The wifi or wi-fi (delivery) is a technology of Data-processing network without wire installation to function in internal network and, since, become a means of access to High banc with Internet. It is based on the standard IEEE 802.11 (ISO/CEI 8802-11).

Presentation

The standard IEEE 802.11 (ISO/CEI 8802-11) is an international standard describing the characteristics of a lan without wire (WLAN). The name wifi corresponds initially to the name given to the certification delivered by WECA ( Wireless Ethernet Compatibility Alliance ), the organism in charge to maintain interworking between the materials meeting standard 802.11. By abuse language (and for reasons of Marketing) the name of the standard merges today with the name of certification (it is at least the case in France, in Spain, in Canada and the United States). Thus a network wifi is actually a network meeting standard 802.11. In other countries (in Germany for example) such networks are correctly named WLAN.

Thanks to the wifi, it is possible to create lans without wire with high banc. In practice, the wifi makes it possible to connect laptops, machines of office, personal assistants (PDA), objects communicating or even of the peripherals to a bond high banc (of 11 Mbit/s in 802.11b to 54 Mbit/s in 802.11a/g and 540 Mbit/s for the future 802.11n) on a ray of several tens of meters in interior (generally between a score and about fifty meters). In an opened environment, the range can even reach several hundred meters under optimal conditions several tens of kilometers (for “alternative” WIMAX or with directional antennas).

Thus, of the suppliers of access Internet start to irrigate zones with strong concentration of users (stations, airports, hotels, trains, etc) with wireless networkings connected to Internet. These zones or access point are called terminals wifi and in English “ hot spots ”.

The IBook S of Apple, Inc. was, in 1999, among the first computers general public to propose equipment wifi integrated (under the name of AirPort), followed soon by the remains range. Starting from 2003, one also sees appearing models of PC portable frames around the technology Intel Centrino, which allow them a similar integration. The other models of PC must be still equipped with an adapted expander card (PCMCIA, USB, Compact Flash, SD, NCV, MiniPCI, etc).

Significance of the term

The term wifi is largely known to be the contraction of Wi reless Fi delity , but it is an erroneous explanation, as Phil Belanger explains it, one of the founders of WECA, silent partner of this denomination.

In fact the group had required at an advertizing agency to propose a name easier to him to use than " IEEE 802.11b Direct Sequence". The agency their proposed ten names; among those, WECA chooses that-of wifi which sounded a little like Hi-fi . Initially, that did not have a precise direction; but so that the general public includes/understands better what it acted, the group presents this name with the slogan: “ Standard For Wireless Fidelity ” (“the standard for fidelity wireless telegraphy”).

The idea that wifi means wireless fidelity is from now on widespread. Thus, even if to associate " fidélité" with the technology of wireless networking does not have technically direction, the goal marketing to diffuse a popular term for the standard " IEEE 802.11b" was reached.

When the wifi term appeared, certain manufacturers and retailers associated it with equipment not supporting technology " IEEE 802.11" but being compatible with a technology wireless telegraphy, for example:

• wireless networkings personnel WPAN Wireless Personal Area Networks: Bluetooth technologies, Infra-reds, ZigBee

• local wireless networkings WLAN - Local Wireless Area Networks: Technologies WiFi, Hyperlan
• metropolitan wireless networkings WMAN - Metropolitan Wireless Area Networks: Technology BLR (Radio operator Loop Local), WiMax
• national wireless networkings WWAN - Wireless Wide Area Networks: Technology GSM, GPRS, UMTS (3G)

Certain laptops were sold with the word wifi in their characteristics whereas for any equipment wireless they had only one simple infra-red port.

The word WiFi , with the W and the F capital means compatibility with technology " IEEE 802.11". (One also finds it written all in capital letter). It is represented by a logo deposited, the " Wi " is written in white on a black bottom, the " Fi " is written in black on a white zone. The whole with small TM (Trademark: registered trademark)

On equipment which one wishes to buy, only white and black the WiFi logo, or the specification " IEEE 802.11" , can guarantee that the material is compatible with technology " IEEE 802.11". The WiFi logo with a black round in background, means that one is in a zone where a network WiFi " IEEE 802.11" is present.

Availability

In practice, for a data-processing use of the network wifi, it is necessary to have two equipment wifi at least, for example computer, and router ADSL " Wifi". The computer must be equipped with a chart wifi, which contain an antenna, and with drivers which makes it possible to make function this chart. The types, numbers, flow and distances between the equipment vary according to technical details, of which some are specified in this article.

A Routeur wifi can also be useful.

