In the family of networks 'Wireless', are those which are built according to the 802.11 standards family networks WiFi.
The most common fields of application are :
• in the personal environment, the deployment of a small network, mainly intended to share an internet connection broadband.
• in-house, to allow easy connection of mobile workstations (laptop) to the network or a part of the network of the company.
• in rural areas, to distribute to the public internet access most often obtained by a satellite solution.
• in public places \high-tech\, to provide customers with portable digital access.
Before the multitude of solutions proposed, it is probably necessary to make the point on this
technology has certainly advantages, but is not free of disadvantages.
We will try to make the point on Wi - Fi, without going too much into the details of the Protocol in
levels 1 and 2, (the physical layers and data binding, which are of the order of the manipulation of carrier waves), or on other layers, since from the level 3 is everything
in the same way on a wired network, but rather on the topology and security constraints that must absolutely be account.
A few reminders on electromagnetic waves :
Period, frequency, wavelength.
The \wavelength\ involves a spatial dimension. The (electromagnetic) radio waves propagate in a vacuum
(and in the air, with a negligible error) at the speed of 300 000 Km/s (3 x 10 8 m/s). In the case that interests us, the frequency is of the order of 2.5 Ghz for the standards 802 .11b and 802.11 g, the most used currently, which gives us a period of 4 x 10-10 s.
The wavelength is the distance travelled by the wave during a period, so it is here in the order of 12 cm (3 x 10 8 x 4 x 10-10 = 12 x 10-2).
It will be assumed that an object can be a barrier to the propagation of a wave when this barrier reached a higher dimension or equal to the length of the wave.
When a wave encounters an obstacle, unless this obstacle has very special characteristics,
This wave is partially reflected (returned by the obstacle in another direction),
refracted (a part of the wave crosses the barrier) and absorbed (the barrier absorbs some of the energy of the wave).
Special cases are :
• reflective barrier, which makes almost all of the incident wave is reflected.
• absorbent barrier, which makes almost all of the wave energy is absorbed.
It's pretty easy to observe these phenomena in the acoustic field. The waves are more electromagnetic, but still suffer the effects of refraction, reflection and absorption. We will see later what happens in a closed Chamber.
In free atmosphere (without obstacles), there is generally no problem.
Generally, the Wi - Fi can be used in walls and there are many obstacles.
Imagine a transmitter and a receiver placed in adjoining rooms. The transmitter emits in all directions,
so although there will be a multitude of reflected waves, which some will reach the receiver.
In the example, the wave 1 directly reaches the receiver, through the bulkhead, the wave 2 reached it after consideration, the wave 3 after 3 thoughts.
For a single source of emission, the receiver will receive several times the same information, more or less attenuated and more or less offset in time.
In acoustics, the problem is well known under the name of \Reverb\.
In addition, at any given point, two waves can reach in phase opposition. They will probably not have the same amplitude, but their mathematical sum will tend to give a result, this will lead to a loss of the carrier, on this specific point.
Treatment of the reverberation is a complex thing to study, but empirically, we know well until a certain point,
This is hardly embarrassing to retrieve information, or even, it can be beneficial. On the other hand, if the \Reverb\ becomes too large, the signal becomes unusable (\Cathedral\ effect).
For the electromagnetic waves that we use for Wi - Fi, it also applies. This is to explain a major weakness of the system :
in a building, it is very difficult, if not impossible to predict the optimum position of the issuers based on the desired listening points.
In most cases, be required tests to get the coverage you need.
Each channel corresponds to a well-defined carrier frequency and each channel is far from its neighbors by a constant difference in frequency.
For example, in 802 .11b and 802.11 g standards, there in France 13 possible channels, 2.412 GHz to 2,472 GHz, spaced from each other of 5 MHz.
Each channel uses a certain frequency band (width of the channel, due to the modulation of the carrier).
The width of each channel is 22 MHz, so that the channels overlap.
|CHANNEL 802.11 B OR G||FREQUENCY CENTRAL||±11 MHZ FREQUENCY RANGE|
|1||2,412 GHZ||2.401 - 2.423 GHZ|
|2||2.417 GHZ||2.406 - 2.428 GHZ|
|3||2.422 GHZ||2.411 - 2,433 GHZ|
|4||2427 GHZ||2.416 - 2.438 GHZ|
|5||2,432 GHZ||2.421 - 2.443 GHZ|
|6||2.437 GHZ||2.426 - 2.448 GHZ|
|7||2.442 GHZ||2,431 - 2,453 GHZ|
|8||2.447 GHZ||2.436 - 2.458 GHZ|
|9||2.452 GHZ||2.441 - 2,463 GHZ|
|10||2.457 GHZ||2.446 - 2,468 GHZ|
|11||2.462 GHZ||2.451 - 2.473 GHZ|
|12||2.467 GHZ||2,456 - 2478 GHZ|
|13||2.472 GHZ||2.461 - 2.483 GHZ|
It is of course important. For example, we all know that with two ears, one hears better than with one.
Not only from spotting spatial binaural listening allows, but also because the brain implements techniques of correlations between the signals received by each ear.
who to eliminate, to a certain extent, the disturbances by the reverberation and noise.
Wi - Fi systems can be equipped with similar techniques, which allow more or less effectively to treat a tainted reverb signal.
It is not possible to act on the Protocol, and it is not possible to act at level 1 of an Ethernet network.
What it is important to understand here is that the propagation issues are important and can greatly influence the outcome.
There is not in the point network transceiver with a particular role.
This is typically the mode that you will choose if you just want to communicate with each other two or
three machines with a Wi - Fi interface. It's a rudimentary operating mode, which can quickly become complicated
If the number of machines in a network increases.
Each station can communicate with stations that are within reach. In the example :
• station C can communicate with all other stations.
• stations A, B and C can communicate with each other.
• the D station can communicate with station C.
In any case, station C cannot serve as a relay so that, for example, D can connect with A.
This example clearly shows, this type of network has interest to allow close (and few) machines communicate with each other outside any structure.
In this mode, there is at least one transmitter/receiver WiFi which plays a special role, that of AP (Access Point).
This is typically the mode used when one wishes to extend a cable network like Ethernet, with coverage WiFi for laptops, or machines which ' we do not wish to wire.
Modems WiFiIE the modems ADSL or cable, which offer connectivity WiFi typically operate in this mode. It is this method that we will look at more detail.
Here is a typical representation :
Here, all the functions are distinct, but nothing prohibits only the modem, NAT router functions and access point to be focused in the same case. In such a case, the fixed and mobile stations will be able to communicate with each other because we make sure that they are on the same IP network (with all the risks that security level).
The access point acts as a HUB for mobile stations.
Usually this access point (which we will call later AP, to use common terminology) has itself an IP address, which allows to administer it remotely by various means (telnet, http, dedicated application server mini).
Here, the AP serves as a relay between mobile stations, but also between the mobile and fixed stations. For a mobile station, everything happens as if it was connected to the local network through a wire. If there is a DHCP server on the LAN, mobile stations can even receive their automatic IP configuration.
The mode ad-hoc has no interest that occasionally, outside of any structure.
If you want to get proper coverage on a given site, it will be probably necessary to place multiple access points.