
Coding: Barker 11 and CCK.
Modulations: DBPSK and DQPSK,
Maximum data transfer rates (transfer) in the channel: 1, 2, 5.5, 11 Mbps,
IEEE 802.11a- describes significantly higher transfer rates than 802.11b.
Frequency channels in the 5GHz frequency spectrum are used. ProtocolNot compatible with 802.11 b.
Ratified in 1999.
RF technology used: OFDM.
Coding: Conversion Coding.
Modulations: BPSK, QPSK, 16-QAM, 64-QAM.
Maximum data transfer rates in the channel: 6, 9, 12, 18, 24, 36, 48, 54 Mbps.
IEEE 802.11g
- describes data transfer rates equivalent to 802.11a.
Frequency channels in the 2.4GHz spectrum are used. The protocol is compatible with 802.11b.
Ratified in 2003.
RF technologies used: DSSS and OFDM.
Coding: Barker 11 and CCK.
Modulations: DBPSK and DQPSK,
Maximum data transfer rates (transfer) in the channel:
- 1, 2, 5.5, 11 Mbps on DSSS and
- 6, 9, 12, 18, 24, 36, 48, 54 Mbps on OFDM.
IEEE
802.11n- the most advanced commercial WiFi standard, currently officially approved for import and use in the Russian Federation (802.11ac is still being developed by the regulator). 802.11n uses frequency channels in the 2.4GHz and 5GHz WiFi frequency spectrums. Compatible with 11b/11 a/11g . Although it is recommended to build networks targeting only 802.11n, because... requires configuration of special protective modes if backward compatibility with legacy standards is required. This leads to a large increase in signal information anda significant reduction in the available useful performance of the air interface. Actually, even one WiFi 802.11g or 802.11b client will require special configuration of the entire network and its immediate significant degradation in terms of aggregated performance.
The WiFi 802.11n standard itself was released on September 11, 2009.
WiFi frequency channels with a width of 20MHz and 40MHz (2x20MHz) are supported.
RF technology used: OFDM.
OFDM MIMO (Multiple Input Multiple Output) technology is used up to the 4x4 level (4xTransmitter and 4xReceiver). In this case, a minimum of 2xTransmitter per Access Point and 1xTransmitter per user device.
Examples of possible MCS (Modulation & Coding Scheme) for 802.11n, as well as the maximum theoretical transfer rates in the radio channel are presented in the following table:
Here SGI is the guard intervals between frames.
Spatial Streams is the number of spatial streams.
Type is the modulation type.
Data Rate is the maximum theoretical data transfer rate in the radio channel in Mbit/sec.
It is important to emphasize that the indicated speeds correspond to the concept of channel rate and are the maximum value using a given set of technologies within the framework of the described standard (in fact, these values, as you probably noticed, are written by manufacturers on the boxes of home WiFi devices in stores). But in real life, these values are not achievable due to the specifics of the WiFi 802.11 standard technology itself. For example, “political correctness” in terms of ensuring CSMA/CA is strongly influenced here (WiFi devices constantly listen to the air and cannot transmit if the transmission medium is busy), the need to confirm each unicast frame, the half-duplex nature of all WiFi standards and only 802.11ac/Wave-2 will be able to start bypassing this, etc. Therefore, the practical efficiency of legacy 802.11 b/g/a standards never exceeds 50% under ideal conditions (for example, for 802.11g the maximum speed per subscriber is usually no higher than 22Mb/s), and for 802.11n efficiency can be up to 60%. If the network operates in protected mode, which often happens due to the mixed presence of different WiFi chips on different devices on the network, then even the indicated relative efficiency can drop by 2-3 times. This applies, for example, to a mix of Wi-Fi devices with 802.11b, 802.11g chips on a network with WiFi 802.11g access points, or a WiFi 802.11g/802.11b device on a network with WiFi 802.11n access points, etc. Read more about .
In addition to the basic WiFi standards 802.11a, b, g, n, additional standards exist and are used to implement various service functions:
. 802.11d. To adapt various WiFi standard devices to specific country conditions. Within the regulatory framework of each state, ranges often vary and may even differ depending on geographic location. The IEEE 802.11d WiFi standard allows you to adjust frequency bands in devices from different manufacturers using special options introduced into the media access control protocols.
. 802.11e. Describes QoS quality classes for the transmission of various media files and, in general, various media content. Adaptation of the MAC layer for 802.11e determines the quality, for example, of simultaneous transmission of audio and video.
. 802.11f. Aimed at unifying the parameters of Wi-Fi access points from different manufacturers. The standard allows the user to work with different networks when moving between coverage areas of individual networks.
. 802.11h. Used to prevent problems with weather and military radars by dynamically reducing the emitted power of Wi-Fi equipment or dynamically switching to another frequency channel when a trigger signal is detected (in most European countries, ground stations tracking weather and communications satellites, as well as military radars operate in ranges close to 5 MHz). This standard is a necessary ETSI requirement for equipment approved for use in the European Union.
. 802.11i. The first iterations of the 802.11 WiFi standards used the WEP algorithm to secure Wi-Fi networks. It was believed that this method could provide confidentiality and protection of the transmitted data of authorized wireless users from eavesdropping. Now this protection can be hacked in just a few minutes. Therefore, the 802.11i standard developed new methods for protecting Wi-Fi networks, implemented at both the physical and software levels. Currently, to organize a security system in Wi-Fi 802.11 networks, it is recommended to use Wi-Fi Protected Access (WPA) algorithms. They also provide compatibility between wireless devices of different standards and modifications. WPA protocols use an advanced RC4 encryption scheme and a mandatory authentication method using EAP. The stability and security of modern Wi-Fi networks is determined by privacy verification and data encryption protocols (RSNA, TKIP, CCMP, AES). The most recommended approach is to use WPA2 with AES encryption (and don't forget about 802.1x using tunneling mechanisms, such as EAP-TLS, TTLS, etc.). .
