zed interface between a terminal and its network enables subscribers to purchase terminal equipment from multi- ple vendors. Standardized interfaces among systems in the network enable network operators to use multiple competing suppliers for systems. This improves the availability and quality of systems and reduces their cost. Introduction to Telecommunications Standards make international services available. Standardization plays a key role in the provision of international services. Official global standards de- fine, for example, telephone service, ISDN, and facsimile. The standards of some systems may not have official worldwide acceptance, but if the system becomes popular all around the world, a worldwide service may become available. Recent examples of these services are GSM and the Internet with WWW. Internet specifications have no official status, and GSM was origi- nally specified for Europe only. Their specifications have been openly avail- able, which has supported their expansion. To clarify and understand the influence of standardization on our eve- ryday lives, consider these examples of international standardization: Screw thread pitches (ISO, Technical Committee 1): This was one of the first activity areas of standardization. In the 1960s, a bolt from one car would not fit another. Currently, bolts are internationally standard- ized and most often compatible. International telephone numbering and country codes: Without glob- ally unique identification of subscribers, automatic international telephone calls would not be available. Telephone subscriber interfaces. PCM coding and primary rate frame structure: This coding and struc- ture make national and international digital connections between networks possible. Television and radio systems. Frequencies used for satellite and other radio communications. Connectors and signals for PC, printer, and modem interfaces. LANs: These enable people to use computers from any manufacturer in a company network. Cellular telephone systems: Enable users to cho