Le Wi-Fi

Le Wi-Fi TRAVAUX PERSONNELS ENCADRES Clément SICARD, Vincent GUILLEE, Harvey BARON 1S6 Wi-fi is a pretty recent wireless radiocommunication technology used in particular in computing activities. It is used in particular for wireless local networks, and nowadays it’s the first way to access the internet. ? What are the advantages and the drawbacks of Wi-Fi technology, and how safe is it? SUMMARY I - General Presentation ................................................................................................................................ 2 a) Why was it invented? ................................................................................................................... 2 b) History of Wi-Fi............................................................................................................................... 3 II – How does it work? ................................................................................................................................... 4 A) Explanations of electro-magnetic waves .............................................................................. 4 B) The different kinds of standards and their specifics ........................................................ 8 C) The structure of the signal....................................................................................................... 10 D) Characterization of the connected devices and different modes within a Wi-Fi network ..................................................................................................................................................... 13 III – Making of the antenna ....................................................................................................................... 15 A) Crafting of the Antenna ............................................................................................................. 15 B) Tests ................................................................................................................................................. 18 IV – Safety ........................................................................................................................................................ 20 A) From a technological side ........................................................................................................ 20 B) From a medical side ................................................................................................................... 21 V – Advantages & drawbacks ................................................................................................................... 25 A) The advantages ............................................................................................................................ 25 B) The drawbacks ............................................................................................................................. 26 Conclusion ....................................................................................................................................................... 27 Glossary ............................................................................................................................................................ 28 Sources ............................................................................................................................................................. 30 ~1~ I - General Presentation A) WHY WAS IT INVENTED? Wireless data transmission by electro-magnetic waves was established in Seattle universities, where the students wanted to find a way to exchange data using this method. They introduced the WLAN notion, which means Wireless Local Area Network and which uses radio link between the devices equipped with this technology. It allows to create local networks, without wires for the links between the computers. This diagram represents the importance of this technology. ~2~ B) HISTORY OF WI-FI The IEEE, the Institute of Electrical and Electronics Engineers established in 1998 a basic standard named 802.11. It will be later called Wi-Fi with the IEEE 802.11b standard, in 1999. This standard was a very important step in wireless networks development, because it made this wireless technology and a real wireless network available for everyone. It led to the implementation of a digital terrestrial network for computing activities. The WECA commission (Wireless Ethernet Compatibility Alliance) was created in order to check the devices compatibility with the first IEEE 802.11 standards and also to allow future compatible evolutions. We will explain those standards more specifically in the second part. Wi-Fi was first used by Apple under the name airport. It started becoming more widely used in June 2000 as a group in Seattle started up the first community of computers connected with Wi-Fi. Now well known, this initiative had a snowball effect in the USA and in the rest of the world. In June 2002 the 2.4GHZ frequency band was accessible in 38 departments in France. In November 2002, 20 other departments will allow Wi-Fi to be used. In March 2003 the first “hotspots” appeared in France. In July 2003 all the French departments could finally use the 2.4 GHZ frequency band. Nowadays, the regions are gradually provided by the Internet providers with the new IEEE 802.11ac standard, born in January 2014, which uses a 5 GHz frequency. A standard named IEEE 802.11ad was studied by an organization called WiGig (Wireless Gigabit Alliance) which promoted a 3 frequency band, and was supposed to deliver up to 7 Gbit/s data transfer rates, which is about ten times more than what the 802.11ac standard is able to deliver. Unfortunately, it wasn’t delivered to the general public and is even very rarely used in the world. The 802.11ad standard will probably be released in 2016, and should be the democratized version of the 802.11ay standard. ~3~ II – How does it work? A) EXPLANATIONS OF ELECTRO-MAGNETIC WAVES First, let’s recap what a wave is. A wave can be described as a disturbance that travels through a medium from one location to another location. This disturbance moves without deformation, and without removing any material. The wave transports the information; our bodies are containing several wave receptors, like the eyes to receive the light, ears to receive the sound and the noises. Representation of a sinusoidal wave, CC An electromagnetic wave is a sinusoidal wave, that can be characterized by its wavelength and its frequency. Both are linked by this equation. λ stands for the wavelength (in m), ν is the frequency (in Hz) c is the celerity (=3.108 m.s-1) and T is the period (in s) λ= c ν with ν= 1 T We know that Wi-Fi networks frequency is about 2,45 GHz for the older standards (till 802.11n), and 5 GHz for the newer ones (802.11ac). So we can make the calculation: ~4~ ? For 2,45 GHz Wi-Fi: Let’s convert this first: 2,45 GHz = 2,45.109 Hz c 3,0. 108 λ= = = 0,1224 m = ??, ?? ?? ν 2,45. 109 2,45 GHz Wi-Fi has then 12,24 cm for wavelength. ? For 5 GHz Wi-Fi: Let’s convert this first: 5 GHz = 5,0.109 Hz So we have then: c 3,0. 108 λ= = = 0.06 m = ? ?? ν 5,0. 109 5 GHz Wi-Fi has then 6 cm for wavelength. Indeed, Wi-Fi waves belong to the electromagnetic waves. Here is the electromagnetic spectrum: The EM spectrum (Wikimedia Commons) ~5~ An electromagnetic wave contains both an electrical field and a magnetic field oscillating at the same frequency. These two fields are perpendicular to each other and are spreading in a medium following an orthogonal (perpendicular to a surface in order to make a right-angled corner) direction, like the following schema shows: Electromagnetic waves (and more globally waves) are submitted to phenomena in their environment. We are going to talk about six of them, probably the most important ones: ? The attenuation is a phenomenon which has to be considered, because a wave won’t travel through a medium to the infinite; the further the source of the signal, the worse the signal. It’s due to spatial dispersion, which is applicable to light waves too. It can be represented this way: ~6~ Because off this phenomenon, the signal declines and causes losses; the received power decreases, as showed by this graphics (with the received power in decibel (dB) and the distance (in feet)): Wi-Fi losses depending to distance (Wikipedia Commons) ? The wave which travels is hitting electrons and will excite them. These electrons are then emitting a radiation, which are going to disturb the signal and by the way attenuate it. This phenomenon is the absorption. The higher the frequency is, the higher this absorption is and then the shorter the range of the signal is. That’s why radio communications have most of the time hundreds of MHz for frequencies. But we have to note that...