Chapter 3

General Principles of the IMS Architecture

In Chapter 1 we introduced the circuit-switched and the packet-switched domains and described why we need the IMS to provide rich Internet services. Chapter 2 introduced the players standardizing the IMS and defining its architecture. In this chapter we will describe the history of the circuit-switched and the packet-switched domains. In addition, we will introduce the design principles that lay behind the IMS architecture and its protocols. We will also tackle in this chapter the IMS network nodes and the different ways in which users are identified in the IMS.

3.1 From Circuit-switched to Packet-switched

Let us look at how cellular networks have evolved from circuit-switched networks to packet-switched networks and how the IMS is the next step in this evolution. We will start with a brief introduction to the history of the 3G circuit-switched and packet-switched domains.

The Third Generation Partnership Project (3GPP) is chartered to develop specifications for the evolution of GSM. That is, 3GPP uses the GSM specifications as a design base for a third generation mobile system.

GSM has two different modes of operation: circuit-switched and packet-switched. The 3G circuit-switched and packet-switched domains are based on these GSM modes of operation.

3.1.1 GSM Circuit-switched

Not surprisingly, the GSM circuit-switched network uses circuit-switched technologies, which are also used in the PSTN (Public Switched Telephone Network). ...

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