 Let us go into the details of the core parts of IMS and we start with the proxy CSCF. In the overall architecture the core IMS starts with the proxy call session control function. It means that the user equipment, a mobile phone, a tablet or a laptop starts to communicate with PCSCF to interact subsequently with other nodes. It is interesting, just realize that the relationship or interaction between the proxy call session control function and the serving call session control function is one on one. However, the most important element that we will discuss later, the serving call session control function has many interactions with other nodes. It means the proxy call session control function is mainly responsible to get the user equipment through to the IP multimedia subsystem. So we say that it is the first contact point of the user equipment with IMS. Since this is proxy, it can be considered analogous to the SIP proxy. We haven't gone into detail on how exactly SIP protocol works, but for gravity I'll say here that session initiation protocol is all about establishing, managing and terminating voice and video calls. Since this is an ITF standard, it had to be adopted under a different framework and brand or NGN. Let's get back to the proxy CSCF. The proxy CSCF interacts with admission control subsystem to authorize the media components. These media components can only be admitted as long as a certain level of quality of service can be provisioned. While doing so, the proxy CSCF does not alter the request initiated by the user agent. It means it is not authorized to modify the SIP message for initiating the session. Since it is a node, so it has to be recognized by the user equipment, for that an IP address is allotted or assigned to the proxy CSCF by the service provider. And since the user equipment is coming from the access network, so at the time of configuration it could be pre-configured user equipment or it could be user equipment being configured while it makes a request to establish the connection. So it means the user equipment would recognize a proxy CSCF through its IP address. Other features related to the proxy CSCF include that proxy CSCF may even act as a user agent. It means under abnormal or unusual conditions such as a user equipment enters into a mobile network and leaves the mobile network into a no coverage area. In that case, the proxy CSCF may act as a user agent itself. It means it can generate SIP transactions that is SIP messages and it can terminate SIP messages like an endpoint, though we know that the endpoint is supposed to be the user equipment. But since the user equipment is not available, so it has to act on behalf of the user equipment. So this is an abnormal situation that can also be handled by the proxy CSCF. proxy CSCF has additional role of providing end-to-end security association between the IMS and the user equipment. It means that all the SIP sessions which would be established between the user equipment and IMS would now be encrypted and the communication would remain confidential. The physical location of proxy CSCF is typically in a home network, but if a user is roaming, it can leave its home network and enter into a foreign network or a visited network. In that case, the proxy CSCF has to be there in both these networks. From the user perspective, it has to have an entry point into the IMS. Let's quickly have a look at it again. Here we see that on the leftmost bottom side, the user equipment enters into the IMS interaction for associating first for security and then for exchanging SIP messages through the proxy CSCF.