Materials:![]() Objectives: The student should become familiar with: ![]() ![]() ![]() ![]() Competency: The student will become familiar with the general concepts of Wireless LAN technologies including the engineering problems concerning WLANs and how they were solved. The student will become familiar with the IEEE specifications as well as the various technologies that have emerged supporting them and how they work. |
In order to set up a wireless network the engineers had to deal with a variety of different issues that are all unique to the problems involved with connecting PC's using radio transceivers. These problems include (but are certainly not limited to):
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What frequencies can be used? The FCC controls all radio frequency transmissions to prevent unauthorized transmissions from blocking authorized transmissions. |
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Once a frequency band is found, how can the network remain reliable? How can the network transceivers deal with interference from other radio sources? |
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How can a roaming station stay connected to the network even though it passes out of range of one "base station" and then into range of another "base station." |
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How can the network remain secure? Since the network's transmissions are traveling through the air they are easily intercepted by any radio receiver in the vicinity. |
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How can the network deal with hidden nodes? This term is used to refer to nodes that are within range of some of the nodes but beyond the range of some of the others. In this case a node in the middle would be caught in the cross fire of two nodes that can't hear each other transmitting simultaneously and interfering with each other. |
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How can the network deal with phase shifting reflections? This term is used to refer to a common radio communication problem in which a transmission heading out in all directions from the transmitter intended for a specific target is inadvertantly reflected off of a third object and to the intended target. The two radio waves arrive out of phase and if shifted 180° will actually cancel out and completely kill the original signal. |
Clearly then there were plenty of problems for the engineers to solve that are all related directly to the physical layer medium (radio waves) before they even began to actually design the transceivers and the Data Link layer technologies. The first problem was the frequency band. The engineers chose the 2.4Ghz to 2.4835Ghz radio frequency range because the FCC does not license this range. It is open and free to use for anyone which leads to the second problem that the engineers had to solve: how to deal with interference since anyone and everyone is allowed to transmit in this range.
Not only is this band wide open for any transmitter to use, it is the band that microwave ovens use. And as a matter of fact, wireless networks do have major problems with microwave ovens and now you know why. So what the engineers decided to do is the following: