Wireless mode 802.11. Wi-Fi standards and their differences from each other. Health safety

Protocol wireless communication Wi-Fi (Wireless Fidelity) was developed back in 1996. It was originally intended to build local networks, but gained the greatest popularity as effective method Internet connections of smartphones and other portable devices.

Over the course of 20 years, the alliance of the same name has developed several generations of the connection, introducing faster and more functional updates every year. They are described by 802.11 standards published by the IEEE (Institute of Electrical and Electronics Engineers). The group includes several versions of the protocol, differing in data transfer speed and support for additional functions.

The very first Wi-Fi standard did not have a letter designation. Devices that support it communicate at a frequency of 2.4 GHz. The information transfer speed was only 1 Mbit/s. There were also devices that supported speeds of up to 2 Mbit/s. It was actively used for only 3 years, after which it was improved. Each subsequent Wi-Fi standard is designated by a letter after the common number (802.11a/b/g/n, etc.).

One of the first updates to the Wi-Fi standard, released in 1999. By doubling the frequency (up to 5 GHz), engineers were able to achieve theoretical speeds of up to 54 Mbit/s. It was not widely used, since it itself is incompatible with other versions. Devices that support it must have a dual transceiver to operate on 2.4 GHz networks. Smartphones with Wi-Fi 802.11a are not widespread.

Wi-Fi standard IEEE 802.11b

The second early interface update, released in parallel with version a. The frequency remained the same (2.4 GHz), but the speed was increased to 5.5 or 11 Mbit/s (depending on the device). Until the end of the first decade of the 2000s, it was the most common standard for wireless networks. Compatible with more old version, as well as a fairly large coverage radius, ensured its popularity. Despite being superseded by new versions, 802.11b is supported by almost all modern smartphones.

Wi-Fi standard IEEE 802.11g

A new generation of Wi-Fi protocol was introduced in 2003. The developers left the data transmission frequencies the same, making the standard fully compatible with the previous one (old devices operated at speeds of up to 11 Mbit/s). The information transfer speed has increased to 54 Mbit/s, which was sufficient until recently. All modern smartphones work with 802.11g.

Wi-Fi standard IEEE 802.11n

In 2009, a large-scale update to the Wi-Fi standard was released. A new version interface received a significant increase in speed (up to 600 Mbit/s), while maintaining compatibility with previous ones. To be able to work with 802.11a equipment, as well as combat congestion in the 2.4 GHz band, support for 5 GHz frequencies has been returned (parallel to 2.4 GHz).

Network configuration options have been expanded and the number of simultaneously supported connections has been increased. It has become possible to communicate in multi-stream MIMO mode (parallel transmission of several data streams on the same frequency) and combine two channels for communication with one device. The first smartphones supporting this protocol were released in 2010.

Wi-Fi standard IEEE 802.11ac

In 2014 it was approved new standard Wi-Fi IEEE 802.11ac. It became a logical continuation of 802.11n, providing a tenfold increase in speed. Thanks to the ability to combine up to 8 channels (20 MHz each) simultaneously, the theoretical ceiling has increased to 6.93 Gbit/s. which is 24 times faster than 802.11n.

It was decided to abandon the 2.4 GHz frequency due to the congestion of the range and the impossibility of combining more than 2 channels. The IEEE 802.11ac Wi-Fi standard operates in the 5 GHz band and is backward compatible with 802.11n (2.4 GHz) devices, but is not guaranteed to work with earlier versions. Today, not all smartphones support it (for example, many budget smartphones on MediaTek do not have support).

Other standards

There are versions of IEEE 802.11 labeled with different letters. But they either make minor amendments and additions to the standards listed above, or add specific functions (such as the ability to interact with other radio networks or security). It is worth highlighting 802.11y, which uses a non-standard frequency of 3.6 GHz, as well as 802.11ad, designed for the 60 GHz range. The first is designed to provide a communication range of up to 5 km, through the use of pure range. The second (also known as WiGig) is designed to provide maximum (up to 7 Gbit/s) communication speed over ultra-short distances (within a room).

