#2 Radio Transmission

1. Radio waves are easy to generate, can travel long distances, and can penetrate buildings easily, so they are widely used for communication, both indoors and outdoors. 
2. Radio waves also are omnidirectional, meaning that they travel in all directions from the source, so the transmitter and receiver do not have to be carefully aligned physically.
3. The properties of radio waves are frequency dependent. At low frequencies, radio waves pass through obstacles well, but the power falls off sharply with distance from the source, roughly as 1/r2 in air. 
4. At high frequencies, radio waves tend to travel in straight lines and bounce off obstacles. They are also absorbed by rain. 
5. At all frequencies, radio waves are subject to interference from motors and other electrical equipment.
6. Due to radio’s ability to travel long distances, interference between users is a problem. For this reason, all governments tightly license the use of radio transmitters, with one exception, discussed below.
7. AM radio broadcasting uses the MF band, which is why the ground waves from Boston AM radio stations cannot be heard easily in New York. Radio waves in these bands pass through buildings easily, which is why portable radios work indoors.
8.  The main problem with using these bands for data communication is their low bandwidth.

Microwave Transmission

1. Before fiber optics, for decades these microwaves formed the heart of the long-distance telephone transmission system. 
2. In fact, MCI, one of AT&T’s first competitors after it was deregulated, built its entire system with microwave communications going from tower to tower tens of kilometres apart. 
3. Since the microwaves travel in a straight line, if the towers are too far apart, the earth will get in the way. Consequently, repeaters are needed periodically. 
4. The higher the towers are, the farther apart they can be. The distance between repeaters goes up very roughly with the square root of the tower height. For 100-metre-high towers, repeaters can be spaced 80 km apart.
5. Unlike radio waves at lower frequencies, microwaves do not pass through buildings well. In addition, even though the beam may be well focused at the transmitter, there is still some divergence in space. 
6. Microwave communication is so widely used for long-distance telephone communication, mobile phones, television distribution, and other uses that a severe shortage of spectrum has developed. 
7. It has several significant advantages over fiber. The main one is that no right of way is needed, and by buying a small plot of ground every 50 km and putting a microwave tower on it, one can bypass the telephone system and communicate directly. 
8. This is how MCI managed to get started as a new long-distance telephone company so quickly. (Sprint went a completely different route: it was formed by the Southern Pacific Railroad, which already owned a large amount of right of way and just buried fiber next to the tracks.)
9. Microwave is also relatively inexpensive. Putting up two simple towers (may be just big poles with four guy wires) and putting antennas on each one may be cheaper than burying 50 km of fiber through a congested urban area or up over a mountain.
10. And it may also be cheaper than leasing the telephone company’s fiber, especially if the telephone company has not yet even fully paid for the copper it ripped out when it put in the fiber.

Infrared and Millimetre Waves

1. Unguided infrared and millimetre waves are widely used for short-range communication. The remote controls used on televisions, VCRs, and stereos all use infrared communication. 
2. They are relatively directional, cheap, and easy to build but have a major drawback: they do not pass through solid objects.
3.  In general, as we go from long-wave radio toward visible light, the waves behave more and more like light and less and less like radio.
4. On the other hand, the fact that infrared waves do not pass through solid walls well is also a plus. It means that an infrared system in one room of a building will not interfere with a similar system in adjacent rooms or buildings: you cannot control your neighbour’s television with your remote control.
5. Furthermore, the security of infrared systems against eavesdropping is better than that of radio systems precisely for this reason. 
6. Therefore, no government licence is needed to operate an infrared system, in contrast to radio systems, which must be licensed outside the ISM bands. 
7. Infrared communication has a limited use on the desktop, for example, connecting notebook computers and printers, but it is not a major player in the communication game.