Mobile Communication) GSM was first developed in the 1980s. From 1982 to 1985, in the GSM group (originally hosted by CEPT) discussions were held to decide between building an analogue or a digital system. After multiple field tests, etc., it was decided to build a digital system and a narrowband TDMA solution was chosen. The modulation scheme chosen was Gaussian minimum shift keying (GMSK). The technical fundamentals were ready by 1987 and by 1990 the first specification was produced. By 1991, GSM was the first commercially operated digital cellular system with Radiolinja in Finland. GSM is by far the most popular and widely implemented cellular system with more than a billion people using the system (by 2005). Features such as prepaid calling, international roaming, etc., enhanced the popularity of the system. Of course, this also led to the development of smaller and lighter handsets with many more features. The system became more user friendly with many services also provided apart from just making calls. These services included voice mail, SMS, call waiting, etc. SMS was a phenomenal success, with almost 15 billion SMS sent every month by the year 2000. The key advantage of GSM systems has been higher Cellular Networks digital voice quality and low cost alternatives to making calls, such as text messaging. The advantage for network operators has been the ability to deploy equipment from different vendors because the open standard allows easy interoperability. The GSM system operates at various radio frequencies, with most them operating at 900 MHz and/or 1800 MHz. In the US and Canada, the operation is at 850 MHz and/or 1900 MHz. The uplink frequency band in the 900 MHz band is 935-960 MHz and the downlink frequency is 890-915 MHz. Thus, in both the uplink and downlink the band is 25MHz, which is subdivided into 124 carriers, each being 200 kHz apart. Each radio frequency channel contains eight speech channels. The cell radius in the GSM network varies depending upon the antenna height, antenna gains, propag