illion. Although the history of cellular networks has been rather brief, it has already seen three generations, the fourth is emerging, and a fifth generation is on the drawing boards: First generation (1G)-The first generation, which initially debuted in Japan in 1979, is characterized by analog transmission systems. Key 1G standards included AMPS, TACS, JTACS, and NMT. Second generation (2G)-The second generation introduced digital transmission, and the first 2G networks were operational in 1992. GSM, UWC-136, cdmaOne, and PDC are the four major 2G cellular standards, along with the Personal Communications Services (PCS) standards GSM 1800, GSM 1900, and PDH. 2G networks are currently serving the vast majority of mobile subscribers. Intermediate second generation (2.5G)-Anintermediate second genera- tion called 2.5G offers enhancements to the data services on existing 2G digital platforms, achieving many of the same speeds offered by 3G net- works but without requiring the operator to acquire new spectrum and put in an entirely new infrastructure. The key 2.5G standards include GPRS, HSCSD, and EDGE. Third generation (3G)-The third generation, which is now upon us, involves digital transmission but also permits per-user and terminal mobil- ity, providing a single mobile communications service, adjusted for broad- band applications (including voice, data, and multimedia streams) to be supported at higher data speeds, in the range of 144Kbps to 384Kbps, up to 2Mbps in some cases, and with visions of supporting 155Mbps. The major 3G standards include W-CDMA, UMTS, CMDA2000, CDMA450, TD-SCDMA, and FOMA. 3G standards deployment is occurring around the world, but universal deployment is still far from complete. The key advances in emerg- ing technologies that greatly improve spectrum utilization and performance threaten 3G. An intermediate third generation, 3.5G, is introducing many enhance- ments designed for services running over an IP backbone, including high- speed data and video. Key 3.5G standards include HSDPA