. 802.11k. This standard is actually aimed at implementing load balancing in the radio subsystem of a Wi-Fi network. Typically, in a wireless LAN, the subscriber device usually connects to the access point that provides the strongest signal. This often leads to network congestion at one point, when many users connect to one Access Point at once. To control such situations, the 802.11k standard proposes a mechanism that limits the number of subscribers connected to one Access Point and makes it possible to create conditions under which new users will join another AP even despite a weaker signal from it. In this case, the aggregated network throughput increases due to more efficient use of resources.
. 802.11m. Amendments and corrections for the entire group of 802.11 standards are combined and summarized in a separate document under the general name 802.11m. The first release of 802.11m was in 2007, then in 2011, etc.
. 802.11p. Determines the interaction of Wi-Fi equipment moving at speeds of up to 200 km/h past stationary WiFi Access Points located at a distance of up to 1 km. Part of the Wireless Access in Vehicular Environment (WAVE) standard. WAVE standards define an architecture and a complementary set of utility functions and interfaces that provide a secure radio communications mechanism between moving vehicles. These standards are developed for applications such as traffic management, traffic safety monitoring, automated payment collection, vehicle navigation and routing, etc.
. 802.11s. A standard for implementing mesh networks (), where any device can serve as both a router and an access point. If the nearest access point is overloaded, data is redirected to the nearest unloaded node. In this case, a data packet is transferred (packet transfer) from one node to another until it reaches its final destination. This standard introduces new protocols at the MAC and PHY levels that support broadcast and multicast (transfer), as well as unicast delivery over a self-configuring Wi-Fi access point system. For this purpose, the standard introduced a four-address frame format. Examples of implementation of WiFi Mesh networks: , .
. 802.11t. The standard was created to institutionalize the process of testing solutions of the IEEE 802.11 standard. Testing methods, methods of measurement and processing of results (treatment), requirements for testing equipment are described.
. 802.11u. Defines procedures for interaction of Wi-Fi standard networks with external networks. The standard must define access protocols, priority protocols and prohibition protocols for working with external networks. At the moment, a large movement has formed around this standard, both in terms of developing solutions - Hotspot 2.0, and in terms of organizing inter-network roaming - a group of interested operators has been created and is growing, who jointly resolve roaming issues for their Wi-Fi networks in dialogue (WBA Alliance ). Read more about Hotspot 2.0 in our articles: , .
. 802.11v. The standard should include amendments aimed at improving the network management systems of the IEEE 802.11 standard. Modernization at the MAC and PHY levels should allow the configuration of client devices connected to the network to be centralized and streamlined.
. 802.11y. Additional communication standard for the frequency range 3.65-3.70 GHz. Designed for latest generation devices operating with external antennas at speeds up to 54 Mbit/s at a distance of up to 5 km in open space. The standard is not fully completed.
802.11w. Defines methods and procedures for improving the protection and security of the media access control (MAC) layer. The standard protocols structure a system for monitoring data integrity, the authenticity of their source, the prohibition of unauthorized reproduction and copying, data confidentiality and other protection measures. The standard introduces management frame protection (MFP: Management Frame Protection), and additional security measures help neutralize external attacks, such as DoS. A little more on MFP here: . In addition, these measures will ensure security for the most sensitive network information that will be transmitted over networks that support IEEE 802.11r, k, y.
802.11ac.
A new WiFi standard that operates only in the 5GHz frequency band and provides significantly faster O
higher speeds both for an individual WiFi client and for a WiFi Access Point. See our article for more details.
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The new IEEE 802.11n wireless standard 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 finally 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.
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. The entire frequency range, however, is usually divided into 11 overlapping frequency channels of 22 MHz, spaced 5 MHz from each other. 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, considering the physical layer of the 802.11 protocol, we note that at 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.
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.
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 signal states, and taking into account the value of the input bit, we obtain 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.
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 maximum emission power limits. 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, the four upper frequency channels, which provide the highest transmission power, 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 the 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 a standard scrambling operation. 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.
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. In the 802.15.3a group, there was a confrontation between Motorola and all other members of the group, and 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. In each of the groups, for a long time, none of the alternative options could get the 75% of votes necessary for its 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 obtain 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 the use of 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.
There are several types of WLAN networks, which differ in the signal organization scheme, data transmission rates, network coverage radius, as well as the characteristics of radio transmitters and receiving devices. The most widely used wireless networks are IEEE 802.11b, IEEE 802.11g, IEEE 802.11n, IEEE 802.11ac and others.
The 802.11a and 802.11b specifications were the first to be approved in 1999, however, devices made according to the 802.11b standard are the most widespread.
Standard 802.11b based on the Direct Sequence Spread Spectrum (DSSS) modulation method. The entire operating range is divided into 14 channels, spaced by 25 MHz to eliminate mutual interference. Data is transmitted over one of these channels without switching to others. Only 3 channels can be used simultaneously. The data rate may change automatically depending on the level of interference and the distance between the transmitter and receiver.
The IEEE 802.11b standard implements a maximum theoretical transfer rate of 11 Mbps, which is comparable to a 10 BaseT Ethernet cable network. Please note that this speed is possible when transmitting data with one WLAN device. If a larger number of subscriber stations operate simultaneously in an environment, the bandwidth is distributed among all and the data transfer rate per user drops.
Standard 802.11a was adopted in 1999, however, it found its application only in 2001. This standard is used mainly in the USA and Japan. It is not widely used in Russia and Europe.
The 802.11a standard uses a signal modulation scheme - Orthogonal Frequency Division Multiplexing (OFDM). The main data stream is divided into several parallel sub-streams at a relatively low bit rate, and then an appropriate number of carriers are used to modulate them. The standard defines three mandatory data transfer rates (6, 12 and 24 Mbit/s) and five additional ones (9, 18, 24, 48 and 54 Mbit/s). It is also possible to use two channels simultaneously, which increases the data transfer speed by 2 times.
