2.1 Circuit-Switched Data Transmission over GSM
As discussed in Chapter 1, the GSM network was initially designed as a circuit-switched network. All resources for a voice or data session are set up at the beginning of the call and are reserved for the user until the end of the call, as shown in Figure 2.1. The dedicated resources assure a constant bandwidth and end-to-end delay time. This has a number of advantages for the subscriber:
- Data that is sent does not need to contain any signaling information such as information about the destination. Every bit simply passes through the established channel to the receiver. Once the connection is established, no overhead, for example, addressing information, is necessary to send and receive the information.
- As the circuit-switched channel has a constant bandwidth, the sender does not have to worry about a permanent or temporary bottleneck in the communication path. This is especially important for a voice call. As the datarate is constant, any bottleneck in the communication path would lead to a disruption of the voice call.
- Furthermore, circuit-switched connections have a constant delay time. This is the time between sending a bit and receiving it at the other end. The greater the distance between the sender and receiver, the longer the delay time. This makes a circuit-switched connection ideal for voice applications as they are extremely sensitive to a variable delay time. If a constant delay time cannot be guaranteed, a buffer at the ...