 Now, we are going to look at the control components, which are there in Evolved Packet Core, which help us to determine if a certain call is admissible. And if that call is admitted into the network, then depending upon the profile, it is built. So we have two important elements, specifically we see here, we have the Home Subscriber Server and the Policy and Charging Rule Function. Now these two control network elements in the Evolved Packet Core are working together with the Gateway nodes in the Evolved Packet Core. So we see that essentially there is a signalling path which is established between the HSS-PCRF with other network elements in the APC because as such they are not responsible for providing circuit for bearer path. So the Home Subscriber Server is like a database. It is a database which is centralized in the Evolved Packet Core to provide useful user-related information depending upon the context, so it means once a certain connection is admitted, then Home Subscriber Server is invoked to carry out the activities step by step. For instance, the Authentication Center is now integrated. Remember in 2G Networks, Authentication Center was part of the HLR, VLR and OSS Server Center. Here it is integrated into the HSS. So the Authentication Center provides the necessary hash function, the SIM-related information which is validated and the user is now allowed to enter into the network and avail services. The location is also provided by the HSS, which is essentially nothing but the current mobility management entity to which the user is at the moment connected to. The subscriber profile determines the level of mobility and the data rate and other service-related information that a user is entitled to. That means certain services. At the same time, the packet data networks to which a certain user is allowed to be connected to are also listed. The Home Subscriber Server actually has a very important relationship with the mobility management entity because it is going to provide functionality in the wake of roaming if roaming is permissible for this particular user. Another important entity which is used for providing the triple A kind of service for the LTE, LTEA networks in release 8 onwards is the Policy and Charging Rules functions. Now the Policy and Charging Rules functions is a server that provides a variety of billing, identification, subscription, notification service. So it is more of a software-based network node that provides policy repository that is a number of policies can be hosted and a user can be treated depending upon the activation and enforcement of a certain policy and then certain charging and billing model like a prepaid customer or a postpaid customer. The PCRF now also has an important role because if you remember the original 3GPP architecture for the next generation networks, we had the transport stratum and above it we had the services stratum. So the Policy and Charging Rules functions is also important because it integrates and combines these. PCRF now detects if a certain call is admitted. So it depending upon the flow ID, the service flow, it invokes relevant and associated rules for it in real time. It means that whatever is taking place is taking place in a matter of 2 milliseconds because you know that for audio, visual and multimedia content delivery in next generation networks, the overall delay should not preferably exceed 150 milliseconds otherwise we'd have a jittery and unpleasant kind of experience. So real time actually means that PCRF works very fast. So it enforces the charging policy. It means that if a user has an account or a wallet that we already have known by now, deduction would start to be made from the user wallet in real time. Now there's another important role of this charging policy and charging rule function that it implements the deduction or the transaction deduction model through the policy and charging enforcement function. Now we should not be going too much into the details but expectedly PCRF is a large server with a lot of functionality. PCEF is a service which is available on that server once a user is admitted into the network. Now this enforcement function is required whenever and wherever a certain data path or the better path is established. In EPC, since we are talking about IP multimedia subsystem that is SIP based services, if a certain application entity bypasses the direct, the intermediate nodes and connects to the serving gateway directly, then the application function or the user API uses again the services available within this PCRF server to apply the policy and charging control rules and functions.