**2.3.1 IMS architecture**

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


Design and Analysis of IP-Multimedia Subsystem (IMS) 71

sender increases the waiting time and tries again and so up to a certain number of retransmissions, (Rosenberg et al.2002) (29.328 2008).In general one can distinguish between

This behavior applies for INVITE requests as well as some other messages exchanged during session establishment. If this mode is used then the sender retransmits a message if no confirmation was received after T1 seconds. The retransmission timer is then increased exponentially up to a maximum retransmission timer (Timer B). Once this timer is reached

This behavior applies to all requests other than INVITE. In this mode the sender retransmits a message if no confirmation was received after T1 seconds. The retransmission timer is then increased exponentially up to a maximum retransmission timer called T2. Once this timer is reached the sender continues retransmitting the request every T2 up to a maximum timer (TimerF). Once this timer is reached the sender drops the message and stops the

Figure 2 illustrates a basic session establishment in the IMS with the caller and callee roaming to foreign networks. The example is based on the scenario provided in (Sisalem et al. 2009) .The session establishment in IMS is triggered by the sending of an INVITE request. In general, the session establishment can be considered as consisting of five phases, namely: Phase 1: Initiation (INVITE / 100 Trying): This phase is initiated with the sending of an INVITE request and is terminated when the client receives a provisional or final

 Phase 2: Negotiation (Session Progress 183 / PRACK / 200 OK) : In this phase the caller and callee negotiate the audio and video codes to be used as well as the QoS criteria. The 183 provisional responses is sent reliably. Session Progress 183 (INVITEretransmission mode) , PRACK and 200 OK (Non-INVITE-retransmission mode) Phase 3: Confirmation (UPDATE / 200 OK): In this phase the caller and callee complete the code and QoS negotiations. UPDATE and 200 OK (Non-INVITE-retransmission

 Phase 4: Ringing: (Ringing 180 / PRACK / 200 OK): In this phase the callee informs the caller that the user is being alerted about the call. Ringing 180 (INVITE-retransmission

Phase 5: Final Response (200 OK / ACK): In this phase the callee informs the caller that

In this section we will provide a theoretical model for SIP retransmission techniques in lossy network, bandwidth calculation for IMS session set up and estimation of IMS session set up delay. Figure.2. shows that the calls traverse five SIP proxies. Each link of the depicted

the call was accepted. 200 OK / ACK (Non-INVITE-retransmission mode)

mode) , PRACK and 200 OK (Non-INVITE-retransmission mode)

network has a loss rate of (l) and has a propagation delay of (D) seconds.

two retransmission modes in SIP: **2.3.2.1 INVITE-retransmissions** 

**2.3.2.2 Non-INVITE-retransmissions** 

**2.4 IMS session establishment phases** 

response. (INVITE-retransmission mode).

**2.5 Modeling IMS session establishment** 

retransmission.

mode)

the sender drops the message and stops the retransmission.

Fig. 1. IMS Functional Elements

 The Home Subscriber Server (HSS) contains all the user related subscription data required to handle multimedia sessions.
