ode fibers but nondispersion-shifted fiber (NZ-DSF), which is optimized for 1.55-um windows and DWDM, is the preferred type for new optic installations. 4.7.3 Radio Transmission The most important advantage of radio transmission over cable transmission is that it does not require any physical medium. Radio systems are quick to install and because no digging of cable into the ground is required, the investment costs are much lower. One important factor that restricts the use of radio transmissions is the shortage of frequency bands. The most suitable frequencies are already occu- pied and there are many systems with a growing demand for wider frequency bands. Examples of other systems using radio waves are public cellular sys- items, professional mobile radio systems, cordless telephones, broadcast radio and TV, satellite communications, and WLANs. The use of radio frequencies is regulated by the ITU-R at the global level and, for example, by ETSI at the European level and the FCC in the United States. To implement a radio system, permission from a national tele- communications authority is required. 4.7.4 Satellite Transmission In satellite communications a microwave repeater is located in a satellite. An Earth station transmits to the satellite at one frequency band and the satellite regenerates and transmits the signal back at another frequency band. The fre- quencies allocated by ITU for satellite communications are in the frequency range of 1 to 30 GHz. Figure 4.31 illustrates point-to-point transmission with the help of a geostationary or geosynchronous satellite using the 6/4-GHz satellite band. The satellites used in the telecommunications network are usually located in a so-called "geostationary" orbit SO that they seem to be in the same location all the time from the point of view of the Earth station, as shown in Figure 4.31. The distance of this orbit is around 36,000 km from the equator on the Earth's surface and this introduces a long transmission delay that is approximately 250 ms from the transmitt