5.4 Logical Architecture

23.0603GPPGeneral Packet Radio Service (GPRS)Release 17Service descriptionStage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5.4.0 General

When based on Gn/Gp interfaces, the GPRS Core Network functionality is logically implemented on two network nodes, the Serving GPRS Support Node and the Gateway GPRS Support Node. When based on S4/S5/S8 interfaces, the GPRS Core Network functionality is logically implemented on three network nodes, the Serving GPRS Support Node, the Serving Gateway and the PDN Gateway. No inference should be drawn about the physical configuration on an interface from figure 2 or figure 2a.

Figure 2: Overview of the GPRS Logical Architecture when based on Gn/Gp interfaces

NOTE: Between S4-SGSN and HSS, the interface is Diameter based (S6d). However, to assist with the SGSN transition the use of MAP based Gr between the S4-SGSN and the HSS is not precluded.

Figure 2a: Overview of the GPRS Logical Architecture when based on S4/S5/S8 interfaces

5.4.1 GPRS Core Network Nodes

5.4.1.1 General

A GPRS Support Node (GSN) contains functionality required to support GPRS functionality for GERAN and/or UTRAN. In one PLMN, there may be more than one GSN.

The SGSN and GGSN functionalities may be combined in the same physical node, or they may reside in different physical nodes. The SGSN and the GGSN contain IP or other (operator’s selection, e.g. ATM-SVC) routeing functionality, and they may be interconnected with IP routers.

5.4.1.2 Gateway GPRS Support Node

The Gateway GPRS Support Node (GGSN) is the node that is accessed by the packet data network due to evaluation of the PDP address. It contains routeing information for PS-attached users. The routeing information is used to tunnel N‑PDUs to the MS’s current point of attachment, i.e. the Serving GPRS Support Node. The GGSN may request location information from the HLR via the optional Gc interface. The GGSN is the first point of PDN interconnection with a PLMN supporting GPRS (i.e. the Gi reference point is supported by the GGSN). GGSN functionality is common for all types of RANs.

For emergency bearer service, the GGSN shall block any traffic that is not from/to addresses of network entities (e.g. P‑CSCF) providing emergency service. The list of allowed addresses may be configured by the operator.

5.4.1.3 Serving GPRS Support Node

The Serving GPRS Support Node (SGSN) is the node that is serving the MS. The SGSN supports GPRS for A/Gb mode (i.e. the Gb interface is supported by the SGSN) and/or Iu-mode (i.e. the Iu interface is supported by the SGSN). At PS attach, the SGSN establishes a mobility management context containing information pertaining to e.g. mobility and security for the MS. At PDP Context Activation, the SGSN establishes a PDP context, to be used for routeing purposes, with the GGSN that the subscriber will be using.

In Iu mode, the SGSN and RNC may be interconnected with one or more IP routers.

In Gn/Gp mode and when the SGSN and the GGSN are in different PLMNs, they are interconnected via the Gp interface. The Gp interface provides the functionality of the Gn interface, plus security functionality required for inter-PLMN communication. The security functionality is based on mutual agreements between operators.

In Gn/Gp mode, the SGSN interworks signalling on the Gn/Gp interface with Iu/Gb interface signalling. In S4 mode, the SGSN interworks signalling on the S4 interface with Iu/Gb interface signalling. One SGSN may have some MSs using Gn/Gp mode and other MSs using S4 mode.

The SGSN may send location information to the MSC/VLR via the optional Gs interface. The SGSN may receive paging requests from the MSC/VLR via the Gs interface.

The SGSN interfaces with the GSM‑SCF for optional CAMEL control using Ge reference point. Depending on the result from the CAMEL interaction, the session and packet data transfer may proceed normally. Otherwise, interaction with the GSM‑SCF continues as described in TS 23.078 [8b]. Only the GSM‑SCF interworking points are indicated in the signalling procedures in this specification.

If there is already an emergency bearer activated, the SGSN shall reject any additional PDP context activation request by the MS for emergency services.

5.4.1.4 Serving Gateway

The functionality of the Serving Gateway is defined in TS 23.401 [89] with the following additions and exceptions:

The Serving Gateway:

‑ terminates the user plane interface towards the UTRAN when the Direct Tunnel feature is in use;

‑ is the local Mobility Anchor point for SRNS relocation when the Direct Tunnel feature is in use;

‑ is the local Mobility Anchor for inter-SGSN routeing area update;

‑ if E‑UTRAN is not in use in that PLMN, need not support functionality related to inter eNodeB mobility.

