Core Network: Architecture and Signaling Procedures

The UE is the subscriber’s device, for example a mobile phone or a data modem, that connects to the UTRAN over the Uu radio interface. It is the endpoint for all mobility management, session management, and user-plane traffic.

The UTRAN is the Universal Mobile Telecommunications System (UMTS) radio access network, made up of Radio Network Subsystems (RNSs), each of which contains a Radio Network Controller (RNC) and the Node Bs that the RNC controls. It provides the Uu radio interface towards the UE and relays packet-switched signaling and user data to the SGSN over IuPs. Two RNCs interconnect over Iur, for example so that a Drift RNS can provide radio resources to the Serving RNS of a UE (TS 25.401 clause 6).

The SGSN is the node that serves the UE, handling mobility management, session management, and the forwarding of user data between the UTRAN and the GGSN. Unlike the Mobility Management Entity (MME) in the EPC, the SGSN usually carries both control-plane signaling and user-plane traffic, and it reaches the GGSN over Gn within the same Public Land Mobile Network (PLMN) or over Gp towards another PLMN. Two SGSNs also interconnect over Gn, for example to transfer the context of a UE when the serving SGSN changes.

The GGSN is the gateway between the GPRS Core and the external packet data network. It anchors the Packet Data Protocol (PDP) address of the UE and routes packets between the SGSN over Gn/Gp and the PDN over Gi. The GGSN requests online credit control from the OCS over Gy and sends offline charging information to the OFCS over Gz (TS 23.203 clauses 5.2.4 and 5.2.5).

The HSS/HLR is the subscriber database of the GPRS Core. It stores subscription data, authentication data, and the identity of the SGSN currently serving each subscriber; the SGSN queries the HSS/HLR over Gr (S6d for an S4-SGSN) for authentication vectors and location updates, and the GGSN can query it over the optional Gc interface for the address of the serving SGSN.

The PCRF is an optional policy and charging control element. It makes policy control decisions and flow-based charging control decisions, and it provides the GGSN with the resulting rules over Gx for enforcement.

The OCS is the real-time charging system for prepaid and quota-based services. It receives credit-control requests from the GGSN over Gy and grants or denies the quota for continued user-plane usage based on the available balance of the subscriber.

The OFCS is the postpaid charging system. It receives over Gz the Charging Data Records (CDRs) that the GGSN generates, and it forwards them to the billing domain without affecting the session of the subscriber in real time. In the GPRS Core, Gz is functionally equivalent to the Ga reference point (TS 32.240 clause 4.2.2 and TS 32.251 clause 4.2).

The PDN is the external packet data network, for example the internet or a corporate intranet, that the UE obtains connectivity to, reached from the GGSN over Gi.