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Self-help Book for Wlan Installers/providers
Author: Michal Lepiarz  | Posted: 08-05-2008 | Comments: 0 | Views: 10 | Rating: (51)  (?) 
WLAN (Wireless Local Area Network) means technology which allows to build wireless data networks with satisfactory parameters and quite large ranges of operation at a comparatively low cost. Additional advantage of this technology is short time needed for its implementation.
Potentiality of WLANs
* wireless access to local network in home, office, business etc.
* wireless access to the Net in public space, e.g. in airports, stations, cafes etc. (hot-spot)
* wireless point-to-point links (connecting LAN networks, telemetry, remote control, remote monitoring)
* wireless access to the Internet (both in cities and in the country)
* emergency communications link (duplication of wired network)
WLAN standards
I will describe some solutions compliant to the next three standards:
* 802.11a - in 5 GHz band: 5.150 - 5.350 GHz and 5.470 - 5.725 GHz, transfer rate up to 54 Mbps;
* 802.11b - in 2.4 GHz band: 2.4 - 2.483 GHz, transfer rate up to 11 Mbps;
* 802.11g - in 2.4 GHz band: 2.4 - 2.483 GHz, transfer rate up to 54 Mbps;
However, there are also used other standards:
* 802.11f - IAPP - Inter Access Point Protocol - for cooperation between access points;
* 802.11i - standard defining new security methods in wireless networks;
* 802.11n - standard for transmitting multimedia in homes using MIMO technology, up to 300 Mbps;
* 802.11e - standard defining QoS - support for high quality of services;
* 802.16 - WiMax standard for backbone networks of high capacity
Wireless network range
It should be realized that the range of a wireless network depends on many factors; we can have an influence on some of them and the rest is unknown. The range of wireless network depends on:
1. Factors related to used devices:
* output power (it has been decided by the manufacturer),
* cable attenuation (depends on the cable and its length),
* gain of antennas (given by the manufacturer),
* sensitivity of devices (given by the manufacturer).
2. External factors:
* attenuation between antennas (can be estimated basing on FSL model);
* interferences from other devices (can't be predicted - some additional margin of safety needs to be provided to compensate these interferences),
* influence of physical barriers (walls, floors, trees etc.)
So, if we want to know what would be the effective range of our network we have to gather the information mentioned above and carry out simple calculations showed in the further part of this self-help book.
Propagation of radio waves
Fresnel zone
Fresnel zone is one of the most important conceptions connected with propagation of electromagnetic waves, which is indispensable to assess parameters of wireless link. It is the area actively participating in transmission of radio signal energy. Shape of this area is an ellipse in longitudinal section, and circle in cross-section. Radius of this circle is a function of the ratio of distances between antennas - it has the maximum value in the middle of the link. The importance of first Fresnel zone comes from the fact that almost all energy of the signal is conveyed via this space.
The shape of Fresnel zone. R1 is the radius of the I zone.
 [m]; where:
* dkm = d1km+d2km, is the distance between masts in km
* d1km - distance from the first antenna in km
* d2km - distance from the second antenna in km
Wrongly made installation. The installer didn't secure mutual visibility of antennas. The radio link does not work.
Another example of wrongly made installation. Presence of barriers in the first Fresnel zone causes that radio link still doesn't work properly.
... it is only the begining... full version of this text you can find on DIPOL library web site (free of charge, no login needed).
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