LTE Advanced: 3GPP Solution for IMT-Advanced

3.5 EPC and IMS

In the core network side two key entities are separated. First of all there is the Evolved Packet Core (EPC) and then there is the IP Multimedia Sub-system (IMS), as shown in Figure 3.15. All user data goes through EPC and the data related to the operator IMS services (such Voice over LTE) goes also via IMS while traffic to the normal internet does not. The key EPC elements and functionalities are as follows:

• Mobility Management Entity (MME) is the main control element in EPC. It is contacted by the eNodeB for the UE authentication and MME obtains the necessary information from the registers about the subscription. The MME gets the subscription profile from the home network and based on that determines what kind of connections can be allowed for the UE, including parameters such as the maximum data rate.
• The MME is involved in the mobility managements as well. The MME will track where the UE is in the network and instructs the gateways where the data needs to be provided for the UE, updating this information in connection with the handover operation. In RRC_IDLE the MME follows in which tracking area the UE is in so that the network knows from which cell to page the UE in case incoming call.
• Serving Gateway (S-GW) key functionality is to handle the user data tunnelling based on the instructions received from the MME. When the UE moved in the network with active connection, the S-GW stays the same, like anchor point, and just routed the data to the correct eNodeB. ...