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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. | 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 . Circuit-switched networks have two different planes the signaling plane and the media plane. The signaling plane includes the protocols used to establish a circuit-switched path between terminals. In addition service invocation also occurs in the signaling plane. The media plane includes the data transmitted over the circuit-switched path between the terminals. The encoded voice exchanged between users belongs to the media plane. Signaling and media planes followed the same path in early .