Standard 802.11g was finally approved in June 2003. It is a further improvement of the IEEE 802.11b specification and implements data transmission in the same frequency range. The main advantage of this standard is increased throughput - the data transfer rate in the radio channel reaches 54 Mbit/s compared to 11 Mbit/s for 802.11b. Like IEEE 802.11b, the new specification operates in the 2.4 GHz band, but to increase speed it uses the same signal modulation scheme as 802.11a - orthogonal frequency division multiplexing (OFDM).
The 802.11g standard is compatible with 802.11b. Thus, 802.11b adapters can work on 802.11g networks (but not faster than 11 Mbps), and 802.11g adapters can reduce the data transfer rate to 11 Mbps to work on older 802.11b networks.
Standard 802.11 n was ratified on September 11, 2009. It increases the data transfer rate by almost 4 times compared to standard devices 802.11g (the maximum speed of which is 54 Mbps), subject to use in 802.11n mode with other 802.11n devices. The maximum theoretical data transfer rate is 600 Mbit/s, using data transmission over four antennas at once. One antenna – up to 150 Mbit/s.
802.11n devices operate in the frequency ranges of 2.4 – 2.5 or 5.0 GHz.
The IEEE 802.11n standard is based on OFDM-MIMO technology. Most of the functionality is borrowed from the 802.11a standard, however, the IEEE 802.11n standard has the ability to use both the frequency range adopted for the IEEE 802.11a standard and the frequency range adopted for the IEEE 802.11b/g standards. Thus, devices that support the IEEE 802.11n standard can operate in either the 5 GHz or 2.4 GHz frequency range, with the specific implementation varying by 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 by doubling the channel width from 20 to 40 MHz, as well as due to the implementation of MIMO technology.
The 802.11ac standard is a further development of the technologies introduced in the 802.11n standard. In the specifications, 802.11ac devices are classified as VHT (Very High Throughput) - with veryhigh throughput. 802.11ac networks operate exclusively in the 5 GHz band. The radio channel band can be 20, 40, 80 and 160 MHz. It is also possible to combine two 80 + 80 MHz radio channels.
|
802.11 n |
802.11ac |
|
Bandwidth 20 and 40 MHz |
Added 80 and 160 MHz channel width |
|
2.4 GHz and 5 GHz bands |
5 GHz only |
|
Supports modulation |
256-QAM has been added to 2-PM, 4-PM, 16-QAM and 64-QAM modulations |
|
Single-user MIMO transmission |
Multi-user MIMO transmission |
|
Aggregation of MAC frames: A-MSDU, A-MPDU |
Advanced MAC frame aggregation capabilities |
Sources:
1. A.N. Steputin, A.D. Nikolaev. Mobile communications on the way to 6G . In 2 T. – 2nd ed. - Moscow-Vologda: Infra-Engineering, 2018. – 804 p. : ill.
2. A.E. Ryzhkov, V. A. Lavrukhin Heterogeneous radio access networks: a tutorial. - St. Petersburg. : SPbSUT, 2017. – 92 p.
The ubiquity of wireless networks, the development of hotspot infrastructure, and the emergence of mobile technologies with a built-in wireless solution (Intel Centrino) have led to the fact that end users (not to mention corporate clients) began to pay increasing attention to wireless solutions. Such solutions are considered primarily as a means of deploying mobile and fixed wireless local networks and as a means of rapid access to the Internet.
However, the end user who is not a network administrator is generally not very network savvy, making it difficult to make the right choice when purchasing a wireless solution, especially given the variety of products offered today. The rapid development of wireless communication technology has led to the fact that users, not having time to get used to one standard, are forced to switch to another, with even higher transmission speeds. We are, of course, talking about the family of wireless communications protocols known as IEEE 802.11, which includes the 802.11, 802.11b, 802.11b+, 802.11a, 802.11g, 802.11g+ protocols, with a new standard 802.11n already looming on the horizon. And if you add to this large family such security and QoS protocols as 802.11e, 802.11i, 802.11h, etc., then it becomes clear that understanding this is not at all easy.
To make life easier for those who want to join the world of wireless communications, but do not know where to start, we decided to compile a short guide, after reading which the reader will be able to understand the main differences between the wireless communications protocols of the 802.11 family and understand the basic principles of the functioning of wireless networks.
The main difference between the standards of the 802.11 family lies in the methods of encoding information and the resulting differences in reception/transmission speeds. All wireless protocols are based on spectrum broadening technology (Spread Spectrum, SS), which implies that the initially narrow-band (spectrum width) useful information signal is converted during transmission in such a way that its spectrum is significantly wider than the spectrum of the original signal, that is, the signal spectrum as if spread across the frequency range. Simultaneously with the broadening of the signal spectrum, a redistribution of the spectral energy density of the signal occurs; the signal energy is also “smeared” across the spectrum. As a result, the maximum power of the converted signal is significantly lower than the power of the original signal. In this case, the level of the useful information signal can literally be compared with the level of natural noise, as a result of which the signal becomes, in a sense, “invisible” - it is simply lost at the level of natural noise.
For license-free use in Europe and the USA (it is in this spectral range that the 802.11 family protocols operate), the radio range from 2400 to 2483.4 MHz is allocated, intended for use in industry, science and medicine (Industry, Science and Medicine, ISM) and called ISM- range), as well as from 5725 to 5875 MHz, but the transmitter power is strictly regulated, which is limited to 100 mW in Europe (ETSI restrictions) and 1 W in the USA (FCC restrictions). To organize the sharing of the radio spectrum in such harsh conditions, spectrum broadening technology is used. 802.11b/g protocols use Direct Sequence Spread Spectrum (DSSS) technology.
The very first wireless networking standard, which served as the basis for a whole family of wireless communication protocols, was IEEE 802.11. Today there are no longer solutions based solely on this protocol, but it deserves a separate discussion, if only because it is included as a subset of the 802.11b and 802.11g protocols.
The 802.11 standard provides for the use of a frequency range from 2400 to 24835 MHz and transmission rates of 1 and 2 Mbit/s. The DSSS method with 11-chip Barker codes is used to encode data. With an information speed of 1 Mbit/s, the speed of individual Barker sequence chips is 11×106 chip/s, and the spectrum width of such a signal is 22 MHz.
