 IMS or IP multimedia subsystem is one of the most useful and widely deployed network elements that forms the center stage of next generation network for providing multimedia services. It would be interesting to devote couple of lectures for IMS to understand what are the internal details and functional aspects of IMS. Let us start with understanding its history and we quickly take the overview of the architecture that we shall understand in detail in due course of time. IMS was first adopted with release 5 by 3GPP. It was conceived to be basically an application development and execution environment. Later in release 7, IMS was refocused to provide classical PSTN kind of services on IP based networks. A significant advancement took place when in release 8, IMS was adopted by taking into consideration all functional and technical requirements from the international bodies such as ITU, 3GPP-2, T-SPAN, cable labs, etc. Interestingly, up till this stage, it was primarily the voice for IP kind of services that remained largely the focus of IMS. However, as we are experiencing, there is much more to IMS than what was initially conceived although it was 3GPP that brought in and promoted the idea of IMS. However, with the emergence of NGN as the unifying architecture for the services for all different kinds of access networks, it was adopted from 3GPP into the service spectrum. When we say service spectrum, you may recall that it relates to all the functional aspects and network elements which are related to provisioning of services to the end user. While we were talking about the fixed mobile convergence, we actually were interested in coming up with a mechanism that allows us to integrate multiple services from different access networks. IMS provides support for integrating FMC as a legacy technology because it has widely been deployed. So IMS not only provides new and emerging services, it also provides backward compatibility. IMS is essentially SIP-based mechanisms. Session initiation protocol that was recommended and standardized by the Internet Engineering Task Force is all about providing mechanisms for signaling, whether in-band or out-of-band, to manage and control the voice sessions and IMS is all about that. The concept of service provisioning here in IMS is actually referred to as sessions which can be categorized as multimedia sessions or there is something known as non-session-based services which are present service or localization service, messaging service or message passing service and the PSTN and ISDN simulation to relate how exactly these classical services could be provided into the NGN network architecture. Let's quickly look at the seemingly complex structure of IMS. The IMS architecture contains the core nodes shown to you in the innermost rectangle with certain entities known as the call session control functions, certain variants of it such as SCSCF, ICSCF, PCSCF. These are the core IMS nodes supported by BGCF, MRFC and MGCF. Let's look at their abbreviations. We are going to spend considerable time in discussing these network and node elements. However, for the sake of brevity, we say that we start with the core including the call session control functions including P for proxy, S for serving, I for interrogating. Then we have the breakout gateway control function, media gateway control function and media resource function MFRC, multimedia resource function controller. The overall architecture is represented primarily by the core of IMS and then certain nodes which are into the support node role. It means that the primary responsibility lies with these core IMS nodes which are supported by non-core entities. In 3GPP, these were referred to as functions but in NGN, these are simply known as the functional entities and as I just said, the most important responsibility lies with the functional entities known as the call session control functions including the proxy, serving and interrogating. In subsequent lectures, we are going to go into more detail.