Which Wi-Fi standard is better for a smartphone?

All modern smartphones are equipped with a Wi-Fi module designed to work with several versions of 802.11. In general, all mutually compatible standards are supported: b, g and n. However, work with the latter can often be realized only at a frequency of 2.4 GHz. Devices that are capable of operating on 5 GHz 802.11n networks also feature support for 802.11a as backwards compatible.

An increase in frequency helps to increase the speed of data exchange. But at the same time, the wavelength decreases, making it more difficult for it to pass through obstacles. Because of this, the theoretical range of 2.4 GHz will be higher than 5 GHz. However, in practice the situation is a little different.

The 2.4 GHz frequency turned out to be free, so consumer electronics use it. In addition to Wi-Fi, Bluetooth devices and transceivers operate in this range wireless keyboards and mice, it also emits magnetrons from microwave ovens. Therefore, in places where there are several Wi-Fi networks, the amount of interference cancels out the range advantage. The signal will be caught even from a hundred meters away, but the speed will be minimal, and the loss of data packets will be large.

The 5 GHz band is wider (from 5170 to 5905 MHz) and less congested. Therefore, waves are less able to overcome obstacles (walls, furniture, human bodies), but in direct visibility conditions they provide a more stable connection. The inability to effectively overcome walls turns out to be an advantage: you won’t be able to catch your neighbor’s Wi-Fi, but it won’t interfere with your router or smartphone.

However, it should be remembered that to achieve maximum speed, you also need a router that works with the same standard. In other cases, you still won’t be able to get more than 150 Mbit/s.

Much depends on the router and its antenna type. Adaptive antennas are designed in such a way that they detect the location of the smartphone and send it a directional signal that reaches further than other types of antennas.

You will also like:

Possibility of setting up a smartphone through the engineering menu

If you're looking for the fastest WiFi, you need 802.11ac, it's as simple as that. Essentially, 802.11ac is an accelerated version of 802.11n (the current WiFi standard used on your smartphone or laptop), offering link speeds ranging from 433 megabits per second (Mbps), up to several gigabits per second. To achieve speeds that are tens of times higher than 802.11n, 802.11ac operates exclusively in the 5GHz band and uses a huge throughput(80-160MHz), works with 1-8 spatial streams (MIMO), and uses a unique technology called "beamforming". We'll talk more about what 802.11ac is and how it will eventually replace wired Gigabit Ethernet in your home and work networks.

How 802.11ac works.

A few years ago, 802.11n introduced some interesting technology that significantly increased speed compared to 802.11b and g. 802.11ac works almost the same as 802.11n. For example, while the 802.11n standard supported up to 4 spatial streams, and a channel width of up to 40 MHz, 802.11ac can use 8 channels, and a width of up to 80 MHz, and combining them can generally produce 160 MHz. Even if everything else remained the same (and it won't), this means that 802.11ac handles 8x160MHz spatial streams, compared to 4x40MHz. A huge difference that will allow you to squeeze huge amounts of information out of radio waves.

To improve throughput even further, 802.11ac also introduced 256-QAM modulation (compared to 802.11n's 64-QAM), which literally compresses 256 different signals of the same frequency, shifting and interweaving each one into a different phase. Theoretically, this increases the spectral efficiency of 802.11ac by 4 times compared to 802.11n. Spectral efficiency is a measure of how well a wireless protocol or multiplexing technique uses the bandwidth available to it. In the 5GHz band, where the channels are quite wide (20MHz+), spectral efficiency is not so important. In the cellular bands, however, channels are most often 5 MHz wide, making spectral efficiency extremely important.

802.11ac also introduces standardized beamforming (802.11n had it but was not standardized, making interoperability an issue). Beamforming essentially transmits radio signals in such a way that they are aimed at specific device. This can improve overall throughput and make it more consistent, as well as reduce power consumption. Beam shaping can be done by using a smart antenna that physically moves in search of the device, or by modulating the amplitude and phase of the signals so that they destructively interfere with each other, leaving a narrow, non-interfering beam. 802.11n uses the second method, which can be used by both routers and mobile devices. Finally, 802.11ac, like previous versions 802.11 is fully backward compatible with 802.11n and 802.11g, so you can buy an 802.11ac router today and it will work great with your older WiFi devices.