Wifi and free software

• the systems BSD (FreeBSD, NetBSD and OpenBSD) had a support for the majority of the adapters since the end 1998. Code for the chips Atheros, PRISM, Harris/Intersil and Aironet (of the salesmen wifi éponymes) is mainly shared by the 3 BSD. Darwin and Mac OS X, in spite of their overlapping with FreeBSD, have their characteristic and single implementation. In OpenBSD 3.7, other drivers for Chipset S wireless telegraphies are available, including RealTek RTL8180L, Ralink RT25x0, Atmel AT76C50x and Intel 2100/2200BG/2225BG/2915ABG. This is due, at least partly, with the effort of OpenBSD to support the drivers open source for the chipsets wireless telegraphy. It is possible that such drivers can be implemented by other BSDs if they already do not exist. The NdisWrapper is also available under FreeBSD.

• Linux : Since version 2.6, certain materials wifi are supported nativement in the kernel. The support for Orinoco, PRISM, Aironet and Atmel is included in the principal branch of the tree structure of the core, whereas ADMtek and Realtek RTL8180L are both supported by drivers of closed code provided by the manufacturers and of the open drivers source written by the community. The radios Intel Calexico are supported by open drivers source available on Sourceforge. Atheros and Ralink RT2x00 are supported through open projects source. Since the core Linux 2.6.17, the chipsets Broadcom, used on charts such as Extreme Apple Airport, are supported thanks to the free driver bcm43xx. In the other cases, the support for other materials wireless telegraphy is available through the use of the driver NdisWrapper open source, which makes it possible Linux to make turn on architectures Intel x86 in wrappant a driver of the salesman, envisaged for Windows. FSF recommended certain charts.

• the Free software also made it possible to lay out routers wifi IPv6 to 80 € including all taxes approximately Paris (60 € (EURO) or USD net of tax).

Structure (layers of the protocol)

Standard 802.11 attempts to define the low layers of OSI model for a connection without wire using of the electromagnetic waves, i.e.:

• the physical Layer (noted sometimes layer PHY), proposing three types of coding of information;

• the layer connection of data, made up of two underlayers:
• the control of the logical connection (Logical Link Control, or LLC);
• access control to the support (Media Access Control, or MAC).

The physical layer defines the modulation of the radio waves and the characteristics of indication for the data transmission, while the data link layer defines the interface between the bus of the machine and the physical layer, in particular an access method close to that used in the standard Ethernet and the rules to communication between the various stations. Standard 802.11 proposes actually three physical layers, defining alternative modes of transmission:

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It is possible to use any protocol of transport on a wireless networking wifi as well as on an Ethernet network.

Modes of setting in network

Infrastructure

The Infrastructure mode is an operating process which makes it possible to connect the computers equipped with a chart network wifi between them via one or more Access point (AP) which acts like concentrators (example: Hub/Switch in telegraphic network). This mode is primarily used in company. The installation of such a network obliges to pose with regular interval terminals (AP) in the zone which must be covered by the network. The terminals, as well as the machines, must be configured with same SSID (name of network) in order to be able to communicate. The advantage of this mode is to guarantee a required passage by a. It is thus possible to check which enters on the network. On the other hand, the network cannot increase, except by posing new terminals.

Ad hoc

The “ad hoc” mode is an operating process which makes it possible to directly connect the computers equipped with a chart network wifi, without using a third material such as an Access point (Access Not in English). This mode is ideal to quickly inter-connect machines between them without additional material (example: file-swapping between portables in a train (TGV…), division of its access to Internet in its house, the street, with the coffee, in friends…). The installation of such a network is restricted to configure the machines in Ad-Hoc mode (instead of the Infrastructure mode), the selection of a channel (frequency) and a SSID (name of network) common to all. The advantage of this mode is to free itself from expensive third materials and is easier to implement. Thanks to the addition of a simple software of dynamic routing (examples: OLSR , AODV …), it is possible to create autonomous mesh networks in which the range is not limited to its neighbors (all the participants play the part of the router).

• Ad hoc Networks

The various standards wifi

The standard IEEE 802.11 is actually the initial standard offering of the flows of 1 or 2 Mbit/s. Revisions were brought to the original standard in order to optimize the flow (it is the case of the standards 802.11a, 802.11b and 802.11g, called standards 802.11 physics) or to specify elements in order to ensure a better safety or a better interworking. Here a table presenting the various revisions of standard 802.11 and their significance:

Linksys, division general public of Cisco Systems, developed technology SRX for " Speed and Arranges Expansion " (Speed and Wide Range). This one superimposes the signal of two signals 802.11g to double the rate of transfer of the data. The highest rate of transfer of the data via a wireless networking SRX400 thus exceeds the capacities of a telegraphic network Ethernet 10/100 which one finds in the majority of the networks.

