s of corporate offices in a region. Circuit-switched networks dedicated to data transmission are not widely used today. Packet-switched data service is provided by the X.25 network worldwide. It operates according to the X-series recommenda- tion of ITU-T, but the marketing names of X.25 networks differ from The Telecommunications Network: An Overview country to country, for example, Auspak is used in Australia and Finpak in Finland. These networks were developed to provide commercial data com- munication service and they provide charging functionality SO that the cus- tomer bill may be based on the amount of transferred data. The importance of these networks has been reduced because of expansion of the Internet. Internet e-mail has replaced X.25 e-mail. Public wireless data networks, such as general packet radio service (GPRS), have been implemented to provide data services for mobile users. Wireless LAN (WLAN) is another technology that is used to provide data service in hot spots, such as airports. 2.10.1.6 Internet The Internet is a worldwide packet-switched network developed from the ARPANET, which in turn was developed in the late 1960s by the U.S. Department of Defense. The ARPANET grew until it became a wide-area computer network called the Internet, which was used in the 1970s and 1980s mainly by academic institutes such as universities. Because of its his- tory the Internet does not provide charging functions, and customer billing is usually based on the access data rate and fixed monthly fee. In the first half of the 1990s the user-friendly graphical user interface WWW was intro- duced; since then the use of the Internet has expanded very rapidly. Cur- rently, the Internet is the major information network in the world, and many Internet service providers (ISPs) have sprung up to provide Internet services for both businesses and residential customers. The expansion of the Internet continues, and the evolving commercial services (e.g., electronic shopping), the new access technologies (such as xDSL, disc