802.11ac range

Theoretically, at a frequency of 5 MHz, and using beamforming, 802.11ac should have the same as 802.11n, or more best range(white radiation). The 5 MHz band, due to its lower penetrating power, does not have the same range as 2.4 GHz (802.11b/g). But that's a trade-off we're forced to make: we simply don't have enough spectral bandwidth in the heavily used 2.4GHz band to allow 802.11ac's peak gigabit-level speeds. As long as your router is in the perfect location, or you have several of them, there is no need to worry. As always, the more important factor is the power transmission of your devices, and the quality of the antenna.

How fast is 802.11ac?

And finally, the question everyone wants to know: how fast is 802.11ac WiFi? As always, there are two answers: the speed theoretically achievable in the lab, and the practical speed limit you'll likely be content with in a real-world home environment surrounded by a bunch of signal-jamming obstacles.

The theoretical maximum speed of 802.11ac is 8 channels of 160MHz 256-QAM, each capable of 866.7Mbps, giving us 6.933Mbps, or a modest 7Gbps. Transfer speed of 900 megabytes per second is faster than transfer to a SATA 3 drive. In the real world, due to channel clogging, you most likely will not get more than 2-3 160 MHz channels, so the maximum speed will stop somewhere at 1.7-2.5 Gbit/s. Compared to theoretical maximum speed 802.11n at 600Mb/s.

Apple Airport Extreme at 802.11ac, disassembled by iFixit's most powerful router today (April 2015), includes D-Link AC3200 Ultra Wi-Fi Router (DIR-890L/R), Linksys Smart WiFi Router AC 1900 (WRT1900AC), and Trendnet AC1750 Dual-Band Wireless Router (TEW-812DRU), as reported by PCMag. With these routers, you can definitely expect impressive speeds from 802.11ac, but don't bite yours just yet. Gigabit Ethernet cable.

In Anandtech's 2013 test, they tested a WD MyNet AC1300 802.11ac router (up to three streams) paired with a number of 802.11ac devices that supported 1-2 streams. Fastest transfer speed has been achieved Intel laptop 7260 s wireless adapter 802.11ac, which used two streams to achieve 364Mbps over a distance of just 1.5m. At 6m and through the wall, the same laptop was the fastest, but the maximum speed was 140Mb/s. The fixed speed limit for the Intel 7260 was 867Mb/s (2 streams of 433Mb/s).

In a situation where you don't need maximum performance and the reliability of wired GigE, 802.11ac is truly compelling. Instead of cluttering your living room with an Ethernet cable running to home theater from a PC under a TV, it makes more sense to use 802.11ac, which has enough bandwidth to wirelessly deliver the highest definition content to your HTPC. For all but the most demanding cases, 802.11ac is a very worthy replacement for Ethernet.

The future of 802.11ac

802.11ac will become even faster. As we mentioned earlier, the theoretical maximum speed of 802.11ac is a modest 7Gbps, and until we hit that in the real world, don't be surprised by the 2Gbps mark in the next few years. At 2Gbps, you get 256Mbps transfer speeds, and suddenly Ethernet will be used less and less until it disappears. To achieve such speeds, chipset and device manufacturers will have to figure out how to implement four or more channels for 802.11ac, given how software, and hardware.

We see Broadcom, Qualcomm, MediaTek, Marvell, and Intel already making strong moves to provide 4-8 channels for 802.11ac to integrate the latest routers, access points, and mobile devices. But until the 802.11ac specification is finalized, a second wave of chipsets and devices is unlikely to appear. Device and chipset manufacturers will have a lot of work to do to ensure that advanced technologies like beamforming are compliant with the standard and are fully compatible with other 802.11ac devices.

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 widespread wireless network 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.

Wi-Fi standard 802.11b

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.

