Indeed, despite the fact that wireless Wi-Fi networks have gained widespread recognition and distribution, they still have three main disadvantages: low (compared to wired Ethernet) real data transfer speed, difficulties with uniform coverage (and the presence of so-called dead zones - dead spots) and problems of data security and unauthorized access. Now let's look at the main advantages of devices created according to the 802.11n specification. This means a noticeably higher data transfer rate, improved security thanks to the introduction of the new WPA2 encryption algorithm, as well as a significant expansion of the coverage area and greater noise immunity. But, of course, we have long been accustomed to the fact that advertising and marketing figures that promise multiple improvements in a variety of indicators, of course, have something in common with real characteristics, but do not always coincide with them even in order of magnitude. And in order to correctly assess new opportunities and their limitations, it always makes sense to imagine how, in fact, these new opportunities are achieved.
A little theory. The theoretical connection speed for 802.11n devices is 300 Mbit/s, and for devices of the previous and most common 802.11g - 54 Mbit/s. Both figures correspond to ideal conditions that do not exist in nature. But still, how can an increase in speed of more than 5 times be achieved? If you ask this question to an inquisitive child, who, fortunately, does not yet have to demonstrate deep knowledge of radio physics, he will definitely speak in the spirit that new devices have more antennas sticking out, which means they work faster. And in general, this is approximately how it is, an increase in speed and stable coverage area is achieved largely thanks to multipath propagation technology (MIMO - Multiple Input Multiple Output), in which data is divided between several transmitters operating at the same frequency.
The developers did not abandon another simple and clear way to increase speed - using two frequency channels instead of one. If 802.11g uses one frequency channel with a width of 20 MHz, then 802.11n uses technology that links two channels located next to each other into one with a width of 40 MHz (information about using two channels instead of one will be very useful to us in practice when setting up devices for maximum performance).
One of the reasons why the actually observed speed in network applications is always less than that declared by the manufacturer is that in addition to the actual transmitted data, devices also exchange service information through the same communication channel. Thus, the network connection speed at the application level is always lower than at the physical level. Well, on the box, for obvious reasons, it is customary to indicate a larger absolute value without any additional clarification. Accordingly, another opportunity to increase the real data transfer speed is to optimize the “overhead”, i.e. the volume of transferred service data, primarily by combining several data frames into one at the physical level.
Of course, these are just some of the major innovations in the 802.11n standard. But, strictly speaking, a complete and final specification for 802.11n devices does not exist until today. And this is another, much less joyful reason for the close attention to the new standard and the large number of conversations about it. Adoption of its final IEEE 802.11n specification has been delayed for several years and is currently scheduled for the second half of 2008, but there is no guarantee that approval of the document will not be delayed again. At the same time, many manufacturers tried to be among the first to introduce devices to the market based on preliminary versions of the standard, which at some point led to the emergence of crude and poorly compatible devices, which, moreover, often lost in speed compared to non-standardized ones. solutions from other manufacturers (see “Draft-N: don’t rush with speed”, “PC World”, ). Since then, a preliminary version of the 802.11n Draft 2.0 standard has been approved, the Wi-Fi Alliance took over the certification without waiting for the official approval of IEEE 802.11n, and the developers have had enough time to eliminate the shortcomings characteristic of the first device models. The list of certified devices is available on the website www.wifialliance.org, and it was this list that we relied on when planning to test the first 802.11n Draft 2.0 devices.
