A key difference from 2G/3G Core networks is that the EPC is defined to support packet-switched traffic only with IP based protocols’ Interfaces. It means that all services will be delivered through packet connections, including voice; and it means saving for operators, because they use a single network for all services.
There are two principle functionalities in the Evolved Packet Core, one is Packet routing and transfer functions, the other one is mobility management. These functionalities are applied in below main network elements.
- SGW – Serving Gateway; router, packet marking, anchor for inter-eNodeB handover, some accounting
- MME – Mobility Management Entity; NAS signalling point, admission control, bearer setup, authentication, roaming functions, selects SGW
- P-GW – Packet Gateway; date entry/exit point, packet inspection/filtering, IP address allocation, mobility anchor for non-3GPP handover
EPC Network Architecture
There are several functional entities within the core Evolved Core Network. Within the user plane the core network is the gateway between the access network and the PDNs (e.g., the Internet) that support the interfaces, mobility needs and the differentiation of QoS flows. The gateway may be split in two separate nodes with an optional S5 interface. The two logical gateway entities are the Serving Gateway (S-GW) and the PDN Gateway (P-GW).
The Serving Gateway (S-GW) acts as a local mobility anchor, forwarding and receiving packets to and from the eNodeB currently serving the UE.
The PDN Gateway (P-GW) interfaces with the external PDNs, such as the Internet and IMS. It is also responsible for several IP functions, such as address allocation, policy enforcement, packet classification and routing, and it provides mobility anchoring for non-3GPP access networks.
The control plane functions are performed by the MME which is connected to the gateway via the S11 interface.
The PCRF makes policy decisions based on information obtained via the Rx interface. It confirms that the information received is consistent with policies defined in the PCRF. The PCRF will authorize QoS resources and will decide if new resources are required for existing connections. PCRF mechanism is used also in 3G network.
HSS (Home Subscriber Server)
The HSS has a combination function of the HLR (Home Location Register) and the AuC (Authentication Centre).
The SGSN enables:
- Inter EPC node signalling for mobility between 2G/3G and E-UTRAN 3GPP access networks;
- PDN and Serving GW selection: the selection of S GW/P GW by the SGSN is as specified for the MME;
- MME selection for handovers to E-UTRAN 3GPP access network.
Serving Gateway (S-GW)
The S-GW terminates the interface towards the E-UTRAN and is the main packet routing and forwarding node in the EPC. It is able to provide transport level packet marking in the uplink and downlink by setting the Code Point dependent upon the QoS class identifier (QCI) of the associated EPS bearer which may be used for QoS management by other network elements. The S-GW provides accounting functions based on the user and QCI inter-operator charging and uplink and downlink charging per UE, PDN, and QCI for roaming within home routed traffic. The S-GW acts as a local anchoring point for inter eNodeB handover and assists in the reordering function by sending one or more “end marker” packets to the source eNodeB immediately after switching the path. The S-GW acts as a 3GPP anchoring point for inter-RAT handovers by providing the termination point for the S4 interface and relaying traffic between 2G/3G systems and the PDN-GW (P-GW). It provides idle mode functions such as packet buffering and initiation of network triggered service request.
The S-GW is one of the Lawful Interception points in the network. There are a number of interfaces associated with the S-GW including:
- S11 interface that connects to the MME
- S1-Uinterface connection to the eNodeB
- The interface between the S-GW and P-GW is the S5/S8 interface.
Packet Data Network (PDN) Gateway (P-GW)
The PDN Gateway is the link between the mobile device and the services that reside in an external packet network such as IMS. The P-GW provides an entry and exit point for UE connectivity with external data networks and terminates the SGi interface towards the PDN and acts as a mobility anchor between 3GPP and non-3GPP technologies such as 3GPP2 CDMA2000 and WiMAX. The P-GW is also responsible for the allocation of user IP-address. The P-GW also provides support for charging, packet filtering and lawful interception.
There are a number of interfaces associated with the P-GW including:
Mobility Management Entity (MME)
The Mobility Management Entity (MME) is the primary signalling node in the EPC, Non Access Stratum (NAS) signalling is terminated at this point and responsible for signalling related to bearer establishment and authentication of the UEs through interaction with the Home Subscriber Server (HSS). It is also the decision point for SGW selection, and MME, SGW selection during handover where EPC node change is necessary. The MME handles roaming functions such as allocation of temporary identities, admission control and communication with the home HSS on the S6a interface. There are a number of interfaces associated with the MME including:
- MTN Academy
- Telecoms Academy
- SAE and the Evolved Packet Core, Author(s): Magnus Olsson, Shabnam Sultana, Stefan Rommer, Lars Frid and Catherine Mulligan, ISBN: 978-0-12-374826-3