Differential Binary Phase Shift Key (DBPSK) is used to modulate a sinusoidal carrier signal (a process necessary to inform the carrier signal).
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, DSSS technology with 11-chip Barker codes is used, but differential quadrature phase shift key is used to modulate the carrier wave.
At an information speed of 2 Mbit/s, the speed of individual chips of the Barker sequence remains the same, that is, 11×106 chip/s, and therefore the width of the spectrum of the transmitted signal does not change.
The IEEE 802.11b protocol, adopted in July 1999, 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. To operate at speeds of 5.5 and 11 Mbit/s, instead of noise-like Barker sequences, so-called eight-chip CCK sequences (Complementary Code Keying, CCK) are used to broaden the spectrum.
Using CCK codes allows you to encode 8 bits per character at a speed of 11 Mbit/s and 4 bits per symbol at a speed of 5.5 Mbit/s. In both cases, the symbol transmission rate is 1.385 x 106 symbols per second (11/8 = 5.5/4 = 1.385).
The phase values that determine the elements of the CCK sequence depend on the sequence of input information bits. At a transmission rate of 11 Mbit/s, knowledge of 8 bits (4 dibits) of input data is required to unambiguously determine the CCK sequence. The first dibit of the input data determines the phase shift of the entire symbol relative to the previous one, and the remaining 6 bits are used to specify the CCK sequence itself. Since 6 bits of data can have 64 different combinations, the IEEE 802.11b protocol uses one of 64 possible eight-bit CKK sequences when encoding each character, and this allows 6 bits to be encoded in one transmitted symbol. Since each symbol is additionally shifted in phase relative to the previous symbol depending on the value of the first dibit and the phase shift can take four values, we obtain that 8 information bits are encoded in each symbol.
CCK sequencesCCK sequences are characterized by the fact that the sum of their autocorrelation functions for any cyclic shift other than zero is always zero. The IEEE 802.11b standard deals with complex complementary sequences containing elements with different phases. Each element of such a sequence is a complex number from the set of the following eight values: 1, –1, j, –j, 1+j, 1–j, –1+j, –1–j. A complex representation of a signal is just a convenient mathematical apparatus for representing a phase-modulated signal. Thus, a sequence value equal to 1 corresponds to a signal that is in phase with the generator signal (that is, there is no phase shift), and a sequence value equal to –1 corresponds to a signal that is in phase with the generator signal, etc. The main difference between CCK sequences and the previously discussed Barker codes is that here there is not a strictly defined sequence through which either a logical zero or one could be encoded, but a whole set of sequences. And since each element of the sequence can take one of eight values depending on the value of the phases, it is possible to combine a fairly large number of different CCK sequences. This allows several information bits to be encoded in one transmitted symbol, thereby increasing the information transmission rate. |
||
At a transmission speed of 5.5 Mbit/s, 4 bits are already encoded in one symbol, which determines the information speed two times lower. At this transmission rate, the already discussed CCK sequences are used, formed according to the same rules; the only difference is the number of CCK sequences used and the rule for their selection.
To specify all members of the CCK sequence, 4 input information bits are used, that is, 2 dibits. The first dibit, as before, sets the phase shift value of the entire symbol, and the second dibit is used to select one of four possible CCK sequences. If we take into account that each symbol is also additionally shifted in phase relative to the previous one by one of four possible values, then this allows 4 information bits to be encoded in one symbol.
Considering the possible transmission speeds of 5.5 and 11 Mbit/s in the 802.11b protocol, we have so far left without addressing the question of why a speed of 5.5 Mbit/s is needed if the use of CCK sequences allows data to be transmitted at a speed of 11 Mbit/s . Theoretically, this is true, but only if you do not take into account the interference environment. In real conditions, the noise level of transmission channels and, accordingly, the ratio of noise and signal levels may be such that transmission at a high information speed (that is, when many information bits are encoded in one symbol) becomes impossible due to their erroneous recognition. Without going into mathematical details, we only note that the higher the noise level of communication channels, the lower the information transmission speed. It is important that the receiver and transmitter correctly analyze the interference environment and select an acceptable transmission rate.
In addition to CCC sequences, the 802.11b protocol optionally provides an alternative coding method at transmission rates of 5.5 and 11 Mbit/s - packet convolutional coding PBCC. And it was this encoding mode that formed the basis of the 802.11b+ protocol an extension of the 802.11b protocol. Actually, the 802.11b+ protocol as such does not officially exist, but this extension was once supported by many manufacturers of wireless devices. The 802.11b+ protocol provides another data rate of 22 Mbps using PBCC technology.
Binary Burst Convolutional Coding PBCCThe idea of convolutional coding (Packet Binary Convolutional Coding, PBCC) is as follows. The input sequence of information bits is transformed in the convolutional encoder so that each input bit corresponds to more than one output bit, that is, the convolutional encoder adds some 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 rate of 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. The dibit generated in the convolutional encoder is subsequently used as a transmitted symbol, but this dibit is first subjected to phase modulation, and depending on the transmission speed, binary, quadrature and even eight-position phase modulation is possible. As you can see, PBCC technology is quite simple. Unlike DSSS technologies (Barker codes, SSK sequences), spectrum broadening technology is not used here through the use of noise-like sequences, however, spectrum broadening to standard 22 MHz is also provided in this case. For this purpose, variations of possible QPSK and BPSK signal constellations are used. The PBCC method uses two signal constellations, QPSK and BPSK, to broaden the spectrum of the output signal. |
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At a transmission rate of 5.5 Mbit/s, binary phase modulation BPSK is used to modulate the dibit generated by a convolutional encoder with a convolutional coding rate of 1/2, and at a transmission rate of 11 Mbit/s, quadrature phase modulation QPSK is used. At the same time, for a speed of 11 Mbit/s, one input bit is encoded in each symbol and the bit rate corresponds to the symbol transmission rate, and at a speed of 5.5 Mbit/s, the bit rate is equal to half the symbol rate (since each input bit in this case correspond to two output symbols). Therefore, for both a speed of 5.5 Mbit/s and a speed of 11 Mbit/s, the symbol rate is 11×106 symbols per second.
