The LTE related core network evolution is referred to as Evolved Packet Core (EPC). LTE architecture is based on the system architecture evolution (SAE) model defined by the 3G Partnership Project (3GPP). EPC consists of the following network elements:
Mobility Management Entity
The Mobility Management Entity (MME) is the LTE mobility management and session management entity of the evolved packet core. MME is responsible for selection of the P-GW, triggering and enabling authentication, and saving the subscriber profile downloaded from the HSS.
The MME handles signaling traffic from the UE/eNodeB through any of the following:
- S1-MME interface
- MME talks to other MMEs through the S10 interface
In the evolved packet core, the MME terminates the control plane with the mobile device. MME is responsible for terminating Non Access Stratum (NAS) signaling such as Mobility Management (MM) and Session Management (SM) information as well as coordinating Idle Mode procedures. The MME also includes the gateway selection inter MME Mobility and authentication of the mobile device.
Serving Gateway
The Serving Gateway (S-GW) is responsible for anchoring the user plane for inter-eNodeB handover and inter-3GPP mobility. S-GW in LTE terminates the LTE RAN and a UE that has only one S-GW at any instance. S-GW handles the user data functionality and is involved in routing and forwarding the data packets to P-GW through S5 interface.
Packet Data Network Gateway
The Packet Data Network Gateway (PDN-GW) is responsible for IP address allocation to the UE. The PDN-GW is also the policy enforcement point to enforce Quality of Service (QOS) specific rules on traffic packets. The PDN-GW terminates the signaling gateway (SG) interface in evolved packet core network.
PDN-GW is responsible for functions such as policy enforcement based on the traffic monitoring characteristics on a subscriber by subscriber basis and ensures the appropriate traffic policy. The PDN-GW connects the UE to external PDNs (Packet Data Networks) and acts as the UE’s default router.
Policy Control and Charging Rules Function
The Policy Charging Rules Function (PCRF) is a functional entity of the 3GPP PCC (Policy and Charging Control) architecture. The PCRF plays a vital role and makes Quality of Service (QOS) and charging policy decisions. The Home PCRF (HPCRF) interfaces with the CSCF, it retrieves IMS layer QOS and makes policy decisions. These Policies are passed down to the P-GW, S-GW, and H-SGW for policy enforcement through the visited PCRF in the regional center.