ories. Today, GSM technology is used by more than 20% of the world's pop- ulation and almost 80% of the world's cellular market. There are close to 1,700 models of GSM phones available from more than 100 vendors. And the progress hasn't stopped there. Today's GSM platform is living, growing, and evolving and already offers an expanded and feature-rich family of voice and multimedia services. GSM uses TDMA and FDD, and it is independent of underlying analog sys- items. In principle, GSM can be implemented in any frequency band. However, GSM terminals are currently available in several bands: GSM 850-This is the newest GSM band implemented in North America. It operates over 824MHz to 849MHz, paired with 869MHz to 894MHz. GSM 900-This is the first and traditional implementation of GSM, and it operates over 890MHz to 915MHz, paired with 935MHz to 960MHz. 2G: Digital Cellular Radio GSM 1800-This band is also referred to as DCS (Digital Cellular System) 1800, and it is the PCS implementation of GSM globally. It operates over 1,710MHz to 1,785MHz, paired with 1,805MHz to 1,880MHz. GSM 1900-This band, also called PCS 1900, is the North American implementation of GSM. It operates over 1,850MHz to 1,910MHz, paired with 1,930MHz to 1,990MHz. GSM mobile stations transmit on the lower-frequency subband, and base stations transmit on the higher-frequency subband. Triband and quadband phones are now available, and they enable roaming between countries that use these different fre- quency bands. GSM supports 124 channel pairs with a 200KHz spacing to prevent channel interference and is based on an eight-time-slot technique, which means it can sup- port eight callers per channel. GSM supports both voice and data, but with basic GSM, data rates reach only as high as 9600bps. Among the features of GSM are international roaming with a single invoice and the ability to handle SMS, which can offset the need for paging by enabling text messages up to 160 alphanumeric characters in length to be sent to and from a GSM phone and an extern