For a speed of 22 Mbit/s, compared to the PBCC scheme we have already discussed, data transmission has two differences. Firstly, 8-position phase modulation (8-PSK) is used, that is, the signal phase can take on eight different values, which allows you to encode 3 bits in one symbol. Secondly, in addition to the convolutional encoder, a puncture encoder (Puncture) was added to the circuit for the following reason: 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 purpose, you can, of course, develop a corresponding convolutional encoder with a convolutional coding speed of 2/3, but it is better to add a special puncture encoder to the circuit, which will simply destroy extra bits.
Having understood the principle of operation of the puncture encoder, let's return to the consideration of PBCC encoding at a speed of 22 Mbit/s in the 802.11b+ protocol.
The convolutional encoder (r = 1/2) receives data at a speed of 22 Mbit/s. After adding redundancy in the convolutional encoder, the 44 Mbps bits are fed into the puncture encoder, which reduces the redundancy so that for every four input bits there are three output bits. Consequently, after the puncture encoder, the flow rate will already be 33 Mbit/s (not the information speed, but the total speed, taking into account the added redundant bits). The resulting sequence is sent to an 8-PSK phase modulator, where every three bits are packed into one symbol. In this case, the transmission speed will be 11×106 characters per second, and the information speed will be 22 Mbit/s.
The relationship between transmission rates and coding type in the 802.11b/b+ standard is given in table. 1.
*22 Mbps speed applies to 802.11b+ only.
The 802.11g standard 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 transfer speed in the 802.11g standard is 54 Mbit/s.
Two competing technologies were considered during the development of 802.11g: the orthogonal frequency division OFDM method and the PBCC binary packet convolutional coding method, which is optionally implemented in the 802.11b standard. As a result, the 802.11g standard is based on a compromise solution: OFDM and CCK technologies are used as base technologies, and the use of PBCC technology is optionally provided.
In the 802.11g protocol, PBCC coding technology can optionally (but not necessarily) be used at speeds 5.5; eleven; 22 and 33 Mbit/s. In general, the standard itself requires transmission rates of 1; 2; 5.5; 6; eleven; 12 and 24 Mbps, with higher transfer rates of 33, 36, 48 and 54 Mbps optional. In addition, the same transmission rate can be realized with different modulation techniques. For example, a transmission rate of 24 Mbit/s can be achieved with both multi-frequency OFDM coding and the hybrid CCK-OFDM coding technique.
The only thing we haven't mentioned yet is the hybrid coding technique. 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, when using OFDM technology, the frame header and data are transmitted using OFDM coding. With 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, CCK-OFDM technology is not the only hybrid technology: with PBCC packet encoding, the frame header is transmitted using CCK codes and the frame data is encoded using PBCC.
The 802.11b and 802.11g standards discussed above refer to the 2.4 GHz frequency range (from 2.4 to 2.4835 GHz), and the 802.11a standard involves the use of a higher frequency range (from 5.15 to 5.350 GHz and from 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 maximum emission power limits. The low range (5.15 to 5.25 GHz) provides only 50 mW of power, the mid range (5.25 to 5.35 GHz) 250 mW, and the high range (5.725 to 5.825 GHz) up to 1 W. The use of three frequency subbands with a total width of 300 MHz makes the 802.11a standard the most broadband in the 802.11 family of standards and allows the entire frequency range to be divided into 12 20 MHz wide channels, eight of which lie in the 200 MHz range from 5.15 to 5.35 GHz , and the remaining four in the 100-MHz range from 5.725 to 5.825 GHz. At the same time, the four upper frequency channels, which provide the highest transmission power, are used primarily for transmitting signals outdoors.
The 802.11a protocol is based on the Orthogonal Frequency Division Multiplexing (OFDM) technique. To separate channels, an inverse Fourier transform with a window of 64 frequency subchannels is used. Since each of the 12 channels defined in the 802.11a standard is 20 MHz wide, each orthogonal frequency subchannel 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).
Orthogonal Frequency Division Multiplexing (OFDM)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 as a result of interference, some frequencies add up in phase, which leads to an increase in the signal, while others, on the contrary, are out of phase, causing a weakening of the signal at a given frequency. With regard to multipath interference, there are two extreme cases. In the first case, the maximum delay between different signals does not exceed the duration of one symbol and interference occurs within one transmitted symbol. In the second case, the maximum delay between different signals is greater than the duration of one symbol, and as a result of interference, signals representing different symbols are added, and so-called inter-symbol interference (ISI) occurs. In OFDM technology, the data transmission rate in each of the frequency subchannels can be made not too high, which creates the prerequisites for effective suppression of intersymbol interference. With frequency division of channels, it is necessary that the width of an individual channel be, on the one hand, narrow enough to minimize signal distortion within a separate channel, and on the other hand, wide enough to ensure 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 closely as possible, but at the same time avoid interchannel interference in order to ensure complete independence of the channels from each other. Frequency channels that satisfy the listed requirements are called orthogonal. The carrier signals of all frequency subchannels (or rather, the functions that describe these signals) are orthogonal to each other. And although the frequency subchannels themselves may partially overlap each other, the orthogonality of the carrier signals guarantees the frequency independence of the channels from each other, and therefore the absence of interchannel interference. One of the key advantages of the OFDM method is the combination of high transmission speed with effective resistance to multipath propagation. More precisely, OFDM technology as such 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) cyclic repetition of the end of the symbol, attached at the beginning of the symbol. The guard interval is redundant information and in this sense reduces the useful (information) transmission rate, but it is precisely this interval that serves as protection against the occurrence of intersymbol interference. This redundant information is added to the transmitted symbol at the transmitter and discarded when the symbol is received at the receiver. The presence of a guard interval creates time pauses between individual symbols, and if the duration of the guard interval exceeds the maximum signal delay time as a result of multipath propagation, then intersymbol interference does not occur. |
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In terms of modulation technology, the 802.11a protocol is not much different from 802.11g. At low transmission rates, binary and quadrature phase modulation BPSK and QPSK are used, and at high transmission rates, quadrature amplitude modulation 16-QAM and 64-QAM are used. In addition, the 802.11a protocol provides for the use of convolutional coding to improve noise immunity. Since the convolutional coding rate can be different, when using the same type of modulation, the transmission speed is different.