Wi-Fi standard 802.11a

Standard 802.11a was adopted in 1999, however, it found its application only in 2001. This standard 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.

Wi-Fi standard 802.11g

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.

Wi-Fi standard 802.11n

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.

An 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.

Wi-Fi standard 802.11ac

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.

Comparison of 802.11n and 802.11ac

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 2-FM, 4-FM, 16-QAM and 64-QAM	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: tutorial. - St. Petersburg. : SPbSUT, 2017. – 92 p.

The IEEE 802 Standards Committee formed the 802.11 Wireless LAN Standards Working Group in 1990. This group began developing a universal standard for radio equipment and networks operating at 2.4 GHz, with access speeds of 1 and 2 Mbps (Megabits-per-second). Work on creating the standard was completed after 7 years, and the first 802.11 specification was ratified in June 1997. The IEEE 802.11 standard was the first standard for WLAN products from the independent international organization that develops most standards for wired networks. However, by that time, the originally designed data transfer speed in the wireless network no longer satisfied the needs of users. In order to do Wireless technology LAN is popular, cheap, and most importantly, meets today's stringent requirements of business applications, developers were forced to create a new standard.

In September 1999, IEEE ratified an extension of the previous standard. Called IEEE 802.11b (also known as 802.11 High rate), it defines a standard for wireless networking products that operate at speeds of 11 Mbps (similar to Ethernet), allowing these devices to be successfully deployed in large organizations. Product Compatibility various manufacturers is guaranteed by an independent organization called the Wireless Ethernet Compatibility Alliance (WECA). This organization was founded by wireless industry leaders in 1999. Currently, WECA members are more than 80 companies, including such well-known manufacturers as, , etc. Products that meet Wi-Fi requirements (WECA term for IEEE 802.11b) can be found on the website.

Need in wireless access to local networks is growing as the number of mobile devices such as laptops and PDAs increases, as well as with the growing desire of users to be connected to the network without the need to “plug” a network cable into their computer. It is predicted that by 2003 there will be more than a billion mobile devices in the world, and the market value of WLAN products by 2002 is projected to be more than $2 billion.

IEEE 802.11 standard and its extension 802.11b

Like all IEEE 802 standards, 802.11 operates at the bottom two layers of the ISO/OSI model, the physical layer and the data link layer (Figure 1). Any network application, network operating system, or protocol (such as TCP/IP), will work just as well on an 802.11 network as on an Ethernet network.

Rice. 1. ISO/OSI model levels and their compliance with the 802.11 standard.

The basic architecture, features, and services of 802.11b are defined in the original 802.11 standard. The 802.11b specification only addresses the physical layer, adding only higher access speeds.

802.11 operating modes

802.11 defines two types of equipment - a client, which is usually a computer equipped with a wireless Network Interface Card (NIC), and an access point (AP), which acts as a bridge between the wireless and wired networks. An access point usually contains a transceiver, a wired network interface (802.3), and software that processes data. The wireless station can be ISA, PCI or PC Card LAN card in the 802.11 standard, or built-in solutions, for example, an 802.11 telephone headset.

The IEEE 802.11 standard defines two modes of network operation: Ad-hoc mode and client/server mode (or infrastructure mode). In client/server mode (Fig. 2), a wireless network consists of at least one access point connected to a wired network and a certain set of wireless end stations. This configuration is called the Basic Service Set (BSS). Two or more BSSs forming a single subnet form an Extended Service Set (ESS). Since most wireless stations need to access file servers, printers, and the Internet available on a wired LAN, they will operate in client/server mode.

Rice. 2. Client/server network architecture.

Ad-hoc mode (also called point-to-point or independent basic set of services, IBSS) is a simple network in which communication between multiple stations is established directly, without the use of a special access point (Figure 3). This mode is useful if the wireless network infrastructure has not been created (for example, a hotel, exhibition hall, airport), or for some reason cannot be created.

Rice. 3. Ad-hoc network architecture.

