communications using both forms. Our main concern here is electrical and bidirectional communication, as shown in the upper part of Figure 1.1. The share of mechanical telecommu- nications such as conventional mail and press is expected to decrease, whereas electrical, especially bidirectional, communication will increase and take the major share of telecommunications in the future. Hence, major press corpo- rations are interested in electrical telecommunications as a business opportunity. 1.2 Significance of Telecommunications Many different telecommunications networks have been interconnected into a continuously changing and extremely complicated global system. We look at telecommunications from different points of view in order to understand Introduction to Telecommunications Network Engineering Mobile telephone Telephone Electrical networks Telex Bidirectional Telegraph Telecom- munications Dedicated networks Unidirectional mass communication Press Cable TV Radio Figure 1.1 Telecommunications what a complicated system we are dealing with and how dependent we are on Telecommunications networks make up the most complicated equipment in the world. Let us think only of the telephone network, which includes more than 2 billion fixed and cellular telephones with universal access. When any of these telephones requests a call, the telephone network is able to establish a connection to any other telephone in the world. In addition, many other net- works are interconnected with the telephone network. This gives us a view of the complexity of the global telecommunications network-no other system in the world exceeds the complexity of telecommunications networks. Telecommunications services have an essential impact on the development of a community. If we look at the telephone density of a country, we can esti- mate its level of technical and economical development. In the developing Introduction to Telecommunications countries the fixed telephone density, that is, the teledensity, is fewer than 10 telephones per 1,000 i