In the OFDM method, the duration of one symbol together with the guard interval is 4 μs, and therefore 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 transmission rate will be 250 kHz 48 channels = 12 MHz. If the speed of the convolutional encoder is 1/2, then the transmission rate of information bits will be equal to 6 Mbit/s. If the convolutional coding rate is 3/4, then the transmission rate of information bits will be 9 Mbit/s. In total, the 802.11a protocol provides for the use of eight different transmission modes, differing from each other in speed, modulation type, and convolutional coding rate used (Table 2). At the same time, we emphasize that in the 802.11a protocol itself, only speeds of 6, 12 and 24 Mbit/s are mandatory, and all others are optional.
So far, when considering various wireless communication protocols of the 802.11 family, we have focused specifically on the physical (PHY) layer, which determines the methods of encoding/decoding and modulation/demodulation of the signal during its transmission and reception. However, issues such as regulation of media sharing are determined at a higher level - at the media access level, which is called the MAC layer (Media Access Control). It is at the MAC level that the rules for sharing the data transmission medium simultaneously by several wireless network nodes are established.
The need for regulatory rules is quite obvious. Imagine what would happen if every wireless network node, without following any rules, began transmitting data over the air. As a result of the interference of several such signals, the nodes to which the sent information was intended would not be able not only to receive it, but also to generally understand that this information was addressed to them. That is why it is necessary to have strict regulatory rules that should determine collective access to the data transmission medium. Such rules of collective access can be figuratively compared with traffic rules that regulate the joint use of roads by all road users.
The 802.11 MAC layer defines two types of media sharing: the Distributed Coordination Function (DCF) and the Point Coordination Function (PCF).
At first glance, organizing shared access to a data transmission medium is not difficult: to do this, you just need to ensure that all nodes transmit data only when the medium is free, that is, when none of the nodes is transmitting data. However, such a mechanism will inevitably lead to collisions, since there is a high probability that two or more nodes, trying to access the data transmission medium, will decide that the medium is free and begin simultaneous transmission. That is why it is necessary to develop an algorithm that can reduce the likelihood of collisions and at the same time guarantee equal access to the data transmission medium to all network nodes.
One of the options for organizing such equal access to the data transmission medium is the distributed coordination function (DCF), based on the carrier sense multiple access/collision avoidance (CSMA/CA) method. With this organization, each node, before starting transmission, listens to the medium, trying to detect a carrier signal, and only if the medium is free can it begin transmitting data.
However, as we have already noted, in this case there is a high probability of collisions, and in order to reduce the likelihood of such situations occurring, a collision avoidance (CA) mechanism is used. The essence of this mechanism is as follows. Each node on the network, making sure that the medium is clear, waits for a certain period of time before starting transmission. This interval is random and consists of two components: the mandatory DIFS interval (DCF Interframe Space) and a randomly selected countdown interval (Backoff Time). As a result, each network node waits for a random amount of time before starting transmission, which naturally significantly reduces the likelihood of collisions, since the likelihood that two network nodes will wait for the same period of time is extremely small.
To guarantee equal access to the data transmission medium for all network nodes, it is necessary to appropriately determine the algorithm for choosing the duration of the countdown period. This interval, although random, is selected from a set of some discrete time intervals, that is, it is equal to an integer number of elementary time intervals called time slots (SlotTime). To select the countdown interval, each network node generates a so-called Contention Window (CW), which is used to determine the number of time slots during which the station waited before transmitting. The minimum window size is determined to be 31 time slots, and the maximum is 1023 time slots.
When a network node attempts to access the data transmission medium, after the mandatory DIFS waiting period, a countdown procedure is started, that is, the time slot counter starts counting down from the selected window value. If the medium remains free during the entire waiting period, then the node begins transmission.
After successful transmission, the window is formed again. If, during the waiting time, another network node has started transmission, then the countdown counter value is stopped and data transmission is postponed. After the environment becomes free, this node begins the countdown procedure again, but with a smaller window size determined by the previous value of the countdown counter, and accordingly with a smaller waiting time. It is obvious that the more times a node postpones transmission because the medium is busy, the higher the probability that the next time it will gain access to the data transmission medium.
The considered algorithm for implementing collective access to the data transmission medium guarantees equal access of all network nodes to the medium. However, with this approach, the possibility of collisions still exists. It is clear that the probability of collisions can be reduced by increasing the maximum size of the generated window, but this will increase transmission delays, thereby reducing network performance. Therefore, the DCF method uses the following algorithm to minimize collisions. After each successful reception of a frame, the receiving side, after a short period of SIFS (Short Interframe Space), confirms the successful reception by sending a response acknowledgment ACK frame (ACKknowledgement). If a collision occurs during data transmission, then the transmitting side does not receive an ACK frame indicating successful reception, and then the window size for the transmitting node almost doubles. So, if for the first transmission the window size is 31 slots, then for the second transmission attempt it is already 63, for the third 127, for the fourth 255, for the fifth 511, and for all subsequent 1023 slots. Consequently, the window size increases dynamically as the number of collisions increases, which allows, on the one hand, to reduce time delays, and on the other hand, to reduce the likelihood of collisions.
The considered mechanism for regulating collective access to the data transmission medium has one bottleneck. This is the so-called hidden node problem. Due to the presence of natural obstacles, it is possible that two network nodes cannot hear each other directly; such nodes are called hidden. To solve the problem of hidden nodes, the DCF function optionally provides the ability to use the RTS/CTS algorithm.
