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Wi-Fi Technical Information
From Wikipedia, the free encyclopedia
Evolution of Wi-Fi standards
The IEEE standard that governs Wi-Fi technology is IEEE 802.11; that standard has gone through several generations since its inception in 1997.
802.11. The original version of the standard, released in 1997, specifies two raw data rates of 1 and 2 megabits per second (Mbit/s) to be transmitted via infrared (IR) signals or by either frequency hopping or direct-sequence spread spectrum in the Industrial Scientific Medical frequency band at 2.4 GHz. IR remains a part of the standard but has no actual implementations.
802.11a. The 802.11a amendment to the original standard was ratified in 1999. The 802.11a standard uses the same core protocol as the original standard and yields realistic throughput in the mid Mbit/s. Since the 2.4 GHz band is heavily used, using the 5 GHz band gives 802.11a the advantage of less interference and significantly more channels and network capacity. However, this high carrier frequency also brings slight disadvantages. It restricts the use of 802.11a to slightly shorter overall range.
802.11b. The 802.11b amendment to the original standard was ratified in 1999. 802.11b has a maximum raw data rate of 11 Mbit/s and uses the same CSMA/CA media access method defined in the original standard. The dramatic increase in throughput of 802.11b (compared to the original standard) along with substantial price reductions led to the rapid acceptance of 802.11b as the definitive wireless LAN technology. The 802.11b band (2.4 Ghz)suffers interference and other WLAN overlapping congestion problems due to only a few non-overlapping channels, other devices operating in this range include microwave ovens, Bluetooth devices, baby monitors and cordless telephones
802.11g. In June 2003, a third standard was ratified: 802.11g. This works in the 2.4 GHz band (like 802.11b) but operates at a maximum raw data rate of 54 Mbit/s, or about 24.7 Mbit/s net throughputs (like 802.11a). Despite its major acceptance, 802.11g suffers from the same interference as 802.11b in the already crowded 2.4 GHz range. Devices operating in this range include microwave ovens, Bluetooth devices, and cordless telephones.
802.11n. 802.11n builds upon previous standards by adding MIMO (multiple-input multiple-output). MIMO uses multiple transmitter and receiver antennas to allow for increased data throughput through spatial multiplexing and increased range by exploiting the spatial diversity, through coding. On January 19, 2007, the IEEE 802.11 Working Group unanimously approved 802.11n to issue a new Draft 2.0 of the proposed standard.
Wi-Fi: How it Works
Wi-Fi networks use radio technologies called IEEE 802.11 to provide secure, reliable, fast wireless connectivity. A typical Wi-Fi setup contains one or more Access Points (APs) and one or more clients. An AP broadcasts its SSID (Service Set Identifier, "Network name") via packets that are called beacons, which are usually broadcast every 100 ms. The beacons are transmitted at 1 Mbit/s, and are of relatively short duration and therefore do not have a significant effect on performance. Since 1 Mbit/s is the lowest rate of Wi-Fi it assures that the client that receives the beacon can communicate at at least 1 Mbit/s. Based on the settings (e.g. the SSID the client may decide whether to connect to an AP. If two APs of the same SSID are in range of the client, the client firmware might use signal strength to decide with which of the two APs to make a connection.
The Wi-Fi standard leaves connection criteria and roaming totally open to the client. This is a strength of Wi-Fi, but also means that one wireless adapter may perform substantially better than another. Since Wi-Fi transmits in the air, it has the same properties as a non-switched wired Ethernet network, and therefore collisions can occur. Unlike a wired Ethernet, and like most packet radios, Wi-Fi cannot do collision detection, and instead uses an acknowledgment packet for every data packet sent. If no acknowledgement is received within a certain time a retransmission occurs. Also, a medium reservation protocol can be used when excessive collisions are experienced or expected (RequestToSend/ClearToSend used for Collision Avoidance or CA) in an attempt to try to avoid collisions.
A Wi-Fi network can be used to connect computers to each other to the internet and to wired networks (which use IEEE 802.3 or Ethernet). Wi-Fi networks operate in the unlicensed 2.4 (802.11b/g) and 5 GHz (802.11a/h) radio bands, with an 11 Mbit/s (802.11b) or 54 Mbit/s (802.11a or g) data rate or with products that contain both bands (dual band). They can provide real world performance similar to the basic 10BaseT wired Ethernet networks.
Channels
Except for 802.11a/n, which operates at 5 GHz, Wi-Fi devices historically primarily use the spectrum in 2.4 GHz, which is standardized and unlicensed by international agreement, although the exact frequency allocations and maximum permitted power vary slightly in different parts of the world. Channel numbers, however, are standardized by frequency throughout the world, so authorized frequencies can be identified by channel numbers. The 2.4 GHz band is also used by microwave ovens, Bluetooth devices, Amateur radio, legal domestic videosenders, Security systems (including cordless CCTV also (in North America) cordless phones and baby monitors.
The maximum number of available channels for Wi-Fi enabled devices are:
7 for Israel (only Channels 3 through 9 are permitted)
11 for North America. Only channels 1, 6, and 11 are recommended for 802.11b/g to minimize interference from adjacent channels.[1] (Note: In Mexico Channels 1 through 8 are reserved "for indoor use only")
13 for China and most countries in Europe. A typical channel layout for 802.11b would be 1/7/13 (or 1/6/11 for compatibility to devices bought in North America). For traffic that is predominantly 802.11g, 1/5/9/13 provides a fourth frequency enabling a much better frequency plan. (Note: In France only Channels 10 through 13 are available)
14 for Japan.[2]
From Wikipedia, the free encyclopedia
Evolution of Wi-Fi standards
The IEEE standard that governs Wi-Fi technology is IEEE 802.11; that standard has gone through several generations since its inception in 1997.