802.11 Physical Layer

At the physical layer, two broadband radio frequency transmission methods and one in the infrared range are defined. RF methods operate in the 2.4 GHz ISM band and typically use the 83 MHz band from 2.400 GHz to 2.483 GHz. Broadband signal technologies used in RF methods increase reliability, throughput, and allow many unrelated devices to share the same frequency band with minimal interference to each other.

The 802.11 standard uses Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). These methods are fundamentally different and incompatible with each other.

FHSS uses Frequency Shift Keying (FSK) technology to modulate the signal. When operating at a speed of 1 Mbps, FSK Gaussian modulation of the second level is used, and when operating at a speed of 2 Mbps, the fourth level is used.

The DSSS method uses Phase Shift Keying (PSK) modulation technology. In this case, at a speed of 1 Mbps, differential binary PSK is used, and at a speed of 2 Mbps, differential quadratic PSK modulation is used.

Physical layer headers are always transmitted at 1 Mbps, while data can be transmitted at 1 and 2 Mbps.

Infrared (IR) transmission method

The implementation of this method in the 802.11 standard is based on the emission of a non-directional (diffuse IR) signal by the IR transmitter. Instead of directional transmission, requiring appropriate orientation of the emitter and receiver, the transmitted IR signal is emitted into the ceiling. Then the signal is reflected and received. This method has obvious advantages over the use of directional emitters, but there are also significant disadvantages - a ceiling is required that reflects IR radiation in a given wavelength range (850 - 950 nm); The range of the entire system is limited to 10 meters. In addition, IR rays are sensitive to weather conditions, so the method is recommended for use only indoors.

Two data transfer rates are supported - 1 and 2 Mbps. At a speed of 1 Mbps, the data stream is divided into quartets, each of which is then encoded into one of 16 pulses during modulation. At 2 Mbps, the modulation method is slightly different - the data stream is divided into bit pairs, each of which is modulated into one of four pulses. The peak power of the transmitted signal is 2 W.

FHSS method

Using the frequency hopping method, the 2.4 GHz band is divided into 79 1 MHz channels. The sender and receiver agree on a channel switching scheme (there are 22 such schemes to choose from) and data is sent sequentially over different channels using this scheme. Each data transmission on an 802.11 network follows a different switching pattern, and the patterns themselves are designed to minimize the chances of two senders using the same channel at the same time.

The FHSS method allows for very simple diagram transceiver, however, is limited to a maximum speed of 2 Mbps. This limitation is due to the fact that exactly 1 MHz is allocated for one channel, which forces FHSS systems to use the entire 2.4 GHz band. This means that frequent channel switching must occur (for example, in the US the minimum speed is 2.5 switches per second), which in turn leads to increased overhead.

DSSS method

The DSSS method divides the 2.4 GHz band into 14 partially overlapping channels (only 11 channels are available in the US). In order for multiple channels to be used simultaneously in the same location, they must be spaced 25 MHz apart (not overlap) to avoid mutual interference. Thus, a maximum of 3 channels can be used simultaneously in one place. Data is sent using one of these channels without switching to other channels. To compensate for extraneous noise, an 11-bit Barker sequence is used, where each bit of user data is converted into 11 bits of transmitted data. Such high redundancy for each bit can significantly increase transmission reliability, while significantly reducing the power of the transmitted signal. Even if part of the signal is lost, in most cases it will still be restored. This minimizes the number of repeated data transmissions.

Changes made by 802.11b

The main addition made by 802.11b to the main standard is support for two new data transfer rates - 5.5 and 11 Mbps. The DSSS method was chosen to achieve these speeds because the frequency hopping method cannot support higher speeds due to FCC restrictions. This means that 802.11b systems will be compatible with 802.11 DSSS systems, but will not work with 802.11 FHSS systems.

To support very noisy environments, as well as operation over long distances, 802.11b networks use dynamic rate shifting, which allows the data rate to automatically change depending on the properties of the radio channel. For example, a user can connect at a maximum speed of 11 Mbps, but if the level of interference increases or the user moves away a long distance, the mobile device will begin transmitting at a lower speed - 5.5, 2 or 1 Mbps. In the event that stable operation at a higher speed is possible, the mobile device will automatically begin transmitting at a higher speed. high speed. Rate shifting is a physical layer mechanism and is transparent to higher layers and the user.

