GSM connection

In the circuit-switched GSM network, once a connection has been established between two subscribers, it is available exclusively to these two subscribers for the entire duration of the connection, until the connection is terminated. In the GSM network, a distinction is made between several types of connection, namely for radio transmission and for communication via fixed networks.

The communication between the base station and the mobile station is realized by a radio link. Between the radio exchange and the base station, where signaling traffic occurs, the transmission is realized by SS7 signaling. The same configuration is used on the link between the radio switching center( MSC) and the Home Location Register( HLR) and Visitor Location Register( VLR) databases.

The link between the base station and the mobile station is designed to map the functionalities of the OSI reference model. In the lower three layers, the physical layer, the link layer and the network layer, all functionalities of the radio transmission are implemented so that the transport layer can access a transparent link. Radio-specific functions that do not require communication between the base station and mobile station instances, but are part of the lower three layers' functionalities, are performed by the Mobility Management Entity( MME).

Layer 1, known as the physical layer in the OSI reference model, transmits the user and signaling data between the mobile station and the base station using radio signals. For this purpose, the data is packaged in TDMA data frames and transmitted over special radio channels. This layer is also used for traffic measurements and controls channel occupancy.

In the link layer, variants of the well-known LAP(Link Access Procedure) protocols are used for secure transmission. The switching layer has the task of extending, operating and terminating connections from a radio subscriber through the radio telephone network into the public telephone network. In addition, all procedures for location management, subscriber identification and radio resources are handled in this layer. These include channel clearance, function assignment to channels, traffic control and handover.