In accordance with the RTS/CTS algorithm, each network node, before sending data, first sends a special short message called RTS (Ready-To-Send) and means that the node is ready to send data. Such an RTS message contains information about the duration of the upcoming transmission and the recipient and is available to all nodes on the network (unless, of course, they are hidden from the sender). This allows other nodes to delay transmission for a time equal to the advertised message duration. The receiving station, having received the RTS signal, responds by sending a CTS (Clear-To-Send) signal, indicating that the station is ready to receive information. After this, the transmitting station sends a data packet, and the receiving station must transmit an ACK frame confirming error-free reception.
Now consider a situation where the network consists of four nodes: A, B, C and D (Fig. 1). Let's assume that node C is within reach of only node A, node A is within reach of nodes C and B, node B is within reach of nodes A and D, and node D is within reach of only node B, that is, in the network There are hidden nodes: node C is hidden from nodes B and D, and node A is hidden from node D.
In such a network, the RTS/CTS algorithm makes it possible to cope with the problem of collisions, which is not solved by the considered basic method of organizing multiple access in DCF. Let node A try to send data to node B; to do this, it sends an RTS signal, which, in addition to node B, is also received by node C, but is not received by node D. Node C, having received this signal, is blocked, that is, it suspends attempts to transmit the signal until the end of the transmission between nodes A and B. Node B, in response to the received RTS signal, sends a CTS frame, which is received by nodes A and D. Node D, having received this signal, is also blocked for the duration of the transmission between nodes A and B.
The RTS/CTS algorithm, however, has its own pitfalls, which in certain situations lead to a decrease in the efficiency of using the data transmission medium. For example, sometimes it is possible for a phenomenon such as the spread of the effect of false blocking of nodes, which can ultimately lead to a stupor in the network.
Consider, for example, the network shown in Fig. 2. Let node B try to transmit data to node A by sending it an RTS frame. Since node C also receives this frame, the latter is blocked during the transmission between nodes A and B. Node D, trying to transmit data to node C, sends an RTS frame, but since node C is blocked, it does not receive a response and begins the countdown procedure with increased window size. At the same time, the RTS frame sent by node D is also received by node E, which, incorrectly assuming that this will be followed by a data transmission session from node D to node C, is blocked. However, this is a false blocking, since in reality there is no transmission between nodes D and C, and this phenomenon of false blocking of nodes can lead to a short-term stupor of the entire network.
The above-described DCF distributed coordination mechanism is basic for the 802.11 protocols and can be used both in wireless networks operating in Ad-Hoc mode and in networks operating in Infrastructure mode, that is, in such networks whose infrastructure includes an access point (Access Point, AP). ).
However, for networks in Infrastructure mode, a slightly different mechanism for regulating multiple access, known as the Point Coordination Function (PCF), is more natural. Note that the PCF mechanism is optional and is only used in networks with an access point. In the case of using the PCF mechanism, the access point is the interaction coordination center (Point Coordinator, PC). The coordination center is entrusted with managing the collective access of all other network nodes to the data transmission medium based on a specific polling algorithm or based on the priorities of network nodes. The coordination center polls all network nodes included in its list, and based on this poll, organizes data transfer between all network nodes. It should be noted that this approach completely eliminates competing access to the medium, as in the case of the DCF mechanism, and makes it impossible for collisions to occur.
The central coordination function does not replace the distributed coordination function, but rather complements it by overlaying it. Over a certain period of time, the PCF mechanism is implemented, then the DCF mechanism, and then everything is repeated again.
To be able to alternate between PCF and DCF modes, it is necessary that the access point that performs the functions of a coordination center and implements the PCF mode has priority access to the data transmission medium. This can be done by using media contention (as in the DCF method), but allowing the coordination center to use a latency period less than DIFS. In this case, if the coordination center tries to access the medium, it waits for the end of the current transmission, and since the minimum standby mode is determined for it after detecting “silence” on the air, it is the first to gain access to the medium.
The IEEE 802.11 standard, which was completed in 1997, is the core standard and defines the protocols needed to organize wireless local area networks (WLANs). The main ones are the MAC (Medium Access Control) protocol and the PHY protocol for transmitting signals in the physical environment. As the latter, the use of radio waves and infrared radiation is allowed. Media Access Protocol (MAC) The 802.11 standard defines a single MAC sublayer that interfaces with three types of physical layer protocols corresponding to different signaling technologies - over radio channels in the 2.4 GHz band with direct sequence spread spectrum (DSSS) modulation and frequency hopping (Frequency Hopping). Spread Spectrum, FHSS), as well as using infrared radiation. Both of these broadband technologies are offered in two frequency bands: one around 915 MHz and the other between 2400 MHz and 2483.5 MHz. But it is the 2.4 GHz range that is most interesting for use in wireless networks, since it is the least “noisy” with extraneous signals and allows you to expand the transmission bandwidth. FHSS mode uses the entire 2.4 GHz band as one wideband (with 79 subchannels). In DSSS mode, the same range is divided into several wide DSSS channels, of which no more than three can be used simultaneously. The FHSS method involves changing the carrier frequency of the signal when transmitting information. When using FHSS, the transceiver design is very simple, but this method is only applicable if the throughput does not exceed 2 Mbit/s. As noted above, this problem became one of the main reasons for the creation of new versions of the standard. The standard specifications provide two data transfer rates - 1 and 2 Mbit/s. Compared to wired Ethernet LANs, the capabilities of the MAC sublayer are expanded to include a number of functions typically performed by higher-level protocols, in particular, packet fragmentation and relay procedures. This is driven by the desire to increase the effective throughput of the system by reducing the overhead of packet retransmission. The 802.11 standard defines the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) mechanism as the main method of access to the medium. Power management To save energy resources on mobile workstations used in wireless LANs, the 802.11 standard provides a mechanism for switching stations to the so-called passive mode with minimal power consumption. Network architecture and components The 802.11 standard is based on cellular architecture, and the network can consist of either one or several cells. Each cell is controlled by a base station called access point(Access Point, AP), which, together with the user workstations located within its range of action, forms basic service area(Basic Service Set, BSS) Access points of a multi-cell network communicate with each other through distribution system(Distribution System, DS), which is the equivalent of the backbone segment of cable LANs. The entire infrastructure, including access points and distribution system, forms expanded service area(Extended Service Set). The standard also provides for a single-cell version of a wireless network, which can be implemented without an access point, while some of its functions are performed directly by workstations. Roaming To ensure the transition of mobile workstations from the coverage area of one access point to another, multicellular systems provide special scanning procedures (active and passive listening to the airwaves) and association (Association), however, the 802.11 standard does not provide strict specifications for the implementation of roaming. Security To protect WLAN, the IEEE 802.11 standard provides a whole range of data transmission security measures under the general name Wired Equivalent Privacy (WEP). It includes means of preventing unauthorized access to the network (authentication mechanisms and procedures), as well as preventing the interception of information (encryption). IEEE 802.11a
It is the most “broadband” of the 802.11 family of standards, providing data transfer rates of up to 54 Mbit/s (the edition of the standard, approved in 1999, defined three mandatory speeds - 6, 12 and 24 Mbit/s and five optional - 9, 18, 36, 48 and 54 Mbit/s).
