 Similar to the additional functional elements in IMS, there are certain interconnection nodes which sit at the periphery of the overall IMS architecture and have really important functionalities to be performed. These include HSS, service location function, AS, and interconnection water control function. For the sake of visual clarity, I have omitted the inner details of the IMS architecture. Instead, the interconnection nodes at the periphery of the architecture are being shown to you, except for service location function, the other three nodes are visible here. The home subscriber server is a well-known service which has been present in many telecommunication networks. Similar to home location register and authentication center as in PG network architecture, it acts like a database. It contains subscriber profiles, their contract details such as subscription and the current location of the users in terms of network, aka IP address. The home subscriber server acts like triple-A services through the CSCF entities. Since it is not a very new concept, we can think about it as HLR, VLR, AUC in 3GPP and the HSS node which is given in the 3GPP network architecture for system architecture evolution that came from 4G onwards. Server location function is basically a resolution function. It means on the request of the interrogating CSCF or serving CSCF or even the application server at the request, it resolves the location of the user in terms of the HSS that might have information for a certain user. It means it is the identification of the HSS which actually would be containing the user. Since there could be a situation where there is only one HSS, in that case the subscriber location function is not required. Then we have the application server. Application server deals with hosting, offering and executing services at the request of a service subscriber. Since it is not exactly the direct part of the core IMS architecture, it actually has to interact with the serving CSCF using SIP protocol. The serving CSCF on behalf of the user contacts the corresponding application server. Now the application server could be one application server or it could be a myriad of application servers each hosting a specific service. In that case the serving CSCF has to follow the preferential sequence or the order which is supplied by the HSS on behalf of the user. In case multiple application servers are present, for example a SIP based application server as such the OSA open service access server or the IAM instant messaging server switching function. The border control is actually something that is necessary for IMS to interact with external entities. It means the calling the videoconferencing, audio conferencing, multicasting functionality of IMS has to interact with certain other entities which are external to IMS architecture. So we can say it is kind of border control since it is based on sessions that is IMS is heavily influenced by SIP. We say that it is kind of session border controller so the IPCF provides functionalities not limited to but as basic as firewall services and network address translation because we are talking about IP based network so for all the external entities the IPCF takes the responsibility to interact with the IMS.