Core Network: Architecture and Signaling Procedures

The UE is the subscriber’s device, for example a smartphone or an Internet of Things (IoT) module, that attaches to the network over the radio interface. It is the endpoint for all mobility management, session management, and user-plane traffic. The UE reaches the E-UTRAN over LTE-Uu, and over an untrusted non-3GPP access network, such as public Wi-Fi, the UE reaches the ePDG over SWu (TS 23.402 clause 4.2.2).

The E-UTRAN is the Long Term Evolution (LTE) radio access network, made up of E-UTRAN Node Bs (eNodeBs). It provides the radio interface towards the UE over LTE-Uu, relays control-plane signaling to the MME over S1-MME, and relays user-plane traffic to the SGW-U over S1-U. Neighboring eNodeBs interconnect over X2, for example to prepare a handover.

The MME is the control-plane node of the EPC, handling UE authentication, mobility management, tracking area management, paging, and bearer session signaling. The MME carries no user-plane traffic itself and relies on the SGW-C for session establishment towards the user plane. Two MMEs exchange the context of a UE over S10 when the serving MME changes.

The HSS is the master subscriber database of the EPC. It stores subscription profiles, authentication data, and the identity of the MME currently serving each subscriber, and the MME queries the HSS over S6a for authentication vectors and location updates. The 3GPP AAA Server retrieves the authentication data and the subscription data for non-3GPP access from the HSS over SWx (TS 23.402 clause 4.2.2).

The SGW-C is the control-plane function of the Serving Gateway. It terminates S11 signaling from the MME and translates this signaling into Packet Forwarding Control Protocol (PFCP) rules that configure the SGW-U, without handling any user data itself.

The SGW-U is the user-plane function of the Serving Gateway. It forwards the Internet Protocol (IP) packets of the user between the eNodeB and the PGW-U according to the forwarding rules that the SGW-C provisions over Sxa, and it acts as the local mobility anchor for inter-eNodeB handovers.

The PGW-C is the control-plane function of the PDN Gateway, handling session establishment, IP address allocation, and Quality of Service (QoS) and policy signaling. It then provisions the PGW-U with the corresponding PFCP rules.

The PCRF is the policy control node of the EPC. It derives QoS, charging, and traffic-detection rules from subscriber and service policies, and it provisions the PGW-C with these rules over Gx and the TDF-C with application detection and control (ADC) rules over Sd for enforcement in the user plane.

The OCS is the real-time charging system for prepaid and quota-based services. It receives credit-control requests from the PGW-C over Gy and from the TDF-C over Gyn, and it authorizes or denies continued user-plane usage based on the subscriber’s available balance.

The OFCS is the offline, postpaid charging system of the EPC. It collects offline charging information from the PGW-C over Gz and from the TDF-C over Gzn, which are functionally equivalent to the Rf or the Ga reference point (TS 32.240 clause 4.2.2), for billing purposes, without affecting the subscriber’s session in real time.

The PGW-U is the user-plane function of the PDN Gateway. It routes user traffic between the SGW-U and the TDF-U, enforcing the QoS, charging, and packet-filtering rules that the PGW-C provisions over Sxb.

The 3GPP AAA Server is the Authentication, Authorization, and Accounting (AAA) server for non-3GPP access. When the UE connects through an untrusted non-3GPP access network, the 3GPP AAA Server authenticates the UE through the ePDG over SWm, with the authentication data that it retrieves from the HSS over SWx, and it supplies the PGW-C with the required authorization data over S6b (TS 23.402 clause 4.2.2).

The ePDG is the security gateway for untrusted non-3GPP access, such as public Wi-Fi. It terminates the IPsec tunnel that the UE establishes over SWu, relays the authentication of the UE to the 3GPP AAA Server over SWm, and relays control-plane signaling to the PGW-C over S2b-C and user-plane traffic to the PGW-U over S2b-U.

The TDF-C is the control-plane function of the Traffic Detection Function, handling traffic-detection and application-control policy signaling. It receives ADC rules from the PCRF over Sd, it reports usage to the OCS over Gyn and to the OFCS over Gzn, and it provisions the TDF-U with the corresponding PFCP rules over Sxc (TS 23.214 clauses 4.2.3 and 5.3.1).

The TDF-U is the user-plane function of the Traffic Detection Function. Sitting inline between the PGW-U and the external PDN, it performs deep packet inspection and traffic detection on user traffic according to the rules that the TDF-C provisions over Sxc.

The PDN is the external packet data network, for example the internet, that the UE obtains connectivity to, reached from the TDF-U over SGi.