Practice. As usual, of the eight certified devices whose manufacturers are represented in Russia, only three sets of equipment, consisting of an access point and the corresponding adapter, were actually available - DIR-655 and DWA-645 from D-Link, WNR854T and WN511T from Netgear, and also BR-6504n and EW-7718Un from Edimax. By the way, each of the routers under consideration turned out to be equipped with four Gigabit Ethernet ports, and the wired connection, thus, obviously did not limit the connection speed we measured in any way (for details of measurements, see the sidebar “How we tested”). It is hardly worth dwelling in detail on the appearance and configuration of each of the devices (all such information is presented on the corresponding manufacturers’ websites). Of course, appearance is far from the main quality of a router, but it’s not that insignificant either, because for the best signal distribution, it is logical to place this device in a high and visible place. The Netgear model will probably attract the most attention here - it does not have external antennas. From observations while setting up routers, it is perhaps worth mentioning the rather useful function of automatically selecting the most free frequency channel, implemented in the D-Link DIR-655. Note that before installation, it may make sense to download the latest version of drivers from the manufacturer’s website - for example, initially the Netgear adapter fundamentally did not want to establish 802.11n connections with routers from other manufacturers, but updating the drivers completely solved this problem. Let us also mention that these routers can occupy one or two channels. At the same time, the D-Link device is configured by default to work with a 20 MHz channel, while Netgear and Edimax models are configured with a dual channel. To measure maximum performance, we, of course, used the 40 MHz mode, but in this case, the performance of other wireless networks in the immediate vicinity may be impaired. By the way, before discussing performance, let us recall that before the advent of Wi-Fi networks, the 2.4 GHz range belonged to the so-called garbage bands due to the large number of interference of a very different nature, and since then the situation has changed, if at all. not for the better. And to a certain extent, this can explain the significant differences in the speed of data transfer from one measurement to another. Of course, in order to reduce the random error of measurements, we made quite a lot of them and carried out appropriate statistical processing of the results. But in any case, we can confidently say that the arguments that occur from time to time that one device is better than another because its file copying speed was several megabits per second higher are simply meaningless without repeated measurements and the necessary processing of the results .
Average data transfer rates over the TCP/IP protocol are presented in Diagram 1, after studying which we can draw the following conclusion: on average, the connection speed over 802.11n is about 50 Mbit/s, which is approximately 2.5 times higher than the connection speed over 802.11g . In addition, although, as you would expect, using an access point and adapter from the same manufacturer leads to the best speed performance, devices from all three manufacturers demonstrate fairly good compatibility with each other.
In the second series of tests, we measured the speed of a wireless network near a strong source of interference, which was a working microwave oven. The results obtained speak for themselves: if for a standard 802.11g connection the speed drops by an order of magnitude and is about 2 Mbit/s, then devices corresponding to 802.11n demonstrate stable operation with an average speed of more than 10 Mbit/s, i.e. at least 5 times faster.
Accordingly, based on a series of measurements, we come to the conclusion: 802.11n devices provide a real TCP/IP connection speed of about 50 Mbit/s, demonstrate significantly better wireless network performance in the event of severe interference, and in addition, devices from different manufacturers (in in any case, at least three - D-Link, Netgear and Edimax) already interact quite well with each other.
How we tested
A computer based on an Intel Extreme Edition 955 processor with 1 GB of RAM and a WD4000KV hard drive running Windows XP SP2 was connected to the access point under study via wired Ethernet. Using a wireless connection, an Acer TravelMate 3300 laptop running Windows XP SP2, equipped with an Intel Pentium M 1.7 GHz processor, 512 MB of RAM and a Hitachi TravelStar 4K120 hard drive was connected to the access point. Connection speed was measured using the Netperf package (www.netperf.org). To evaluate the performance of the wireless network, the transmission speed of the downlink TCP/IP data stream from a desktop computer to a laptop was measured. The downstream connection speed when connecting computers via a 1 Gbit/s Ethernet network was about 350 Mbit/s. When setting up the access point, the frequency channel was selected that was the most distant from other signal sources and, accordingly, provided maximum throughput. To exclude the possible influence of the location of the access point and other random factors, each measurement was carried out 20 times.
The shelves are full of new devices based on 802.11ac that have already gone on sale, and very soon every user will be faced with the question: is it worth paying extra for a new version of Wi-Fi? I will try to cover the answers to questions regarding the new technology in this article.
802.11ac - background
The last officially approved version of the standard (802.11n) was in development from 2002 to 2009, but its so-called draft version was adopted back in 2007, and as many probably remember, routers supporting 802.11n draft can be was found on sale almost immediately after this event.
The developers of routers and other Wi-Fi devices did exactly the right thing then, without waiting for the approval of the final version of the protocol. This allowed them to release devices providing data transfer rates of up to 300 Mb/s 2 years earlier, and when the standard was finally put on paper and the first 100% standardized routers appeared, the old modules did not lose compatibility by following the draft version of the standard, ensuring compatibility at the hardware level (minor differences could be resolved with a firmware update).
With 802.11ac, almost the same story is now repeating itself as with 802.11n. The timing of the adoption of the new standard is not yet known exactly (presumably no earlier than the end of 2013), but the already adopted draft specification most likely guarantees that all devices currently released in the future will work without problems with certified wireless networks.
