the signal being transmitted. With the help of CW modulation, the spec- trum of the message is transferred to the suitable frequency band of the medium. The use of frequency bands is controlled internationally by the ITU-R and nationally by national telecommunications authorities. Radio systems are often the most economical solution when new connections are required and there are no free cables or fibers between the end points of the connec- tion. Figure 4.10 illustrates the frequency range that is used in telecommuni- cations and it also shows some examples of the usage of different frequencies. In Figure 4.10 the electromagnetic spectrum used in telecommunica- tions is shown together with typical transmission media, the propagation modes, and some application examples. However, radio systemss have one important problem that restricts the use of radio communication, namely, lack of frequency bands. The most suitable bands are overcrowded, and new technical inventions are needed in order to overcome this problem. Among these are, for example, cellular Transmission Wavelength Frequency Transmission Propagation Applications Frequency designations media modes 10-1 -12 m Cosmic rays Gamma rays X-rays 10 -7 m Ultraviolet 10 15 Hz Visible Laser High data rate 10 -6 m Optical beams Infrared optical line fibers systems 10 14 Hz Extra high frequency, EHF Microwave relay 100 GHz Earth-satellite Super high Free space, Line-of- communication frequency, SHF waveguide sight Radar 10 GHz 10 cm radio Micro-oven Ultra high Cellular systems 1 GHz frequency, UHF UHF TV Mobile Mobile Very high VHF TV and radio frequency, VHF 100 MHz Coaxial Mobile radio Skywave cable, radio CB radio frequency, HF 10 MHz free space Amateur radio Medium AM broadcasting 1 MHz frequency, MF Groundwave Low frequency, pairs Navigation 100 kHz Transoceanic radio 10 km Very low pairs 10 kHz frequency, VLF Telephone 100 km Audio Telegraph Figure 4.10 Allocation and applications of electromagnetic spectrum. mobile systems and WLANs with small cell areas that 