1.1 Circuit-Switched Data Transmission
Initially, GSM was designed as a circuit-switched system that establishes a direct and exclusive connection between two users on every interface between all network nodes of the system. Section 1.1.1 gives a first overview of this traditional architecture. Over time, this physical circuit switching has been virtualized and many network nodes are connected over IP-based broadband connections today. The reasons for this and further details on virtual circuit switching can be found in Section 1.1.2.
1.1.1 Classic Circuit Switching
The GSM mobile telecommunication network has been designed as a circuit-switched network in a similar way to fixed-line phone networks. At the beginning of a call, the network establishes a direct connection between two parties, which is then used exclusively for this conversation. As shown in Figure 1.1, the switching center uses a switching matrix to connect any originating party to any destination party. Once the connection has been established, the conversation is then transparently transmitted via the switching matrix between the two parties. The switching center becomes active again only to clear the connection in the switching matrix if one of the parties wants to end the call. This approach is identical in both mobile and fixed-line networks. Early fixed-line telecommunication networks were designed only for voice communication, for which an analog connection between the parties was established. In the mid-1980s, ...