Unlike the base standard, which is focused on the 2.4 GHz frequency range, the 802.11a specifications provide for operation in the 5 GHz range. Orthogonal frequency division multiplexing (OFDM) was chosen as the signal modulation method. The most significant difference between this method and DSSS and FHSS radio technologies is that OFDM involves parallel transmission of the desired signal over several frequencies simultaneously, while spread spectrum technologies transmit signals sequentially. As a result, channel capacity and signal quality increase.
The disadvantages of 802.11a include higher power consumption of radio transmitters for 5 GHz frequencies, as well as a shorter range (equipment for 2.4 GHz can operate at a distance of up to 300 m, and for 5 GHz - about 100 m).
To summarize, we note that this version is like a “side branch” of the main 802.11 standard. To increase channel capacity, the 5.5 GHz transmission frequency range is used here. For transmission in 802.11a, the multi-carrier method is used, when the frequency range is divided into subchannels with different carrier frequencies (Orthogonal Frequency Division Multiplexing), through which the stream is transmitted in parallel, divided into parts. Using the quadrature phase modulation method allows you to achieve a channel capacity of 54 Mbit/s.
IEEE 802.11b Thanks to the high data transfer rate (up to 11 Mbit/s), almost equivalent throughput of conventional wired Ethernet LANs, as well as its focus on the “mastered” 2.4 GHz range, this standard has gained the greatest popularity among manufacturers of equipment for wireless networks. The final version of the 802.11b standard, also known as Wi-Fi (wireless fidelity), was adopted in 1999. It uses DSSS with 8-bit Walsh sequences as its underlying radio technology. Since equipment operating at a maximum speed of 11 Mbps has a shorter range than at lower speeds, the 802.11b standard provides for an automatic reduction in speed when signal quality deteriorates. As with the base 802.11 standard, clear roaming mechanisms are not defined by the 802.11b specifications.This standard is the most popular today and, in fact, it bears the Wi-Fi trademark. Like the original IEEE 802.11 standard, this version uses the 2.4 GHz band for transmission. It does not affect the data link layer and makes changes to IEEE 802.11 only at the physical layer. For signal transmission, the direct sequence method (Direct Sequence Spread Spectrum) is used, in which the entire range is divided into 5 overlapping subranges, for each of which information is transmitted. The values of each bit are encoded by a sequence of complementary codes (Complementary Code Keying). The channel capacity is 11 Mbit/sec.
IEEE 802.11dThe creation of this standard is associated with the use of multimedia. It defines a mechanism for assigning priorities to different types of traffic, such as audio and video applications.
IEEE 802.11fThe 802.11f specifications describe the protocol for exchanging service information between access points (Inter-Access Point Protocol, IAPP), which is necessary for building distributed wireless data networks. The date for approval of these specifications as a standard has not yet been determined.
This authentication standard defines the mechanism for interaction between communication points when a client moves between network segments. Another name for the standard is Inter Access Point Protocol.
IEEE 802.11gIt is expected that the solution to these problems will be based on the use of the Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) protocols proposed by the European Telecommunications Standards Institute (ETSI). These protocols provide for wireless clients to dynamically respond to radio signal interference by switching to another channel, reducing power, or both.
The development of this standard is associated with problems with the use of 802.11a in Europe, where some satellite communication systems operate in the 5 GHz range. To prevent mutual interference, the 802.11h standard has a mechanism for “quasi-intelligent” control of emitted power and selection of the transmission carrier frequency.
IEEE 802.11i Until May 2001, the standardization of information security tools for 802.11 wireless networks was the responsibility of the IEEE 802.11e working group, but then this issue was separated into an independent division. The developing 802.1X standard is designed to expand the capabilities of the 802.11 MAC protocol by providing means for encrypting transmitted data, as well as centralized authentication of users and workstations. As a result, the scale of wireless local networks can be increased to hundreds and thousands of workstations. 802.1X is based on the Extensible Authentication Protocol (EAP), which is based on PPP. The authentication procedure itself involves the participation of three parties - the caller (client), the callee (access point) and the authentication server (usually a RADIUS server). At the same time, the new standard, apparently, will leave the implementation of key management algorithms to the discretion of manufacturers. The data protection tools being developed should find application not only in wireless, but also in other local networks - Ethernet and Token Ring. That's why the future standard is numbered IEEE 802.1X, and the 802.11i group is developing it jointly with the IEEE 802.1 committee.The purpose of this specification is to improve the security level of wireless networks. It implements a set of security functions when exchanging information via wireless networks - in particular, AES (Advanced Encryption Standard) technology - an encryption algorithm that supports keys with lengths of 128, 192 and 256 bits. Compatibility of all currently used devices - in particular, Intel Centrino - with 802.11i networks is provided.
IEEE 802.11j
