ternet-related activities. Its headquarters is located in the state of Virginia in the United States [3, 5]. The Internet has been used by academics for more than 20 years. It used to be difficult to use, only some organizations had access to it, and the only users were academic specialists who were familiar with it. Because there were no commercial applications, usage charges were not considered at all for Internet technology. The academic information exchanged over the Internet Introduction to Telecommunications Network Engineering was public by nature and neither security nor charging functions were con- sidered in the development of the Internet. The development of a graphical user interface exploded onto the scene in the mid-1990s and the use of the Internet grew exponentially. This new graphical user interface is called the World Wide Web (WWW) browser and it has made the Internet easy to use for anyone. Nowadays many commercial Internet service providers (ISPs), who have access to the worldwide Internet service, provide Internet access for ordinary telephone and ISDN subscribers. Anyone who has a personal computer can access the Internet via the tele- phone or ISDN network. New higher data rate access systems, such as cable modems and ADSL discussed earlier, have become popular because they essentially improve customer access to Internet. The Internet was originally designed for data applications only and it uses the genuine packet-switched transmission principle explained in Section 6.2. This is a very efficient method because the transmission connec- tions in the network are used on demand. There is no circuit and fixed share of capacity for each user as is the case, for example, in ISDN. Because of this efficiency, the Internet will be used more and more for voice communica- tions instead of PSTN. The usage of the Internet for international calls is very attractive because the international section of the call is often free of charge. No method exists to charge inside the Internet a certain type of usag