Data Link level 802.11

The 802.11 link layer consists of two sublayers: Logical Link Control (LLC) and Media Access Control (MAC). 802.11 uses the same LLC and 48-bit addressing as other 802 networks, allowing wireless and wired networks to be easily combined, but the MAC layer is fundamentally different.

The MAC layer of 802.11 is very similar to that implemented in 802.3, where it supports multiple users on a shared media where the user verifies the media before accessing it. 802.3 Ethernet networks use the Carrier Sence Multiple Access with Collision Detection (CSMA/CD) protocol, which defines how Ethernet stations access the wired line and how they detect and handle collisions that occur when multiple devices attempt to connect simultaneously. network communication. To detect a collision, a station must be able to both receive and transmit simultaneously. The 802.11 standard requires the use of half-duplex transceivers, so in 802.11 wireless networks, a station cannot detect a collision during transmission.

To accommodate this difference, 802.11 uses a modified protocol known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), or Distributed Coordination Function (DCF). CSMA/CA attempts to avoid collisions by using an explicit packet acknowledgment (ACK), which means that the receiving station sends an ACK packet to confirm that the packet was received intact.

CSMA/CA works as follows. A station wanting to transmit tests the channel, and if no activity is detected, the station waits for some random amount of time and then transmits if the data medium is still clear. If the packet arrives intact, the receiving station sends an ACK packet, upon receipt of which the sender completes the transmission process. If the transmitting station did not receive the ACK packet, due to the fact that the data packet was not received, or a corrupted ACK arrived, the assumption is made that a collision has occurred, and the data packet is transmitted again after a random period of time.

To determine whether a channel is clear, a Channel Clearance Algorithm (CCA) is used. Its essence is to measure the signal energy at the antenna and determine the received signal strength (RSSI). If the received signal strength is below a certain threshold, then the channel is declared free and the MAC level receives CTS status. If the power is above the threshold, data transmission is delayed according to protocol rules. The standard provides another channel idle detection capability that can be used either alone or in conjunction with RSSI measurement—the carrier probe method. This method is more selective because it tests for the same carrier type as the 802.11 specification. The best method to use depends on the level of interference in the work area.

Thus, CSMA/CA provides a method for separating access over a radio channel. The explicit acknowledgment mechanism effectively solves interference problems. However, it adds some extra overhead that 802.3 doesn't have, so 802.11 networks will always be slower than their equivalent Ethernet local networks.

Rice. 4. Illustration of the "hidden point" problem.

Another MAC layer-specific problem is the “hidden point” problem, where two stations can both “hear” the access point, but cannot “hear” each other, due to distance or obstructions (Figure 4). To solve this problem, 802.11 added an optional Request to Send/Clear to Send (RTS/CTS) protocol at the MAC layer. When this protocol is used, the sending station transmits an RTS and waits for the access point to respond with a CTS. Since all stations on the network can "hear" the access point, the CTS signal causes them to delay their transmissions, which allows the transmitting station to transmit data and receive the ACK packet without the possibility of collisions. Because RTS/CTS adds additional network overhead by temporarily reserving media, it is typically used only for very large packets for which retransmission would be too costly.

Finally, the 802.11 MAC layer provides the ability to calculate CRC and fragment packets. Each packet has its own CRC checksum, which is calculated and attached to the packet. There is a difference here from Ethernet networks, in which error handling is handled by more advanced protocols. high level(eg TCP). Packet fragmentation allows large packets to be broken into smaller ones when transmitted over the air, which is useful in very crowded environments or where there is significant interference, as smaller packets are less likely to be damaged. This method reduces the need for retransmission in most cases and thus increases the performance of the entire wireless network. The MAC layer is responsible for reassembling the received fragments, making this process transparent to higher-level protocols.

