Transmission Systems: Modern long distance transmission systems can be placed under three broad categories:
1. Radio Systems
2. Coaxial cable systems
3. Optical fibre systems
Radio communication deals with electronic radiation of electromagnetic energy from one point to another through the atmosphere or free space. It is possible only in a certain position of the electromagnetic frequency spectrum. Presently, this portion includes frequencies from 9 kHz to 400 GHz. While there are international allocations for the radio spectrum up to 275 GHz, most of the commercial uses take place between 100 kHz and 20 GHz. Very few experimental systems have been operated beyond 100 GHz.
Different layers of the atmosphere play a role in propagating radio waves. The atmosphere consists of four layers. Of the four layers, the ionosphere and troposphere are useful for radio communication in certain frequency ranges. Certain other radio frequencies pass straight through the atmosphere and can be beamed towards satellites placed in the interplanetary space. Depending on the mechanism of propagation, radio communication can be placed under four categories:
1. Sky wave or ionosphere communication
2. Line-of-sight (LOS) microwave communication limited by horizon.
3. Troposphere scatter communication
4. Satellite communication.