ok. As a result, I have made every effort to spot those difficult issues and then give clear explanations. Brevity has been a challenge for me. Telecommunications is explosively developing. My goal has been to hit the high points and leave the details to other texts. xxiii PREFACE A major source of reference material has been the International Telecommunication Union (ITU). The ITU had a major reorganization on January 1, 1993. Its two principal subsidiary organizations, CCITT and CCIR, changed their names to ITU Telecommu- nication Standardization Sector and ITU Radio Communications Sector, respectively. Reference publications issued prior to January 1993 carry the older title: CCITT and CCIR. Standards issued after that date carry ITU-T for Telecommunication Sector publi- cations and ITU-R for the Radio Communications Sector documents. A new edition to a publication is prepared and issued to reflect changes in the industry since the issuance of the prior edition, to correct errors both in substance and format, and to add new material and delete obsolete items. It is truly a challenge to the author to keep up with the modernization and changes taking place in telecommunications. ROGER L. FREEMAN Scottsdale, Arizona INTRODUCTORY CONCEPTS 1.1 WHAT IS TELECOMMUNICATION? Many people call telecommunication the world's must lucrative industry. In the United States, 110 million households have telephones and 50% of total households in the U.S. have Internet access and there are some 170 million mobile subscribers. Long-distance service annual revenues as of 2004 exceeded 100 X 109 dollars. Prior to divestiture (1983), the Bell System was the largest commercial company in the United States. It had the biggest fleet of vehicles, the most employees, and the greatest income. Every retiree with any sense held the safe and dependable Bell stock. Bell System could not be found on the "Fortune 500" listing of the largest companies. In 1982, Western Electric Co., the Bell System manufacturing arm, was number seven on the "Fortu