Until recently, each new version added a new letter to the end of the 802.11 standard (for example, 802.11g), and they increased in alphabetical order. However, in 2011, this tradition was slightly broken and they jumped from the 802.11n version directly to 802.11ac.
Draft 802.11ac was adopted in October last year, but the first commercial devices based on it appeared literally over the past few months. For example, Cisco released its first 802.11ac router at the end of June 2012.
802.11ac improvements
We can definitely say that even 802.11n has not yet had time to reveal itself in some practical tasks, but this does not mean that progress should stand still. In addition to higher data transfer speeds, which may take a few years to become operational, each Wi-Fi improvement brings other benefits: increased signal stability, increased coverage range, and reduced power consumption. All of the above is also true for 802.11ac, so below we will dwell on each point in more detail.
802.11ac belongs to the fifth generation of wireless networks, and in common parlance it may be called 5G WiFi, although this is officially incorrect. When developing this standard, one of the main goals was to achieve gigabit data transfer speeds. While the use of additional, usually not yet used channels, allows even 802.11n to be overclocked to an impressive 600 Mb/s (for this, 4 channels will be used, each of which operates at a speed of 150 Mb/s), the gigabit bar is not suitable for it and will not be destined to take it, and this role will go to his successor.
It was decided to take the specified speed (one gigabit) not at any cost, but while maintaining compatibility with earlier versions of the standard. This means that in mixed networks, all devices will work regardless of which version of 802.11 they support.
To achieve this goal, 802.11ac will continue to operate at up to 6 GHz. But if in 802.11n two frequencies were used for this (2.4 and 5 GHz), and in earlier revisions only 2.4 GHz, then in AC the low frequency is crossed out and only 5 GHz is left, since it is more efficient for data transmission.
The last remark may seem somewhat contradictory, since at a frequency of 2.4 GHz the signal travels better over long distances, avoiding obstacles more efficiently. However, this range is already occupied by a huge number of “household” waves (from Bluetooth devices to microwave ovens and other home electronics), and in practice its use only worsens the result.
Another reason for abandoning 2.4 GHz was that there was not enough spectrum in this range to accommodate a sufficient number of channels with a width of 80-160 MHz each.
It should be emphasized that, despite the different operating frequencies (2.4 and 5 GHz), IEEE guarantees the compatibility of the AC revision with earlier versions of the standard. How this is achieved is not explained in detail, but most likely the new chips will use 5 GHz as a base frequency, but will be able to switch to lower frequencies when working with older devices that do not support this range.
Speed
A noticeable increase in speed in 802.11ac will be achieved due to several changes at once. First of all, due to doubling the channel width. If in 802.11n it has already been increased from 20 to 40 MHz, then in 802.11ac it will be as much as 80 MHz (by default), and in some cases even 160 MHz.
In early versions of 802.11 (before the N specification), all data was transmitted in only one stream. In N their number can be 4, although until now only 2 channels are most often used. In practice, this means that the total maximum speed is calculated as the product of the maximum speed of each channel times their number. For 802.11n we get 150 x 4 = 600 Mb/s.
We went further with 802.11ac. Now the number of channels has been increased to 8, and the maximum possible transmission speed in each specific case can be found depending on their width. At 160 MHz, the result is 866 Mb/s, and multiplying this figure by 8 gives the maximum theoretical speed that the standard can provide, that is, almost 7 Gb/s, which is 23 times faster than 802.11n.
At first, not all chips will be able to provide gigabit, and even more so 7-gigabit data transfer speeds. The first models of routers and other Wi-Fi devices will operate at more modest speeds.
For example, the already mentioned first 802.11ac Cisco router, although superior to the capabilities of 802.11n, nevertheless also did not get out of the “pre-gigabit” range, demonstrating only 866 Mb/s. In this case, we are talking about the older of the two available models, and the younger one provides only 600 MB/s.
However, speeds will not drop noticeably below these indicators even in the most entry-level devices, since the minimum possible data transfer speed, according to the specifications, is 450 Mb/s for AC.
Economical energy consumption
Economical energy consumption will be one of the greatest strengths of AC. Chips based on this technology are already being predicted for all mobile devices, claiming that this will increase autonomy not only at the same, but also at a higher data transfer rate.
