GSM stands for Global System for Mobile Communications. It is developed by the European Telecommunications Standards Institute. It is a second generation digital cellular network. It is a one kind of cellular network. It has different cell sizes of a GSM networks such as pico, macro, micro, femto and umbrella cells. Macro cells are based on the base station antenna. Micro cells are installed in average roof top level. It is used in urban areas. Picocells are small cells. Femtocells are cells and it is designed for residential or small business environments. Umbrella cells are used in shadowed regions. The radius of the cell varies from the antenna height, propagation conditions and antenna gain. The cell radius varies depending on the antenna system. GSM services are provided for indoor coverage. GSM is used for variety of voice codes. It was enhanced in 1997. The network structure is based on the base station, switching sub system and operation support system.