nhabitants; in developed countries in, for instance, North America and Europe, there are around 500 to 600 fixed telephones per 1,000 inhabitants. The economic development of developing countries depends on (in addition to many other things) the availability of efficient telecommunications services. The operations of a modern community are highly dependent on telecommunica- tions. We can hardly imagine our working environment without telecom- munications services. The local area network (LAN) to which our computer is connected is interconnected with the LANs of other sites throughout our company. This is mandatory SO that the various departments can work together efficiently. We communicate daily with people in other organiza- tions with the help of electronic mail, telephones, facsimile, and mobile tele- phones. Governmental organizations that provide public services are as dependent on telecommunications services as are private organizations. Telecommunications plays an essential role on many areas of everyday living. Everyday life is dependent on telecommunications. Each of us uses telecom- munications services and services that rely on telecommunications daily. Here are some examples of services that depend on telecommunications: Banking, automatic teller machines, telebanking; Aviation, booking of tickets; Sales, wholesale and order handling; Credit card payments at gasoline stations; Booking of hotel rooms by travel agencies; Material purchasing by industry; Government operations, such as taxation. 1.3 Historical Perspective Some of the most important milestones in the development of electrical tele- communications systems according to [1] are discussed in this section. Terms and abbreviations used in the chronology are explained in later chap- ters of this book. The development and expansion of some telecommunica- tions services is also illustrated in Figure 1.2. Introduction to Telecommunications Network Engineering Abbreviations: Cable modems e-mail WAN = Wide Area Network Internet WWW LAN = Local Area N