 Now we are going to look at the inside of the world packet core in terms of the gateway nodes and the control nodes. So the gateway nodes are essentially very different from the control nodes because the control nodes are used for controlling the Inlet and the Connection establishment for the caller party and called party, but when it comes to the gateways gateways provide connectivity to the External network that is why these are known as the gateways. So we'll look at some of the well-known and most important gateways that we have briefly looked at the names We have the mobility management entity the packet gateway and we are going to look at the home subscriber server and We'd look at all this functionality one by one. Let's start with mobility management entity Mobility management actually means handling Handoff it it sounds pretty simple because it can be as simple as Intra Base station controller handoff that is a handoff from one tower to another in other words from one base transceiver to another base transceiver station but There are certain higher and more complex forms of mobility Which need to be equally managed so it means that mobility management entity is Introduced as a very important component in in the EPC it is responsible for establishing the signaling network for mobility management that includes Number one and first and foremost authentication because Whatever a user is Authorized to is entitled to Has a very important impact on how the mobility takes place once user leaves its parent network and enters into a For a network so authentication being for one Though it is Enlisted as one of the last functionalities, but it is always activated beforehand then we have the current movement of the user in terms of Wherever the user goes whichever mobility Management entity module it is associated to so the user has to be tracked and Paged as well. So the paging of the user has to be a continuous activity Then after providing the necessary signaling to establish Handoff related functionality then the bearer path has to be realized The bearer path is activated and Deactivated for successful handoff The importance of the mobility management entity is not only there in terms of the Handoff on the radio Network controller and the radio link controller side. It is also important in the Non-axis stratum non-axis stratum actually means it is the side of the core network Which does not necessarily involve the interaction with the radio side, but in terms of mobility it is equally important because the multimedia messaging service or the Certain advanced SMS services are now required to be terminated and In terms of the SMS delivery MMS delivery There is a requirement From the short message service center SMSC to know the current location beforehand so that the exact delivery can be made to the mobile station and The MME also provides connect connectivity for backward compatibility. That is previous 3 gpp core networks including the LTE a and up till release Release 8 and 9 that includes obviously backward connectivity for 3g networks and Non 3 gpp networks including pure 2g which are only voice and messaging service Then we have another important gateway known as the serving gateway It is known as the serving gateway because it is more related to the user so the user Plane or the user path involves this gateway starting from the E node B or the home E node B in LTE a another equivalent of the access Customer premises equipment. Now. This is a very important gateway It establishes the bearer path for user traffic on behalf of the mobile station both with E node B number one and then on the Internet side that is towards the packet gateway because we have seen the diagram the packet Gateway or the packet data network gateway is towards the external side of the EPC that is towards the internet so it means the serving gateway is doing job of providing connectivity between E node B for a mobile station towards the packet gateway It also has Well-defined interfaces. We'd look at different kinds of interfaces over time for the radio network controller in Other um t s like UTRAN and UTRU as the interface And then the serving gateway also has to have an interface With the MME because whenever handover takes place The bearer path also has to be established. So this is what the serving gateway also has to perform Then we have the packet gateway as another important element in EPC it provides connectivity for the mobiles to the Uh external network or the outside network and in this case it's going to be uh the internet cloud So this is the primary responsibility of the packet gateway another important functionality of packet gateway is that it provides an exchange point or an Anchor point between 3 gpp core and non 3 gpp networks For instance, we know that 3 gpp is primarily itf Sorry itu initiative, but in terms of other non 3 gpp efforts coming from the Itf and from from iEEE. We have standards like Wi-Fi Wi max etc. So it means this packet gateway is since it is dealing with ip So it provides a smooth Anchorage point or a center point where these protocols can have smooth transition from one to the other The packet gateway is now the Entity which is providing connectivity to a mobile user in a certain network to the internet So it has to have the functionality of an edge network The edge network router, which is The ingress or egress router connected to a core router in a classical ip network architecture So it has to have that kind of functional modules as well. This includes filtering Deep packet inspection policy enforcement and support for billing and charging when When a packet enters into The core network it leaves the premises of the edge network. So the charging support has to be Integrated in packet gateway as well