 The most important CSEF that other CSEFs interact with and you can say it is at the center of the core of IMS, it won't be an exaggeration. Let's explore the functionality of serving CSEF in more detail. Here you can see that after the proxy CSEF, the serving CSEF comes into play. It has interaction with almost every other node in the IMS architecture. It means serving CSEF not only can interact with other nodes in the architecture, it can even replicate or proxy any other node in case that fails. Because only through these network nodes, only through SCSEF, these IMS architecture nodes interact with each other. Since it is central to the IMS, it performs all the functionality that a typical SIP server performs, that is managing the connections and opening and terminating the sessions. So, serving CSEF acts not only as a SIP server, it acts as a registrar as well. It means like a SIP registrar server, it can receive registration requests from user equipment so that the presence and location of these user equipment be known to interested calling parties through an appropriate location service which in this case is a location server which is positioned or located in a home subscriber server. So, serving CSEF has to deal with the user equipment. Each user equipment comes with an IP address. So, the responsibility of serving CSEF is to assign a SIP address to an IP address. Correspondingly, a binding has to take place or a pair or a tuple has to be created that associates an IP address with SIP address. From a user perspective, for a subscriber, the serving CSEF is always located in the home network that is the parent network in which the user is initially registered. For triple A services and certain other services, serving CSEF adopts diameter protocol which essentially is an extension to the radius server. So, using diameter protocol, serving CSEF actually can access the database in the home subscriber server to query the details of a user such as the subscription, whether it is prepaid, postpaid, balance related information from home subscriber server. However, this access by serving CSEF does not alter the content at the HSS rather it is only fetched and cached, it is not modified. So, serving CSEF can write data or upload data to HSS when it is able to establish user related information which it gets at that particular moment while the user is interacting with serving CSEF to the HSS. Now, this information sometimes is temporarily needed to be stored at HSS in case the user equipment is either roaming or it changes its profile. While performing the essential duties of a SIP server and a registrar server, the serving CSEF actually implements the policies related to QOS, charging, billing, admission control on behalf of the operator. It also provides SIP message forwarding service amongst the IMS nodes as you saw in the figure that it is at such an important location and it has a connection with all the other nodes that it allows these SIP messages to be exchanged between the nodes via itself. It also provides routing of the SIP messages based on the E.164 numbering that is electronic numbering which is used in PSTNs. So, it means SIP translates the enum based addressing into the more familiar and easily implementable IP based addressing. The serving CSEF lies in the home network, agreed, but sometimes for the sake of load balancing or high scalability, adaptability in case of failure to ensure that system remains available, several serving CSEFs can be deployed within one IMS. However, from an individual user equipment or subscriber perspective, only one serving CSEF is assigned to a user. Since serving CSEF is almost like a hub in a star topology, so it can act like any other node. For instance, it can behave like a proxy server, so it can actually replicate the functionality of proxy CSEF. Again, it can act like a user agent like proxy CSEF would do to initiate or terminate sub sessions if needed. Sometimes, when services are available across diverse platforms, each platform can interact with serving CSEF to establish interaction subsequently with other service platforms. It is possible through SIP based IMS service control interface. It means, again, the serving CSEF implements interaction amongst multiple service platforms through service control interface. Let's finally look at the diagram. Here we see that the serving CSEF is located at such an important position that it can interact with other nodes and can replicate the functionality of other nodes.