stimated at an American billion (a British thousand million) subscribers. 9.2.2.2 International intercompatibility. There was some effort to create compatible systems internationally. The United States and Canada agreed to use the same plan for telephony (both landline and wireless), creating a single North American telephony system north of the Rio Grande. With the advent of PCS, the North American cellular network included personal communications numbers (PCNs), allowing a person to keep a mobile telephone number even when changing service provider or changing equipment. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. General Cellular Standards Standards for Cellular Systems GSM took a different, and very clever, approach to personal number assignment and made an effort to achieve truly worldwide interoperability. All the GSM user informa- tion is on a smart card that you can plug into any GSM phone equipment, making your personal user terminal, even if you rent the equipment. GSM services operate on five different frequency bands, GSM900, GSM1800, GSM1900, and more recently, GSM850, and GSM450. Each model of phone may be sin- gle band, dual band, or multiband. A phone that operates on both GSM900 (the primary European carrier) and also on either GSM1800 or GSM1900 (the North American fre- quencies) would offer seamless interoperability wherever there is coverage in Europe and North America without changing phones. 1 However, 2G did not offer seamless global service. In part, this was due to a lack demand. International travelers may be high-profile as well as high-profit customers, but the bread-and-butter customers that grew the 2G client base were the increasingly mobile citizens of each continent, not the globe-trotting executives. This failure caused the ITU to propose an initiative for creating a single global stan- dard. It was call