Chapter 3 - General Principles of the IMS Architecture pot

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 circuit-switched networks.

Nevertheless, at a certain point in time the PSTN started to differentiate the paths the signaling

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The 3G IP Multimedia Subsystem (IMS): Merging the Internet and the Cellular Worlds Third Edition

Gonzalo Camarillo and Miguel A. Garc

© 2008 John Wiley & Sons, Ltd. ISBN: 978- 0- 470- 51662- 1

26 CHAPTER 3. GENERAL PRINCIPLES OF THE IMS ARCHITECTURE

plane and the media plane follow. This differentiation was triggered by the introduction of

services based on IN (Intelligent Network). Calls to toll-free numbers are an example of

an IN service. The GSM version of IN services is known as CAMEL services (Customized

Applications for Mobile network Enhanced Logic).

In both IN and CAMEL the signaling plane follows the media plane until there is a point

where the call is temporarily suspended. At that point the signaling plane performs a database

query (e.g., a query for a routing number for an 800 number) and receives a response. When

the signaling plane receives the response to the query the call setup is resumed and both the

signaling plane and the media plane follow the same path until they reach the destination.

3GPP has gone a step further in the separation of signaling and media planes with the

introduction of the split architecture for the MSC (Mobile Switching Center). The MSC is

split into an MSC server and a media gateway. The MSC server handles the signaling plane

and the media gateway handles the media plane. The split architecture was introduced in

Release 4 of the 3GPP specifications.

We will see that the IMS also keeps signaling and media paths separate, but goes even

further in this separation. The only nodes that need to handle both signaling and media are

the IMS terminals; no network node needs to handle both.

3.1.2 GSM Packet-switched

The GSM packet-switched network, also known as GPRS (General Packet Radio Service,

specified in 3GPP TS 23.060 [35]) was the base for the 3GPP Release 4 packet-switched

domain. This domain allows users to connect to the Internet using native packet-switched

technologies.

Initially, there were three applications designed to boost the usage of the packet-switched

domain:

• the Wireless Application Protocol (WAP) [314];

• access to corporate networks;

• access to the public Internet.

Nevertheless, none of these applications was attracting enough customers to justify the

enormous cost of deploying packet-switched mobile networks.

3.2 IMS Requirements

The situation that operators were facing right before the conception of the IMS was not

encouraging at all. The circuit-switched voice market had become a commodity, and

operators found it difficult to make a profit by only providing and charging for voice calls.

On the other hand, packet-switched services had not taken off yet, so operators were not

making much money from them either.

Thus, operators needed a way to provide more attractive packet-switched services to

attract users to the packet-switched domain. That is, the mobile Internet needed to become

more attractive to its users. In this way the IMS (IP Multimedia Subsystem) was born. With

the vision described in Chapter 1 in mind, equipment vendors and operators started designing

the IMS.

3.2. IMS REQUIREMENTS 27

So, the IMS aims to:

• combine the latest trends in technology;

• make the mobile Internet paradigm come true;

• create a common platform to develop diverse multimedia services;

• create a mechanism to boost margins due to extra usage of mobile packet-switched

networks.

Let us look at the requirements that led to the design of the 3GPP IMS (captured in

3GPP TS 22.228 [53] Release 5). In these requirements the IMS is defined as an architectural

framework created for the purpose of delivering IP multimedia services to end users. This

framework needs to meet the following requirements:

• support for establishing IP Multimedia Sessions;

• support for a mechanism to negotiate Quality of Service (QoS);

• support for interworking with the Internet and circuit-switched networks;

• support for roaming;

• support for strong control imposed by the operator with respect to the services delivered

to the end user;

• support for rapid service creation without requiring standardization.

The Release 6 version of 3GPP TS 22.228 [53] added a new requirement to support access

from networks other than GPRS. This is the so-called access independence of the IMS, since

the IMS provides support for different access networks.

3.2.1 IP Multimedia Sessions

The IMS can deliver a broad range of services. However, there is one service of special

importance to users: audio and video communications. This requirement stresses the need

to support the main service to be delivered by the IMS: multimedia sessions over packet￾switched networks. Multimedia refers to the simultaneous existence of several media types.

The media types in this case are audio and video.

Multimedia communications were already standardized in previous 3GPP releases, but

those multimedia communications take place over the circuit-switched network rather than

the packet-switched network.

3.2.2 QoS

Continuing with the analysis of the requirements we find the requirement to negotiate a

certain QoS (Quality of Service). This is a key component of the IMS.

The QoS for a particular session is determined by a number of factors, such as the

maximum bandwidth that can be allocated to the user based on the user’s subscription or

the current state of the network. The IMS allows operators to control the QoS a user gets, so

that operators can differentiate certain groups of customers from others.