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 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 Internet Protocol packet services in GSM and WCDMA networks. The core network also provides support for other additional 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.
GPRS tunnelling protocol (GTP)
GPRS Tunnelling Protocol is the defining IP-based protocol of the GPRS core network. Primarily it is the protocol which allows end users of a GSM or WCDMA network to move from place to place while continuing to connect to the Internet as if from one location at the Gateway GPRS support node (GGSN). It does this by carrying the subscriber's data from the subscriber's current serving GPRS support node (SGSN) to the GGSN which is handling the subscriber's session. Three forms of GTP are used by the GPRS core network.
GPRS support nodes (GSN)
A GSN is a network node which 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 and Serving GPRS support node.
Gateway GPRS support node (GGSN)
The gateway GPRS support node (GGSN) is a main component of the GPRS network. The GGSN is responsible for the internetworking between the GPRS network and external packet switched networks, like the Internet and X.25 networks.
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 MS (mobile station).
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.
Serving GPRS support node (SGSN)
The Serving GPRS Support Node (SGSN) is the node that is serving the MS/UE. The SGSN supports GPRS and/or UMTS. 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.
Common SGSN functions
- Detunnel GTP packets from the GGSN (downlink)
- Tunnel IP packets toward the GGSN (uplink)
- Carry out mobility management as Standby mode mobile moves from one Routing Area to another Routing Area
- Billing user according to data used.
- Mobile Equipment Identity Check Procedure (Gf/S13' interfaces).
- The SMS GMSCs and SMS IWMSCs support SMS transmission via the SGSN.
- The Offline Charging System (OFCS) collects charging records from SGSNs.
- The SGSN contains mechanisms for avoiding and handling overload situations.
- The SGSN communicate with other SGSN(s) and/or MME(s) (Mobility Management Entity) (Gn/S16/S3 interfaces)
GSM/EDGE specific SGSN functions
Enhanced Data Rates for GSM Evolution (EDGE) specific SGSN functions and characteristics are:
- Maximum data rate of approx. 60 kbit/s (150 kbit/s for EDGE) per subscriber
- Connect via frame relay or IP to the packet control unit using the Gb protocol stack
- Accept uplink data to form IP packets
- Encrypt down-link data, decrypt up-link data
- Carry out mobility management to the level of a cell for connected mode mobiles
WCDMA specific SGSN functions
An access point is:
- An IP network to which a mobile set can be connected
- A set of settings which are used for that connection
- A particular option in a set of settings in a mobile phone
When a GPRS mobile phone sets up a PDP context, the access point is selected. At this point an Access Point Name (APN) is determined
This access point is then used in a DNS query to a private DNS network. This process (called APN resolution) finally gives the IP address of the GGSN which should serve the access point. At this point a PDP context can be activated.
The packet data protocol (PDP; e.g., IP, X.25, FrameRelay) context is a data structure present on both the serving GPRS support node (SGSN) and the gateway GPRS support node (GGSN) which contains the subscriber's session information when the subscriber has an active session. When a mobile wants to use GPRS, it must first attach and then activate a PDP context. This allocates a PDP context data structure in the SGSN that the subscriber is currently visiting and the GGSN serving the subscriber's access point. The data recorded includes
Reference points and interfaces
Within the GPRS core network standards there are a number of interfaces and reference points (logical points of connection which 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.
Interfaces in the GPRS network
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- 3GPP web page including standards
- easy access to different specs
- GPRS attach and PDP context activation sequence diagrams