Unfortunately, it is unlikely that more accurate figures will be obtained before the first devices are released, and when the new models are in hand, it will be possible to compare the increased autonomy only approximately, due to the fact that there are unlikely to be two identical smartphones on the market, differing only in the wireless module. It is expected that such devices will begin to appear on sale en masse towards the end of 2012, although the first signs are already visible on the horizon, for example, the Asus G75VW laptop, presented at the beginning of summer.
Broadcom says the new devices are up to 6 times more energy efficient than their 802.11n counterparts. Most likely, the network equipment manufacturer is referring to some exotic testing conditions, and the average savings figure will be much lower than this, but should still be noticeable in the form of additional minutes, and possibly hours, of mobile devices.
Increased autonomy, as often happens, is not a marketing ploy in this case, since it directly follows from the peculiarities of the technology. For example, the fact that data will be transmitted at higher speeds already causes a reduction in energy consumption. Since the same amount of data can be received in less time, the wireless module will be turned off earlier and therefore stop accessing the battery.
Beamforming
This signal conditioning technique could have been used back in 802.11n, but at that time it was not standardized, and when using network equipment from different manufacturers, it usually did not work correctly. In 802.11ac, all aspects of beamforming are unified, so it will be used in practice much more often, although it still remains optional.
This technique solves the problem of a drop in signal power caused by its reflection from various objects and surfaces. Upon reaching the receiver, all these signals arrive with a phase shift, and thus reduce the total amplitude.
Beamforming solves this problem in the following way. The transmitter approximately determines the location of the receiver and, guided by this information, generates a signal in a non-standard way. In normal operation, the signal from the receiver diverges evenly in all directions, but during beamforming it is directed in a strictly defined direction, which is achieved using several antennas.
Beamforming not only improves signal propagation in an open area, but also helps to “break through” walls. If previously the router did not
“reached” into the next room or provided an extremely unstable connection at a low speed, then with AC the quality of reception at the same point will be much better.
802.11ad
802.11ad, like 802.11ac, has a second, easier to remember, but unofficial name - WiGig.
Despite the name, this specification will not follow 802.11ac. Both technologies began to be developed simultaneously, and they have the same main goal (overcoming the gigabit barrier). Only the approaches are different. While AC strives to maintain compatibility with previous designs, AD starts with a blank sheet of paper, which greatly simplifies its implementation.
The main difference between competing technologies will be the operating frequency, from which all other features follow. For AD it is an order of magnitude higher compared to AC and is 60 GHz instead of 5 GHz.
In this regard, the operating range (the area covered by the signal) will also be reduced, but there will be much less interference in it, since 60 GHz is used less frequently compared to the operating frequency of 802.11ac, not to mention 2.4 GHz.
At what exact distances 802.11ad devices will see each other is difficult to say. Without specifying the numbers, official sources talk about “relatively small distances within the same room.” The absence of walls and other serious obstacles in the signal path is also a mandatory and necessary condition for work. Obviously, we are talking about several meters, and it is symbolic that the limit would be the same limitation as for Bluetooth (10 meters).
The small transmission radius will ensure that AC and AD technologies do not conflict with each other. If the first is aimed at wireless networks for homes and offices, then the second will be used for other purposes. Which ones exactly are still an open question, but there are already rumors that AD will finally replace Bluetooth, which cannot cope with its responsibilities due to the extremely low data transfer speed by today's standards.
The standard is also positioned to “replace wired connections” - it is quite possible that in the near future it will become known as “wireless USB” and will be used to connect printers, hard drives, possibly monitors and other peripherals.
The current Draft version of AD is already ahead of its original target (1 Gb/s), and its maximum data transfer rate is 7 Gb/s. At the same time, the technology used allows us to improve these indicators while remaining within the standard.
What 802.11ac means for ordinary users
It is unlikely that by the time the technology standardizes, Internet providers will already begin to offer tariff plans that require the power of 802.11ac to unlock. Consequently, the real use of faster Wi-Fi at first can only be found in home networks: fast file transfer between devices, watching HD movies while simultaneously loading the network with other tasks, backing up data to external hard drives connected directly to the router.
802.11ac solves more than just the speed problem. A large number of devices connected to a router can already create problems, even if the wireless network bandwidth is not used to the maximum. Considering that the number of such devices in each family will only grow, we need to think about the problem now, and AC is its solution, allowing one network to work with a large number of wireless devices.