Network connection

The 802.11 MAC layer is responsible for how the client connects to the access point. When an 802.11 client comes within range of one or more access points, it selects one of them based on signal strength and observed error rates and connects to it. Once the client receives confirmation that it has been accepted by the access point, it tunes to the radio channel on which it operates. From time to time it checks all 802.11 channels to see if another access point provides more services High Quality. If such an access point is found, then the station connects to it, retuning to its frequency (Fig. 5).

Rice. 5. Connecting to the network and illustrating the correct channel assignment for access points.

Reconnection usually occurs when the station has been physically moved away from the access point, causing the signal to weaken. In other cases, reconnection occurs due to a change in the building's RF characteristics, or simply due to high network traffic through the original access point. In the latter case, this protocol feature is known as “load balancing”, since its main purpose is to distribute the total load on the wireless network as efficiently as possible across the entire available network infrastructure.

The process of dynamic connection and reconnection allows network administrators to establish wireless networks with very wide coverage, creating partially overlapping "cells". The ideal option is one in which neighboring overlapping access points will use different DSSS channels so as not to interfere with each other (Fig. 5).

Streaming support

Streaming data, such as video or voice, is supported in the 802.11 specification at the MAC layer through the Point Coordination Function (PCF). In contrast to Distributed Coordination Function (DCF), where control is distributed among all stations, in PCF mode only the access point controls access to the channel. If a BSS with PCF enabled is installed, the time is evenly split between PCF mode and CSMA/CA mode. During periods when the system is in PCF mode, the access point polls all stations for data. Each station is allocated a fixed period of time, after which the next station is polled. No station can transmit at this time except the one being polled. Since PCF allows each station to transmit in certain time, then maximum latency is guaranteed. The disadvantage of this design is that the access point must poll all stations, which becomes extremely inefficient in large networks.

Power management

In addition to media access control, the 802.11 MAC layer supports power-saving modes to extend the battery life of mobile devices. The standard supports two energy consumption modes, called "continuous operation mode" and "saving mode". In the first case, the radio is always on, while in the second case, the radio is periodically turned on at certain intervals to receive the "beacon" signals that the access point constantly sends. These signals include information regarding which station should receive the data. Thus, the client can receive the beacon, receive the data, and then go back to sleep mode.

Safety

802.11b provides access control at the MAC layer (the second layer in the ISO/OSI model), and encryption mechanisms known as Wired Equivalent Privacy (WEP), which aim to provide a wireless network with security equivalent to that of a wired network. When WEP is enabled, it only protects the data packet, but does not protect the physical layer headers so that other stations on the network can view the data needed to manage the network. To control access, a so-called ESSID (or WLAN Service Area ID) is placed in each access point, without knowledge of which the mobile station will not be able to connect to the access point. Additionally, the access point can maintain a list of allowed MAC addresses, called an Access Control List (ACL), allowing access only to those clients whose MAC addresses are on the list.

For data encryption, the standard provides encryption capabilities using the RC4 algorithm with a 40-bit shared key. Once the station connects to the access point, all transmitted data can be encrypted using this key. When encryption is used, the access point will send an encrypted packet to any station trying to connect to it. The client must use its key to encrypt the correct response in order to authenticate itself and gain access to the network. Above the second layer, 802.11b networks support the same standards for access control and encryption (such as IPSec) as other 802 networks.

Health safety

Since mobile stations and access points are microwave devices, many people have questions about the safety of using Wave LAN components. It is known that the higher the frequency of radio emission, the more dangerous it is for humans. In particular, it is known that if you look inside a rectangular waveguide transmitting a signal with a frequency of 10 or more GHz, with a power of about 2 W, then damage to the retina will inevitably occur, even if the duration of exposure is less than a second. The antennas of mobile devices and access points are sources of high-frequency radiation, and although the power of the emitted signal is very low, you should not be in close proximity to a working antenna. As a rule, the safe distance is a distance of the order of tens of centimeters from the receiving and transmitting parts. A more precise value can be found in the manual for the specific device.

