Центральная сеть GPRS: архитектура и функции

Central part of the general packet radio service
The GPRS core network is the central part of the general packet radio service (GPRS) which allows 2G, 3G and WCDMA mobile networks to transmit Internet Protocol (IP) packets to external networks such as the Internet. The GPRS system is an integrated part of the GSM network switching subsystem. The network provides mobility management, session management and transport for IP packet services in GSM and WCDMA networks. The core network also provides support for other functions such as billing and lawful interception. It was also proposed, at one stage, to support packet radio services in the US D AMPS TDMA system, however, in practice, all of these networks have been converted to GSM so this option has become irrelevant. PRS module is an open standards driven system. The standardization body is the 3GPP.

A GSN is a network node that supports the use of GPRS in the GSM core network. All GSNs should have a Gn interface and support the GPRS tunneling protocol. There are two key variants of the GSN, namely gateway GPRS support node and serving GPRS support node.

The gateway GPRS support node (GGSN) is one of two components of the GPRS PS domain. The GGSN together with the SGSN handle packet transmissions between the GPRS network and external packet switched networks, such as the Internet or an X.25 network. From an external network's point of view, the GGSN is a router to a "sub network", because the GGSN ‘hides’ the GPRS infrastructure from the external network. When the GGSN receives data addressed to a specific user, it checks if the user is active. If it is, the GGSN forwards the data to the SGSN serving the mobile user, but if the mobile user is inactive, the data is discarded. In the other direction, mobile originated packets are routed to the right network by the GGSN. The GGSN is the anchor point that enables the mobility of the user terminal in the GPRS/UMTS networks. In essence, it carries out the role in GPRS equivalent to the home agent in Mobile IP. It maintains routing necessary to tunnel the protocol data units (PDUs) to the SGSN that services a particular mobile station (MS). The GGSN converts the GPRS packets coming from the SGSN into the appropriate packet data protocol (PDP) format (e. g., IP or X.25) and sends them out on the corresponding packet data network. In the other direction, PDP addresses of incoming data packets are converted to the GSM address of the destination user. The readdressed packets are sent to the responsible SGSN. For this purpose, the GGSN stores the current SGSN address of the user and his or her profile in its location register. The GGSN is responsible for IP address assignment and is the default router for the connected user equipment (UE). The GGSN also performs authentication and charging functions. Other functions include subscriber screening, IP pool management and address mapping, QoS and PDP context enforcement. With LTE scenario the GGSN functionality moves to SAE gateway (with SGSN functionality working in MME).

The Serving GPRS Support Node (SGSN) The SGSN keeps track of the location of an individual MS/UE and performs security functions and access control. The SGSN is connected to the GERAN base station system through the Gb or Iu interface and/or to the UTRAN through the Iu interface. A SGSN is responsible for the delivery of data packets from and to the mobile stations within its geographical service area. Its tasks include packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The location register of the SGSN stores location information (e. g., current cell, current VLR) and user profiles (e. g., IMSI, address(es) used in the packet data network) of all GPRS users registered with it.

Within the GPRS core network standards, there are a number of interfaces and reference points (logical points of connection that probably share a common physical connection with other reference points). Some of these names can be seen in the network structure diagram on this page.

Ga The interface serves the CDRs (accounting records) which are written in the GSN and sent to the charging gateway (CG). This interface uses a GTP based protocol, with modifications that supports CDRs (Called GTP' and GTP prime). Gb Interface between the base station subsystem and the SGSN the transmission protocol could be Frame Relay or IP. Iu Interface between the Radio Network Controller and the SGSN. The interface exchanges signaling and payload. Gc Interface between the GGSN and HLR so that the GGSN can get the location details of a mobile station. To avoid implementing MAP/SS7 in the GGSN, this interface is optional. When not present, the GGSN routes inquires to the HLR via an SGSN. Gd Interface between the SGSN and the SMS Gateway. Can use MAP1, MAP2 or MAP3. Ge The interface between the SGSN and the service control point (SCP); uses the CAP protocol. Gf The interface between the SGSN and the Equipment Identity Register (EIR), used for checking the mobile's equipment identity number (IMEI) against a list of reported stolen mobile phones. Gi IP based interface between the GGSN and a public data network (PDN) either directly to the Internet or through a WAP gateway. Gmb The interface between the GGSN and the Broadcast Multicast Service Center (BM SC), used for controlling MBMS bearers. Gn IP based interface between SGSN and other SGSNs and (internal) GGSNs. DNS also shares this interface. Uses the GTP Protocol. Gp IP based interface between internal SGSN and external GGSNs. Between the SGSN and the external GGSN, there is the border gateway (which is essentially a firewall). Also uses the GTP Protocol. Gr Interface between the SGSN and the HLR. Messages going through this interface uses the MAP3 protocol. Gs Interface between the SGSN and the MSC (VLR). Uses the BSSAP+ protocol. This interface allows paging and station availability when it performs data transfer. When the station is attached to the GPRS network, the SGSN keeps track of which routing area (RA) the station is attached to. An RA is a part of a larger location area (LA). When a station is paged this information is used to conserve network resources. When the station performs a PDP context, the SGSN has the exact BTS the station is using. Gx The online policy interface between the GGSN and the charging rules function (CRF). It is used for provisioning service data flow based on charging rules. Uses the diameter protocol. Gy The online charging interface between the GGSN and the online charging system (OCS). Uses the diameter protocol (DCCA application). Gz The off line (CDR based) charging interface between the GGSN and the Charging system Uses GTP'. Lg The interface between the SGSN and the Gateway Mobile Location Center (GMLC), used for location based services. S3 This interface is defined between the SGSN and the MME and it is used to exchange user and bearer information in inter system mobility procedures. This interface is implemented using eGTP C.
S4 This interface is defined between the SGSN and the SGW (Serving Gateway). The term "S4 SGSN" refers to a Release 8 SGSN that has at least one set of S4/S3/S16 interfaces enabled. S6d The interface between SGSN and Home Subscriber Server (HSS). It is a Diameter based interface that is used for transferring subscription and authentication data of the user to HSS for authenticating and authorizing user access. S13' This interface is defined between the SGSN and the EIR (Equipment Identity Register). The S13' interface applies only to the S4 based SGSN. The interface is based on Diameter protocol. S16 This interface is defined between two SGSNs and exclusively uses GTPv2 C.

3GPP TS 23.060 V13.4.0 (2015 09)