Safety (confidentiality of the communications - legal risk)

The wireless gateway with the lans makes necessary the development of a Security policy in the companies and at the private individuals. It is in particular possible to choose a method of coding of the communication on the radio interface. Most common is the use of a key known as Wired Equivalent Privacy (WEP), communicated only to the authorized users of the network.

However, it was shown that such a safety was easy to circumvent, with the assistance of programs such as Aircrack.

New solutions from now on are recommended, like the methods Wi-Fi Protected Access (WPA) or more recently WPA2 since the adoption of the standard 802.11i.

This can be combined with a protected access VPN (Virtual Private Network) to the network in a company to limit the risk of intrusion.

There still exists of many nonprotected access points at the private individuals. More than 20 percent of the networks are not protected. It poses the problem of the responsibility for the holder of connection wifi when the intruder carries out illegal actions on Internet, such as for example downloading pirated contents.

Other methods of security exist, with, for example, a waiter Radius charged to manage the accesses by password and/or name of user.

Interrogations (on the medical impact)

Technology wifi appears at the time when the interrogations as for the impact of technologies of communication without wire develop on the health of the man. Scientific debates multiplied around the cellphone and start today to touch the whole of technology wifi.

Such a step proves to be convenient on the probable assumption where this technology quickly becomes omnipresent in the human environment, which it is with work as in the residence.

However, it is to be raised that the power emitted by the equipment wifi (~30  MW) is thirty times less than that emitted by the cellphones (~1  W). Moreover, the telephone is generally held in the vicinity immediate of the brain, which is not the case of the equipment wifi; and with ten centimetres, the power of the signal is already strongly attenuated (inversely proportional to the square of the distance: $P=1/D^2$).

It is however interesting to note that in general, a telephone transmits a constant flow of information only during one call, much less takes care about it, whereas an access point wifi or machines in Ad hoc transmits screens of beacon all the 125  ms, is permanently.

It as should be noted as the Fréquence S used by the equipment wifi (2,4  Ghz), just like the frequencies of mobile telephony (roughly speaking of 900 MHz with 2100 MHz), are frequencies which do all to vibrate the water molecule, which constitutes the essence of the human body. If the frequency of 2450 MHz is used in the furnaces with Micro-onde S servants, it is for a simple reason of standards, because there exist industrial microwawe ovens functioning at frequencies of 430 MHz and 960 MHz. Concerning the domestic furnaces, the legislation (European Standard IN 60335-1) imposes a safety threshold for the escapes of radiation Hyperfréquence (the escapes should not exceed 5  MW with 5  cm of the walls).

In this attention, a Research center and of Information Independent on the Electromagnetic Radiations was founded.

Cohabitation (division of the wavebands)

The wifi uses narrow a waveband known as ISM, 2,4 to 2,4835 GHz, of type divided with other joint tenants leading to problems of cohabitation which result in interferences, jammings caused by the microwawe ovens, the transmitting S servants, the relays, telemetry, the télémédecine, the cameras without wire and the emissions of Télévision amateur (amateur TV or ATV), etc Inversement, certain systems as technology RFID start to amalgamate with the wifi in order to profit from the infrastructure already in place.

In wifi, one prevents using same or too close frequencies that those used by the cameras without wire and transmitters, namely 2412,2432,2452 and 2472 MHz (+ 2 MHz) what corresponds respectively to preselection 1,2,3 and 4 sometimes 2,3,4 and 1 (or has, B, C and D).

Will it be necessary to find other frequencies free for some of these applications, or the wifi must it migrate towards another less encumbered band?

• to also see: List of the channels Wi-Fi

The antennas wifi

Here 5 principal categories of antenna 2,4 GHz of the trade used by the wifists, the radio hams and the various applications in band ISM:

• the Dipôle, which resembles a pen, is the antenna basic stem (1/4 of wave), the most met. It is omnidirectional, 0 dBd of profit, and is dedicated to the service road of proximity. It equips also the Caméra without wire numerical Wi-Fi 2.4 GHz (conforms " CE") allowing a WORSE (Equivalent Radiated Isotropic Power) maximum authorized of 100 MW, 20 dBm. (D standard indicative = 500 m at sight)

• the antenna external stem, (technology Antenna colinéaire) often installed on the roof. It is omnidirectional, its profit, 7 to 15 dBi, is related to its vertical dimension being able to reach 2 Mr. the profit (in emission) can not be compatible more with the authorized WORSE .

These the first 2 descriptions, functioning in polarization V, can be regarded as antennas station of reception or basic since compatible with an environment 360°.