AC will spread most quickly in the mobile device environment. If the new chip provides at least a 10% increase in autonomy, its use will be fully justified even with a slight increase in the price of the device. The first smartphones and tablets based on AC technology should most likely be expected closer to the end of the year. As already mentioned, a laptop with 802.11ac has already been released, however, as far as we know, this is the only model on the market so far.
As expected, the cost of the first AC routers turned out to be quite high, and a sharp drop in prices in the coming months is unlikely to be expected, especially if you remember how the situation developed with 802.11n. However, at the beginning of next year, routers will cost less than the $150-200 that manufacturers are asking for their first models right now.
According to information leaking out in small doses, Apple will once again be among the first adopters of the new technology. Wi-Fi has always been a key interface for all of the company's devices, for example, 802.11n found its way into Apple technology immediately after the approval of the Draft specification in 2007, so it is not surprising that 802.11ac is also preparing to debut soon as part of many Apple devices: laptops, Apple TV, AirPort, Time Capsule and possibly iPhone/iPad.
In conclusion, it is worth recalling that all speeds mentioned are the maximum theoretically achievable. And just as 802.11n actually runs slower than 300Mbps, actual speed limits for AC will also be lower than what's advertised on the device.
Performance in each case will greatly depend on the equipment used, the presence of other wireless devices, and room configuration, but approximately, a router labeled 1.3 Gb/s will be able to transfer information no faster than 800 Mb/s (which is still noticeably higher than the theoretical maximum of 802.11n) .
One of the most important wireless network settings is “Operation Mode”, “Wireless Network Mode”, “Mode”, etc. The name depends on the router, firmware, or control panel language. This item in the router settings allows you to set a specific Wi-Fi operating mode (802.11). Most often, this is a mixed b/g/n mode. Well, ac if you have a dual-band router.
To determine which mode is best to choose in the router settings, you must first understand what it is and what these settings affect. I think it would be useful to take a screenshot with these settings using the example of a TP-Link router. For the 2.4 and 5 GHz range.
At the moment, there are 4 main modes: b/g/n/ac. The main difference is the maximum connection speed. Please note that the speed that I will write about below is the maximum possible speed (per channel). Which can be obtained in ideal conditions. In real conditions, the connection speed is much lower.
IEEE 802.11 is a set of standards on which all Wi-Fi networks operate. Essentially, this is Wi-Fi.
Let's take a closer look at each standard (essentially these are Wi-Fi versions):
Connection speed
As practice shows, most often the b/g/n/ac settings are changed in order to increase the speed of the Internet connection. Now I will try to explain how it works.
Let's take the most popular standard 802.11n in the 2.4 GHz band, when the maximum speed is 150 Mbit/s. This is the number most often indicated on the box with the router. It may also say 300 Mbit/s, or 450 Mbit/s. This depends on the number of antennas on the router. If there is one antenna, then the router operates in one stream and speeds up to 150 Mbit/s. If there are two antennas, then two streams and the speed is multiplied by two - we get up to 300 Mbit/s, etc.
These are all just numbers. In real conditions, the Wi-Fi speed when connected in 802.11n mode will be 70-80 Mbit/s. The speed depends on a huge number of different factors: interference, signal strength, performance and load on the router, settings, etc.
Since they have many versions of the web interface, let's look at a few of them. If in your case the web interface is light like in the screenshot below, then open the “Wi-Fi” section. There will be a “Wireless Mode” item with four options: 802.11 B/G/N mixed, and separately N/B/G.
Or even like this:
Setting "802.11 Mode".
Open the settings page in your browser at http://netis.cc. Then go to the "Wireless" section.
There will be a menu "Radio frequency range". It allows you to change the Wi-Fi network standard. The default is "802.11 b+g+n".
Nothing complicated. Just don't forget to save the settings.
The settings are located in the "Wireless Mode" - "Basic WIFI Settings" section.
Item "Network mode".
You can install both mixed mode (11b/g/n) and separately. For example, only 11n.
It is simply impossible to provide specific instructions for all devices and software versions. Therefore, if you need to change the wireless network standard, and you did not find your device above in the article, then look at the settings in the section called “Wireless network”, “WiFi”, “Wireless”.
If you don’t find it, write the model of your router in the comments. And it is advisable to attach a screenshot from the control panel. I'll tell you where to look for these settings.
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.
IEEE 802.11- a set of communication standards for communication in the wireless local network zone of the 0.9, 2.4, 3.6 and 5 GHz frequency ranges.
