GSM Mobile Network – mobile network of 2nd generation
Fundamentals of GSM system

At present, GSM is still widely used digital cellular system. This system was built as a European open standard and its deployment has allowed to solve the international roaming, i.e., the operation of the same mobile station with one phone number in all countries that adopt this system.

An important element of GSM system is an identification system based on unique subscriber card called as SIM (Subscriber Identity Module).

The SIM card contains not only basic identification data, but also many other specific individual information, such as subscriber identification number, authentication keys, information on prepaid phone services or the participant list. Mobile station can then be used only with the activation of the specific operator's card. Nonetheless, the exceptions are emergency calls that can be performed without SIM card. Coding and encryption of transmitted information, which significantly complicates the possibility of eavesdropping is other important aspect from a user perspective.

Switching process begins in the moment when the active call begins to send the initial signalling data. One of the most important initial processes is to control of eligibility access of the mobile station to the network.

The MS sends its identification number IMSI (International Mobile Subscriber Identity) through BS and BSC (Base Station Controller) to exchange MSC (Mobile Switching Centre). The block AuC (Authentication Centre) sends towards the MS random number that is converted on the basis of individual data and algorithms of SIM card to another different number that is as original response sent back to the MSC exchange. Subsequently individual data are compared with the data of the database located in the VLR block. If there is a consensus then the mobile station has allowed access into the mobile network.

To ensure anonymity, the subscriber station is communicating with assigned provisional identification number, known as TMSI (Temporary Mobile Subscriber Identity), under which the MS is identified in the MSC. When switching the mobile station to another MSC, it is re-assigned with another TMSI. Self-transfer of user data can only start when these processes are finished.