• the Antenna panel known as also punt (technology interns Antenne quad or Antenne patch, network of dipoles. The profit starts towards 8 dBi (8 × 8 cm) to reach 21 dBi (45 × 45 × 4,5 cm). It is the antenna which presents the best profit ratio/obstruction and also the best output, which turns around 85 to 90%. Beyond this maximum profit, it is not manufactured any more, because emerge the problems of coupling (losses) between stages of the dipoles and it would moreover be necessary to consider the doubling of surface. The volume of an antenna panel is minimal.

• the standard antenna Parabola full or openwork (grid). Its interest of employment is in the research of the profit obtained starting from a theoretical diameter of approach according to:

• 18 dBi = 46 cm,

• 19 dBi = 52 cm,
• 20 dBi = 58 cm,
• 21 dBi = 65 cm,
• 22 dBi = 73 cm,
• 23 dBi = 82 cm,
• 24 dBi = 92 cm,
• 25 dBi = 103 cm,
• 26 dBi = 115 cm,
• 27 dBi = 130 cm,
• 28 dBi = 145 cm,
• 29 dBi = 163 cm,
• 30 dBi = 183 cm.

The output of the parabola is average, 45~ 55%. The volume of the antenna, which takes account of the length of the bracon, therefore of the Focale, is significant.

Foot-note: any parabola (example TPS/CS without head 11-12 GHz) is exploitable in wifi, on the condition of envisaging a adaptée  source;: horn, patch or mono or double quad, etc

• the Antenna with slits, sectoral, with profit.

The directional or omnidirectional antennas with profit are intended for the “longer range”, possible, a few kilometers.

The antennas panels and parabolic are only directional, i.e. they support a privileged direction (more or less open) with the detriment of other not wished.

Panel or parabola :

It is retained that the antennas panels are often preferred (even preferable) when the assessment of connection is favorable, but, as soon as the system must be more powerful, parabolas become necessary. The point of balance, with 21 dBi, is made with side a square panel of 45 cm and other a parabola D = 65 cm.

In conclusion, into directional, or point-to-point, it is more interesting to be equipped initially with a panel, then, if the circumstances require it, of a parabola.

The antennas wifi are generally equipped with connectors SMA, RP-SMA or NR according to the manufacturer.

Caution: the antennas with profit (expressed in dBi or dBd) used with the emission (free reception) must respect the WORSE regulation (Equivalent Radiated Isotropic Power).

Foot-note: there exist other antennas, less known, and those conceived by the wifists, like the Antenna horn, the Antenne horn-funnel, the Yagi, the angles, the dihedrons, the discones etc, but only the stems, the panels and the parabolas are significantly used.

As for the antennas easily “bidouillables”, they are especially the quads and horns.

To improve the exchanges, it can be assembled with more close to the antenna a Préamplificateur of antenna (X-ray) with or without amplifier to power (TX) but always to bidirectional type.

The applications (uses)

Such a technology can open the doors with an infinity of practical applications. It perhaps used with IPv4, even of the IPv6, and allows the development of new distributed algorithms.

Or, the has ccès S years F it with I nternet (ASFI in French or WIA in English), which is today the most current use of the wifi.

One of the most succeeded examples of ASFI is " Ozone". Ozone deploys since 2003 a network, in particular in Paris, built containing Wi-Fi technology. " OzoneParis" propose, indeed, with all the Parisian private individuals like companies an access to high banc with the Wireless Internet. The users can connect themselves to the network of OzoneParis, to reach the Internet but also to profit from all the related services to the Internet (fabric, email, telephony, mobile telephony, Téléchargement S etc). This access is usable in a fixed way as in situation of mobility. This network is also called Réseau Pervasif .

In English, “ pervasive ” means “omnipresent”. The Network Pervasif is a network in which we are connected, everywhere, all the time if we want it, via our traditional communicating objects (computers, PDA, telephones) but also, tomorrow, thanks to multiple objects equipped with an intelligence and storage capacity: walkmans, systems of positioning GPS for car, toys, lamps, apparatuses domestic, etc These objects known as “intelligent” are present right now around us and the phenomenon has to develop with the development of the Network Pervasif . To observe what occurs to Japan, in the USA but also to France, the communicating object is a formidable lever of growth for any type of industry. In parallel of the traditional accesses of type hotspot , Wi-Fi can be used for the technology of last kilometer in the rural areas, coupled to technologies of collection of the satellite type, fiberoptic, Wimax or rented connection.

Telephones Wi-Fi (GSM, DECT, PDA) using technology VoIP start to appear.

In Paris, there exists also an important network of more than 200 coffees offering to the consumers a free Wi-Fi connection. Since July 2007 Paris WI-FI proposes free in Paris 400 access points in 260 municipal places.

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