quency plan was to be re-done to be able to avoid interference related problems. This system not only had capacity related problems, but the security system was also poor. If you are able to get hold of another person's serial code, it would be possible to make illegal calls. Although efforts were made to address these problems, especially the ones related to capacity, the results were not sufficient and the industry started to look into other options, such as the next generation digital systems. The TACS was similar to the AMPS and operated in the 900 MHz frequency range. 1.3 Second Generation Cellular Networks Due to the incompatibility of the various systems in place, the European commission started a series of discussions that tried to change the then existing telecommunication regulatory framework, leading to a more harmonised environment which resulted in the development of a common market for the telecommunication services and equipment. In the early 1990s, digital transmission technology came into force, bringing with it the next generation system, called the 'Second Generation Mobile System'. Digitisation means that the sound of the speaker's voice was processed in a way that imitated a human ear through techniques such as sampling and filtering. This made it possible for many more mobile users to be accommodated in the radio spectrum. Key 2G systems in this generation included GSM (Global Second Generation Cellular Networks Systems for Mobile Communications), TDMA IS-136, CDMA IS-95, PDC (Personal Digital Cellular) and PHS (Personal Handy Phone System). 1.3.1 D-AMPS (Digital Advanced Mobile Phone System) IS 54 and IS 136 (where IS stands for Interim Standard) are the second generation mobile systems that constitute the D-AMPS. This was the digital advancement of the then existing AMPS in America. TDMA (Time Division Multiple Access) was used as the air interface protocol. The D-AMPS used existing AMPS channels and allows for smooth transition between digital and analogue systems in the same area. Capaci