It is better known to users by the name Wi-Fi, which is actually a brand proposed and promoted by the Wi-Fi Alliance. It has become widespread thanks to the development of mobile electronic computing devices: PDAs and laptops.
IEEE 802.11a- Wi-Fi network standard. Uses the 5 GHz U-NII frequency range ( English).
Although this version is not used as often due to the standardization of IEEE 802.11b and the introduction of 802.11g, it has also undergone changes in terms of frequency and modulation. OFDM allows data to be transmitted in parallel on multiple subfrequencies. This improves immunity to interference and, since more than one data stream is sent, high throughput is realized.
IEEE 802.11a can reach speeds of up to 54 Mbps under ideal conditions. In less ideal conditions (or with a clean signal), devices can communicate at speeds of 48 Mbps, 36 Mbps, 24 Mbps, 18 Mbps, 12 Mbps and 6 Mbps.
IEEE 802.11a is not compatible with 802.11b 802.11g.
Contrary to its name, the IEEE 802.11b standard adopted in 1999 is not a continuation of the 802.11a standard, since they use different technologies: DSSS (more precisely, its improved version HR-DSSS) in 802.11b versus OFDM in 802.11a. The standard provides for the use of the unlicensed 2.4 GHz frequency range. Transfer speed - up to 11 Mbit/s.
IEEE 802.11b products from various manufacturers are tested for compatibility and certified by the Wireless Ethernet Compatibility Alliance (WECA), now better known as the Wi-Fi Alliance. Compatible wireless products that have been tested by the Wi-Fi Alliance may be labeled with the Wi-Fi symbol.
For a long time, IEEE 802.11b was a common standard on the basis of which most wireless local area networks were built. Now its place has been taken by the IEEE 802.11g standard, which is gradually being replaced by the high-speed IEEE 802.11n.
The draft IEEE 802.11g standard was approved in October 2002. This standard uses the 2.4 GHz frequency band, providing connection speeds of up to 54 Mbps (gross) and thus surpassing the IEEE 802.11b standard, which provides connection speeds of up to 11 Mbps. In addition, it guarantees backward compatibility with the 802.11b standard. Backward compatibility of the IEEE 802.11g standard can be implemented in DSSS modulation mode, in which the connection speed will be limited to eleven megabits per second, or in OFDM modulation mode, in which the speed can reach 54 Mbit/s. Thus, this standard is the most acceptable when building wireless networks
OFDM(English) Orthogonal frequency-division multiplexing - multiplexing with orthogonal frequency division of channels) is a digital modulation scheme that uses a large number of closely spaced orthogonal subcarriers. Each subcarrier is modulated using a conventional modulation scheme (eg, quadrature amplitude modulation) at a low symbol rate, maintaining the overall data rate of conventional single-carrier modulation schemes in the same bandwidth. In practice, OFDM signals are obtained by using FFT (Fast Fourier Transform).
The main advantage of OFDM over single-carrier design is its ability to withstand challenging channel conditions. For example, combat RF attenuation in long copper conductors, narrowband interference, and frequency-selective attenuation caused by multipath propagation, without the use of complex equalizer filters.
StructureOFDMsignal
In radio access systems, there are types of OFDM signals: COFDM and VOFDM.
SignalsCOFDM use encoding of information on each subcarrier and between subcarriers. Noise-resistant coding allows you to further enhance the useful properties of the OFDM signal.
DesignationVOFDM hides vector modulation, where more than one receiving antenna is used, which can further enhance the effect of combating intersymbol interference.
Physical layer- the first layer of the OSI network model. This is the lowest layer of the OSI model - the physical and electrical medium for data transmission. Typically, the physical layer describes: transmissions using examples of topologies, compares analog and digital encoding, bit synchronization, compares narrowband and wideband transmission, multi-channel communication systems, serial (logical 5-volt) data transmission.
If we look from the point of view that the network includes equipment and programs that control the equipment, then the physical layer will refer specifically to the first part of the definition.
This level, like the channel and network levels, is network dependent.
The unit of measurement used at this layer is Bits, that is, the physical layer transmits a stream of bits over the appropriate physical medium through the appropriate interface.
A set of IEEE 802.3 standards that define the link and physical layer in a wired Ethernet network, as a rule, it is implemented in local area networks (LAN), and in some cases - in wide area networks (WAN).
