enable them to use fre- quencies again in other cells of the same network, narrow beam radio relay systems, sophisticated modulation schemes for radio relays, and digital broadcasting systems. We saw in Section 4.2.4 that we can decrease the modulation rate and, correspondingly, the required bandwidth with the help of more complicated modulation schemes. 4.2.5.1 Wavelength and Frequency The wavelength shown on the left-hand side of Figure 4.10 indicates the propagation distance during one cycle of the radio wave. It is related to the Introduction to Telecommunications Network Engineering frequency and speed of light and electromagnetic wave according to A = clf, where A is the wavelength in meters; C is the propagation speed of light or radio wave in meters per second, approximately 300,000 km/sec; and fis the frequency in Hz = 1/sec. 4.2.5.2 Propagation Modes Radio waves at different frequency bands propagate in different propagation modes. They are very briefly explained as follows: Ground wave: The radio wave follows the surface of the Earth, and thus communication over the horizon is possible. Skywave: The radio wave is reflected from the ionosphere back to Earth. The wave is reflected back from the Earth's surface and back to the Earth again making long-distance communication possible. The communication quality is not stable because the characteristics of the ionosphere vary with time. Line of sight: The radio wave propagates along the straight line from the transmitter to the receiver. A general requirement for good per- formance is that the receiving antenna be visible from the transmitter. The radio frequencies above 100 MHz that propagate in line-of-sight mode are used in most modern communication systems. As the demand for radio communications has increased, higher and higher frequencies have been put into use. However, as we will see in the next section, the attenuation of the radio wave increases with frequency and at extremely high frequencies, beyond 10-100 GHz, even weather conditions affect attenu