GSM (Groupe Special Mobile) is the underlying model and standard for digital cellular communications; the basis of the first 2G digital mobile phone network. This network is a circuit switched network and primarily designed for voice communication It uses narrow band Time Division Multiple Access (TDMA) to provide the voice component. SMS is also provided, by using one of the control channels to send a short message to another subscriber. In terms of data, the GSM model does not really address the issue and any data connection is achieved using dial-up. Thus Thus the phone will dial into the data network and send the data using the same channel as they would a voice call.
This model was developed in the early 1970's. It provides a circuit switched network dividing each 200khz channel into 8 timeslots. The 2G system works at 900/1800MHz. Even for modern 4G phones use the GSM network to phone as there is no phone provision in the 4G network.
The GSM network comprises 4 sub-systems :
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