Global System for Mobile Communications [1], namely GSM, is the most widely used mobile phone standard at present. More than 6,543.8 billion people in more than 200 countries and regions around the world are using GSM mobile phones. The widespread use of GSM standard makes it very common for users to roam internationally after signing a "roaming agreement" between mobile phone operators. The biggest difference between GSM and previous standards is that its signaling and voice channels are digital, so GSM is regarded as the second generation (2G) mobile phone system. The GSM standard is currently formulated and maintained by the 3GPP organization. From the user's point of view, the main advantage of GSM is to provide higher digital voice quality and low-cost new options (such as SMS) to replace calls. From the point of view of network operators, its advantage is that it can deploy devices from different manufacturers, because GSM, as an open standard, provides easier interoperability. Moreover, the standard allows network operators to provide roaming services, and users can use mobile phones around the world. Although the GSM standard is developing (for example, the packet data capability is increased through GPRS in the' 97 version of the standard), it is still backward compatible with the original GSM phone. Higher-speed data transmission was introduced in the' 99 edition standard with EDGE.