4.1 Circuit-switched Mobile Voice Telephony

The main purpose for which 2G wireless systems such as GSM were designed at the beginning was mobile voice telephony. Since it was the only application, each network element was specifically optimized for it. At the time, the state of the art for voice telephony in fixed-line telephony networks was to establish a transparent full duplex channel between two parties. While the connection is established between two parties in such a system, all data sent by the originator was transparently sent to the other side. The only exception are echo cancellation modules which are put into the transmission chain to improve voice quality.

4.1.1 Circuit Switching

Two parties are connected by a switching center which has a switching matrix to connect any telephone line (circuit) with any other. For long-distance calls, several switching centers were daisy-chained. For 2G wireless systems the principle was reused as shown in Figure 4.1. The main difference between a circuit-switched fixed-line telephony system and a wireless circuit-switched telephony system is that a subscriber can no longer be identified by a pair of wires. Instead, a subscriber is now identified with credentials stored on a SIM card. On the network side a database known as the Home Location Register contains a replica of the user credentials and information on which options and supplementary services (e.g. call forwarding) a user is allowed to use. Except for the additional database in the network, most of the hardware and software of a fixed-line switching center could be reused for the design of a mobile switching center. Since the user is no longer identified by a specific pair of wires, a mobilit....