**2.1 Introduction**

By deciding to use session initiation protocol (SIP) as the signaling protocol for session establishment and control in IMS instead of developing its own set of protocols, 3GPP has

Design and Analysis of IP-Multimedia Subsystem (IMS) 69

In this section, a description of the IP Multimedia Sub system (IMS) architecture including

3GPP has standardized the IP Multimedia Subsystem specifications (24.229 2009).IETF also collaborates with them in developing protocols that fulfill their requirements. Figure 1

 CSCF (Call/Session Control Function): CSCF is a SIP server which processes SIP signaling in the IMS. There are three types of CSCFs depending on the functionality

 P-CSCF (Proxy-CSCF): The P-CSCF is the first point of contact between the IMS terminal and the IMS network. All the requests initiated by the IMS terminal or

 I-CSCF (Interrogating-CSCF): It has an interface to the SLF (Subscriber Location Function) and HSS (Home Subscriber Server). This interface is based on the Diameter

The I-CSCF retrieves user location information and routes the SIP request to the

 S-CSCF (Serving-CSCF): It maintains a binding between the user location and the user's SIP address of record (also known as Public User Identity). Like the I-CSCF, the S-CSCF

SIP AS (Application Server): The AS is a SIP entity that hosts and executes IP

ENUM (E.164 NUmber Mapping): The ENUM allows telephone numbers to be resolved

 MRF (Media Resource Function): The MRF provides a source of media in the home network. It is further divided into a signaling plane node called the MRFC (Media Resource Function Controller) and a media plane node called the MRFP (Media Resource Function Processor). The MRFC acts as a SIP User Agent and contains a SIP interface towards the S-CSCF. The MRFC controls the resources in the MRFP via an

BGCF (Breakout Gateway Control Functions): BGCF a SIP server that includes routing

 MGCF (Media Gateway Control Function): MGCF implements a state machine that does protocol conversion and maps SIP to either ISUP (ISDN User part) over IP or BICC (Bearer Independent Call Control) over IP. The protocol used between the MGCF and

 MGW (Media Gateway): The MGW interfaces the media plane of the PSTN. On one side the MGW is able to send and receive IMS media over the Real-Time Protocol (RTP)

into SIP URLs using the Domain Name System (DNS) (Faltstrom 2000).

SGW (Signaling Gateway): SGW performs lower layer protocol conversion.

the function of the key components and SIP function in IMS is also presented.

shows the common nodes included in the IMS .These nodes are:

 Proxy Call Session Control Function (P-CSCF) Interrogating Call Session Control Function (I-CSCF) Serving Call Session Control Function (S-CSCF).

protocol (Calhoun et al. 2003).

Multimedia Services based on SIP.

H.248 interface (ITU-T H.248.1 2005).

functionality based on telephone numbers.

the MGW is H.248 (ITU-T H.248.1 2005)

(Schulzrinne et al. 2003)

destined to the IMS terminal traverse the P-CSCF.

appropriate destination, typically an S-CSCF.

also implements a Diameter interface to the HSS.

**2.3 Background** 

**2.3.1 IMS architecture** 

they provide,

opened the door toward a tight integration of the mobile, fixed and Internet worlds. SIP can be used over various transport protocols such as UDP, TCP or SCTP. To enable the reliable transmission of SIP messages even when used over UDP, SIP supports application level retransmission mechanisms. That is in case no response was received for a sent request then after a timeout the request is retransmitted. Thereby, losses due to overloaded servers or lossy links would cause delays in the session establishment and hence reduce the perceived service quality.

In this part of the chapter we provide a theoretical model that can be used by operators and network designers to determine the effects of introducing IMS to their networks in terms of bandwidth usage for example and the effects of losses and delays on the service quality. This model uses as the input various traffic characteristics such as the number of calls per second and mean holding time and network characteristics, such as losses and propagation delays. The output of the model provides details on the bandwidth needed for successfully establishing a session when using SIP over UDP in IMS networks. In Sec. 2.2 we provide the related work to this chapter and present a brief overview of the literature concerning modeling of SIP. In Sec. 2.3 the IMS and SIP in IMS are presented. In Sec. 2.4 the IMS session establishment phases is presented. The SIP model for IMS session establishment is presented in Sec. 2.5.