5.4.1.5 PDN Gateway

The functionality of the PDN Gateway is defined in TS 23.401 [89].

5.4.2 Packet Domain PLMN Backbone Networks

There are two kinds of backbone networks. These are called:

– intra-PLMN backbone network; and

– inter-PLMN backbone network.

The intra-PLMN backbone network is the IP network interconnecting SGSNs, GGSNs, Serving GWs and PDN GWs within the same PLMN and it interconnects GGSNs and Serving GWs with RNCs if Direct Tunnel functionality is supported.

The inter-PLMN backbone network is the IP network interconnecting SGSNs, GGSNs, Serving GWs and PDN GWs with intra-PLMN backbone networks in different PLMNs.

Figure 3: Intra- and Inter-PLMN Backbone Networks when based on Gn/Gp interfaces

Figure 3a: Intra- and Inter-PLMN Backbone Networks when based on S5/S8 interfaces

Every intra-PLMN backbone network is a private IP network intended for GPRS packet domain data and signalling only. A private IP network is an IP network to which some access control mechanism is applied in order to achieve a required level of security. Two intra-PLMN backbone networks are connected via the Gp interface using Border Gateways (BGs) and an inter-PLMN backbone network. The inter-PLMN backbone network is selected by a roaming agreement that includes the BG security functionality. The BG is not defined within the scope of GPRS. The inter-PLMN backbone can be a Packet Data Network, e.g. the public Internet or a leased line.

5.4.3 HLR/HSS

The HLR/HSS contains GPRS and EPS subscription data and routeing information. The HLR/HSS is accessible from the Gn/Gp SGSN via the Gr interface, from the S4-SGSN via the S6d interface and from the GGSN via the Gc interface. For roaming MSs, the HLR/HSS may be in a different PLMN than the current SGSN.

NOTE: As specified in clause 6.4, "between S4-SGSN and HSS, the interface is Diameter based (S6d); however, to assist with SGSN transition the use of MAP based Gr between the S4-SGSN and HSS is not precluded".

5.4.4 SMS‑GMSC and SMS‑IWMSC

The SMS‑GMSC and SMS‑IWMSC are connected to the SGSN via the Gd or the Gdd interface to enable the SGSN to support SMS.

NOTE: The interface between the SGSN and the SMS GMSC/SMS IWMSC when MAP-based is referred to as Gd and when Diameter-based is referred to as Gdd. Gd and Gdd interfaces are functionally equivalent.

5.4.5 Mobile Stations (A/Gb mode)

An A/Gb mode MS operates in one of three modes of operation. The mode of operation depends on the network domains that the MS is attached to, i.e. only PS or both PS and CS domain, and upon the MS’s capabilities to operate PS and CS domain services simultaneously.

– Class‑A mode of operation: The MS is attached to both PS and CS domain, and the MS supports simultaneous operation of PS and CS domain services.

– Class‑B mode of operation: The MS is attached to both PS and CS domain, but the MS can only operate one set of services, PS or CS services, at a time.

– Class‑C mode of operation: The MS is exclusively attached to the PS domain.

The three modes of operation are defined in TS 22.060 [3].

NOTE: Other technical specifications may refer to the MS modes of operation as GPRS class‑A MS, GPRS class‑B MS, and GPRS class‑C MS.

5.4.6 Mobile Stations (Iu mode)

An Iu mode MS operates in one of three modes of operation. However, these operation modes are different from the ones of an A/Gb mode MS due to the capabilities of an Iu mode RAN to multiplex CS and PS connections, due to paging co-ordination for PS services and CS services that are offered by the CN or the UTRAN/GERAN-Iu, etc. The different Iu mode MS operation modes are defined as follows:

– CS/PS mode of operation: The MS is attached to both the PS domain and CS domain, and the MS is capable of simultaneously signalling with the PS and CS core network domains. This mode of operation is comparable to the class‑A mode of operation defined for A/Gb mode. The ability to operate CS and PS services simultaneously depends on the MS capabilities (for example an A/Gb mode MS of class B, which can not operate simultaneously CS and PS services, may have the same limitations when changing to Iu mode and CS/PS mode of operation).

– PS mode of operation: The MS is attached to the PS domain only and may only operate services of the PS domain. However, this does not prevent CS-like services to be offered over the PS domain (e.g. VoIP). This mode of operation is equivalent to the A/Gb mode GPRS class‑C mode of operation.