802.11. The original version of the standard, released in 1997, specifies two raw data rates of 1 and 2 megabits per second (Mbit/s) to be transmitted via infrared (IR) signals or by either frequency hopping or direct-sequence spread spectrum in the Industrial Scientific Medical frequency band at 2.4 GHz. IR remains a part of the standard but has no actual implementations.
802.11a. The 802.11a amendment to the original standard was ratified in 1999. The 802.11a standard uses the same core protocol as the original standard and yields realistic throughput in the mid Mbit/s. Since the 2.4 GHz band is heavily used, using the 5 GHz band gives 802.11a the advantage of less interference and significantly more channels and network capacity. However, this high carrier frequency also brings slight disadvantages. It restricts the use of 802.11a to slightly shorter overall range.
802.11b. The 802.11b amendment to the original standard was ratified in 1999. 802.11b has a maximum raw data rate of 11 Mbit/s and uses the same CSMA/CA media access method defined in the original standard. The dramatic increase in throughput of 802.11b (compared to the original standard) along with substantial price reductions led to the rapid acceptance of 802.11b as the definitive wireless LAN technology. The 802.11b band (2.4 Ghz)suffers interference and other WLAN overlapping congestion problems due to only a few non-overlapping channels, other devices operating in this range include microwave ovens, Bluetooth devices, baby monitors and cordless telephones
802.11g. In June 2003, a third standard was ratified: 802.11g. This works in the 2.4 GHz band (like 802.11b) but operates at a maximum raw data rate of 54 Mbit/s, or about 24.7 Mbit/s net throughputs (like 802.11a). Despite its major acceptance, 802.11g suffers from the same interference as 802.11b in the already crowded 2.4 GHz range. Devices operating in this range include microwave ovens, Bluetooth devices, and cordless telephones.
802.11n. 802.11n builds upon previous standards by adding MIMO (multiple-input multiple-output). MIMO uses multiple transmitter and receiver antennas to allow for increased data throughput through spatial multiplexing and increased range by exploiting the spatial diversity, through coding. On January 19, 2007, the IEEE 802.11 Working Group unanimously approved 802.11n to issue a new Draft 2.0 of the proposed standard.
Wi-Fi: How it Works
Wi-Fi networks use radio technologies called IEEE 802.11 to provide secure, reliable, fast wireless connectivity. A typical Wi-Fi setup contains one or more Access Points (APs) and one or more clients. An AP broadcasts its SSID (Service Set Identifier, "Network name") via packets that are called beacons, which are usually broadcast every 100 ms. The beacons are transmitted at 1 Mbit/s, and are of relatively short duration and therefore do not have a significant effect on performance. Since 1 Mbit/s is the lowest rate of Wi-Fi it assures that the client that receives the beacon can communicate at at least 1 Mbit/s. Based on the settings (e.g. the SSID the client may decide whether to connect to an AP. If two APs of the same SSID are in range of the client, the client firmware might use signal strength to decide with which of the two APs to make a connection.
The Wi-Fi standard leaves connection criteria and roaming totally open to the client. This is a strength of Wi-Fi, but also means that one wireless adapter may perform substantially better than another. Since Wi-Fi transmits in the air, it has the same properties as a non-switched wired Ethernet network, and therefore collisions can occur. Unlike a wired Ethernet, and like most packet radios, Wi-Fi cannot do collision detection, and instead uses an acknowledgment packet for every data packet sent. If no acknowledgement is received within a certain time a retransmission occurs. Also, a medium reservation protocol can be used when excessive collisions are experienced or expected (RequestToSend/ClearToSend used for Collision Avoidance or CA) in an attempt to try to avoid collisions.
A Wi-Fi network can be used to connect computers to each other to the internet and to wired networks (which use IEEE 802.3 or Ethernet). Wi-Fi networks operate in the unlicensed 2.4 (802.11b/g) and 5 GHz (802.11a/h) radio bands, with an 11 Mbit/s (802.11b) or 54 Mbit/s (802.11a or g) data rate or with products that contain both bands (dual band). They can provide real world performance similar to the basic 10BaseT wired Ethernet networks.
Channels
Except for 802.11a/n, which operates at 5 GHz, Wi-Fi devices historically primarily use the spectrum in 2.4 GHz, which is standardized and unlicensed by international agreement, although the exact frequency allocations and maximum permitted power vary slightly in different parts of the world. Channel numbers, however, are standardized by frequency throughout the world, so authorized frequencies can be identified by channel numbers. The 2.4 GHz band is also used by microwave ovens, Bluetooth devices, Amateur radio, legal domestic videosenders, Security systems (including cordless CCTV also (in North America) cordless phones and baby monitors.
The maximum number of available channels for Wi-Fi enabled devices are:
7 for Israel (only Channels 3 through 9 are permitted)
11 for North America. Only channels 1, 6, and 11 are recommended for 802.11b/g to minimize interference from adjacent channels.[1] (Note: In Mexico Channels 1 through 8 are reserved "for indoor use only")
13 for China and most countries in Europe. A typical channel layout for 802.11b would be 1/7/13 (or 1/6/11 for compatibility to devices bought in North America). For traffic that is predominantly 802.11g, 1/5/9/13 provides a fourth frequency enabling a much better frequency plan. (Note: In France only Channels 10 through 13 are available)
14 for Japan.[2]
Спасибо. Как я понял достаточно взять микроволновку, снять с дверцы металлическую сеточку и врубить, тогда в ее конусе вай-фай заглушится?
если так сделать в конусе гарантированно умрёт вся техника 
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skinner2
На каких частотах передаётся, какой мощности сигнал. Есть ли глушилки для него?