Further development

Two competing standards for next-generation wireless networks are currently being developed - the IEEE 802.11a standard and the European HIPERLAN-2 standard. Both standards operate in the second ISM band, which uses a frequency band around 5 GHz. The declared data transfer speed in new generation networks is 54 Mbps.

802.11b Device Manufacturers

Today, the most famous and popular manufacturers in the WaveLAN solutions market are Lucent (ORiNOCO series) and Cisco (Aironet series). In addition to them, there are quite a large number of companies producing 802.11b compatible equipment. These include companies such as 3Com (3Com AirConnect series), Samsung, Compaq, Symbol, Zoom Telephonics, etc. In the next part of the article, we will look at the characteristics of the ORiNOCO series from Lucent and Aironet from Cisco, and then we will test both series.

Links

• — Working group 802.11
• — WaveLAN in Ukraine
• — Reviews, WaveLAN testing, legal information

The 802.11ac wireless local network standard was introduced back in the winter of 2011, when specialists from the international non-profit association IEEE approved the first test version new high-speed and upgraded Wi-Fi. To everyone's surprise, already in mid-November the manufacturer Quantenna demonstrated a debut, basic chipset that works well in tandem with routers and other network devices. Soon, laptops, smartphones and other devices compatible with this standard appeared in specialized stores.

It is worth noting one of the important events that accelerated the development of high-speed wireless Wi-Fi. After all, it was at the CES exhibition that new controllers were announced by the American corporation Broadcom, which large IT companies such as Lenovo, ZTE, Huawei wanted to implement in their production...

I propose to consider what advantages does the 802.11ac standard have and how does it differ from its previous brother 802.11n?

1. The most important difference is that the new Wi-Fi has three times the speed, which translates positively to streaming media playback.
   
   Thus, transmission and playback of high definition video (HD, FullHD) wireless Wi-Fi the channel, under certain conditions, will be without interruptions and before downloads, if your device is not limited by hardware (applies to). Mobile games and other applications will all the more “pass” through the network at the proper level.
2. Another useful property of gigabyte Wi-Fi is an extended range and a stabilized signal that covers a wider area, which makes it possible to cover an impressively sized apartment with a wireless signal using one router. This is possible thanks to the developed beamforming technology.
   
   Standard n also supported this technology, but at the option level and, moreover, the signal was generated incorrectly. Beamforming technology determines the location of client devices (laptop, tablet, etc.) and sends the signal directly to them.
   
   This approach helped increase the quality of the Wi-Fi wireless signal.
3. It's no secret that electrical engineering using Wi-Fi standard n operates at a frequency range of 2.4 Gigahertz. Not only tablets and smartphones work at the same frequency, but also microwave ovens and other Appliances. Such an intersection at frequency resulted in a search. The 802.11ac standard introduced by the Institute has no interference problems and can operate at speeds of 1.3 Gbps at an effective frequency of 5 GHz.
4. In addition, when conditions do not allow the use of wide channels, the 802.11ac standard has advantages over its older “brother” 802.11n. What does it consist of? The fact is that the new 256-QAM modulation, for example, at 40 MHz with two streams, will provide 400 Mbps, while the previously developed 802.11n provided only 300 Mbps. In addition, devices based on the 802.11n standard are not able to dynamically change the channel width if certain circumstances require it. But 802.11ac contains such a feature, which has been tested by experts and time.
   
   For example, under favorable conditions, the client and network device can start with an 80 MHz channel, and if conditions change for the worse, switch to 40 or 20 MHz. The transition to narrower channels is also carried out under the condition that the signal level does not allow working on a wide channel. From a technical point of view, the narrower the channel and the smaller the flows in space, the lower the requirements for signal level.

For example, the Wi-Fi 802.11ac specification with a channel width of 80 MHz requires at least 76 dBm, and a channel with a width of 20 MHz already requires 82 dBm. Thus, tablets, computers, Smart TVs and other devices at the edge of the coverage area automatically switch to narrower channels. The international association, together with the Wi-Fi Alliance, has created special specifications, and IT experts claim that more than a billion devices are compatible with the technology.