al system such as e-mail, paging, and voicemail systems. It also offers message-handling services in support of the X.400 standard, which was quite important when we were using largely proprietary e-mail systems. Now with the ubiquitous addressing scheme afforded by the Internet, it's not as important, but remember that GSM evolved quite some time ago. GSM also supports Group III and, very importantly, it provisions a subscriber identity module (SIM) card, which is a smart card that defines the accounting, personal details, and phone number of an individual's service. The SIM card can be used in any GSM handset to activate service, SO it can help a user roam globally and still be able to maintain service. You can simply rent a handset tuned to the frequencies of the country you're visiting and use your SIM card to authorize the services and ensure that accounting occurs to the proper address-all without having to change phone numbers. GSM offers spectrum utilization that is much better than in the analog envi- ronment, the ability to improve security by applying encryption to the digital bit- streams, and authentication and fraud prevention via SIM cards. A variety of voice compression devices-both half-rate coders and enhanced full-rate coders where voice quality is the preeminent concern-are being devel- oped so that we can use GSM to make more efficient use of the available spectrum. Another area of development is the ability to create closed user groups and engage in group calls. GSM now offers a number of enhanced data services, commonly referred to as 2.5G; the most important of these services are discussed later in this chapter. Chapter 14 Wireless WANs Universal Wireless Communications (UWC), also known as ANSI-136, was at one time the dominant technology in North America and Latin America. As of Decem- ber 2005, the total number of TDMA subscribers worldwide was approximately 68 million. In comparison, GSM has established itself as the dominant technology on a global basis, with over 1.8 billion subscrib