el, the other as a receive channel. The control channel is used for signaling. Second generation systems (2G), which came about in the early 1990s, were all-digital and still primarily voice oriented, although rudimentary data transport was part of the service package. In 2G systems, digital access became the norm through such technologies as TDMA. In TDMA systems, the available frequency is broken into channels as it is in 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. Access Technologies Chapter 6 FDMA, but here the similarity between the two ends. Each channel is shared among a group of users based on a time-division technique that ensures fair and equal access to the available channel pairs. This increases the so-called packing ratio, that is, the number of simultaneous users that the system can support. An offshoot of 2G TDMA systems is 2.5G, the most common of which is Global System for Mobile Communications (GSM). GSM is widely deployed in Europe and other parts of the world (well over 750 million subscribers globally) and goes beyond 2G by offering a suite of user ser- vices that enhances the value of the technology. These services include not only voice but also two-way radio, paging, short messaging service (SMS, similar to Instant Messaging), and the incorporation of Smart Card technology that permits the customization of the handset and its features. These services are both popular and lucrative: In Europe, where mobile penetration is nearly 85 percent, more people send and receive SMS/IM text messages via their mobile phones than use the Internet for similar functions. The domestic U.S. market is a challenge, however: In the United States, only 5 percent of the 170 million cellular users send or receive text messages-as a result of this and other factors, only 1 per- cent of the $72.8 billion mobile industry comes from data ser