– CS mode of operation: The MS is attached to the CS domain only and may only operate services of the CS domain. However, this does not prevent PS-like service to be offered over the CS domain. The CS mode of operation is outside the scope of this specification.

All combinations of different operation modes as described for A/Gb mode and Iu mode MSs shall be allowed for GERAN and UTRAN multisystem terminals.

5.4.7 Charging Gateway Functionality

The Charging Gateway Functionality (CGF) described in TS 32.251 [70] is a function of the Offline Charging System (OFCS) which is described in TS 32.240 [94].

5.4.8 PCRF

The PCRF is the policy and charging control element. PCRF functions are described in more detail in TS 23.203 [88].

5.4.9 HNB subsystem

A HNB subsystem consists of a HNB, a HNB GW and optionally a Local GW.

The Local IP Access function and SIPTO at the Local Network with L-GW function collocated with HNB is achieved using a Local GW (L-GW) co-located with the HNB.

Figure 5.4.9-1 and figure 5.4.9-2 illustrate LIPA and SIPTO at the Local Network with L-GW function collocated with HNB for HNB connected to respectively EPC and Gn-based SGSN.

Figure 5.4.9-1: LIPA and SIPTO at the Local Network with L-GW function collocated with HNB architecture for HNB subsystem connected to EPC

Figure 5.4.9-2: LIPA and SIPTO at the Local Network with L-GW function collocated with HNB architecture for HNB connected to a Gn-based SGSN

The HNB Subsystem appears as an RNS to the core network and is connected by means of the Iu-CS interface to the MSC and by means of the Iu-PS interface to the SGSN. When LIPA or SIPTO at the Local Network L-GW function collocated with HNB is activated, the HNB subsystem also has following interface to the core network i.e.:

– For S4-SGSN, an S5 interface between the S-GW and the Local GW;

– For Gn-based SGSN, a Gn interface between the SGSN and the Local GW.

NOTE 1: In this specification and for simplification the term RNC (or RNS if used instead) refers to the HNB subsystem if the MS accesses the network via a HNB unless stated otherwise.

NOTE 2: Detailed functions of HNB and HNB GW are described in TS 25.467 [103].

The Local GW is the gateway towards the IP networks (e.g. residential/enterprise networks, Internet) associated with the HNB. The Local GW functions are described in TS 23.401 [89], clauses 4.4.9 and 4.3.15a:

5.4.10 CSG Subscriber Server

CSG Subscriber Server (CSS) is an optional element that stores CSG subscription data for roaming subscribers. The CSS stores and provides VPLMN specific CSG subscription information to the SGSN. The CSS is accessible from the Gn/Gp SGSN via the Ghv interface and from the S4-SGSN via the S7d interface. The CSS is always in the same PLMN as the current SGSN.

If the same CSG ID exists in both CSS subscription data and HLR/HSS subscription data, the CSG subscription data from HLR/HSS shall take precedence over the data from CSS.

Figure 5.4.10-1 illustrates CSS connected to SGSN and MSC/VLR.

Figure 5.4.10-1: CSS connected to SGSN and MSC/VLR

5.4.11 RAN Congestion Awareness Function (Iu mode)

The RAN Congestion Awareness Function (RCAF) is an element that provides RAN User Plane Congestion Information (RUCI) to the PCRF to enable the PCRF to take the RAN user plane congestion status into account for policy decisions.

The RCAF collects information related to user plane congestion from the RAN’s OAM system based on which the RCAF determines the congestion level, the identifier of the impacted UTRAN cell and the IMSIs served by the impacted cell.

Via the Nq’ interface the RCAF retrieves the APNs of the active PDN connections of the UEs in the congested cell.

Via the Np reference point, the RCAF sends the RUCI to the PCRFs serving the UEs’ PDN connections.

The RCAF is available for UTRAN access only.

NOTE 1: The details of congestion reporting to the PCRF and the Np reference point are specified in TS 23.203 [88].

NOTE 2: In case of roaming or RAN sharing as specified in TS 23.251 [83], Np is an inter-operator reference point. Whether Np applies in case of roaming and RAN sharing is subject to inter-operator agreements.

Figure 5.4.11-1 illustrates the RCAF connected to the S4-SGSN or Gn/Gp-SGSN. The RCAF is located in the same PLMN as the serving SGSN except in network sharing scenarios where the RCAF belongs to the RAN operator.

Figure 5.4.11-1: RCAF connected to SGSN