GPRS Core Network

The GPRS core network is the central part of the General Packet Radio Service 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.

General support functions

GPRS core structure

The GPRS core 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.

Like GSM in general, GPRS module is an open standards driven system. The standardization body is the 3GPP.

GPRS tunnelling protocol (GTP)

Main article: GPRS tunnelling protocol

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.

GTP-U

for transfer of user data in separated tunnels for each Packet Data Protocol (PDP) context

GTP-C

for control reasons including:

• setup and deletion of PDP contexts
• verification of GSN reachability
• updates; e.g., as subscribers move from one SGSN to another.

GTP'

for transfer of charging data from GSNs to the charging function.

GGSNs and SGSNs (collectively known as GSNs) listen for GTP-C messages on UDP port 2123 and for GTP-U messages on port 2152. This communication is direct within a single network, or in the case of international roaming, via a GPRS roaming exchange (GRX).

The Charging Gateway Function (CGF) listens to GTP' messages sent from the GSNs on TCP or UDP port 3386. The core network sends charging information to the CGF, typically including PDP context activation times and the quantity of data which the end user has transferred. However, this communication which occurs within one network is less standardized and may, depending on the vendor and configuration options, use proprietary encoding or even an entirely proprietary system.

GTP version zero supports both signalling and user data under one generic header. It can be used with UDP (User Datagram Protocol) or TCP (Transmission Control Protocol) on the registered port 3386. GTP version one is used only on UDP. The control plane protocol GTP-C (Control) using registered port 2123 and the user plane protocol GTP-U (User) using registered port 2152.

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 interworking 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. On the other hand, 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.

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).

Serving GPRS Support Node (SGSN)

A Serving GPRS Support Node (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 this SGSN....

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 data

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

• Carry up to about 42 Mbit/s traffic downlink and 5.8 Mbit/s traffic uplink (HSPA+)
• Tunnel/detunnel downlink/uplink packets toward the radio network controller (RNC)
• Carry out mobility management to the level of an RNC for connected mode mobiles

These differences in functionality have led some manufacturers to create specialist SGSNs for each of WCDMA and GSM which do not support the other networks, whilst other manufacturers have succeeded in creating both together, but with a performance cost due to the compromises required.

Access point

Main article: Access point name

An access point is:

• An IP network to which a mobile 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

Example: aricenttechnologies.mnc012.mcc345.gprs

Example: Internet

Example: mywap

Example: hcl.cisco.ggsn

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.

PDP Context

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

• Subscriber's IP address
• Subscriber's IMSI
• Subscriber's
  • Tunnel Endpoint ID (TEID) at the GGSN
  • Tunnel Endpoint ID (TEID) at the SGSN

The Tunnel Endpoint ID (TEID) is a number allocated by the GSN which identifies the tunnelled data related to a particular PDP context.

Several PDP contexts may use the same IP address. The Secondary PDP Context Activation procedure may be used to activate a PDP context while reusing the PDP address and other PDP context information from an already active PDP context, but with a different QoS profile.^[1] Note that the procedure is called secondary, not the resulting PDP contexts that have no such relationship with the one the PDP address of which they reused.

A total of 11 PDP contexts (with any combination of primary and secondary) can co-exist. NSAPI are used to differentiate the different PDP context.

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

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' or GTP prime).

Gb

Interface between the base station subsystem and the SGSN the transmission protocol could be Frame Relay or IP.

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 on-line 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 on-line 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 GSN and the CG. Uses GTP'.

Lg

The interface between the SGSN and the Gateway Mobile Location Center (GMLC), used for location based services.

See also

• Base station subsystem
  • Packet control unit
• Network switching subsystem

References

1. ^ 3GPP TS 23.060, General Packet Radio Service

External links

• 3GPP web page including standards
• easy access to different specs
• GPRS attach and PDP context activation sequence diagrams