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Detunnel GTP packets from the GGSN (downlink)

Tunnel IP packets toward the GGSN (uplink)

Carry out mobility management as Standby mode mobile moves from Routing Aréa to Routing Aréa.

Carry up to about 60  kbit/s (150 kbit/s for Edge) traffic per subscriber

Connect via frame relay or IP to the PCU using the Gb protocol stack

Accept uplink data to form IP packets

Encrypt downlink data, decrypt uplink data

Carry out mobility management to the level of a cell for connected mode mobiles;

Carry up to about 300 kbit/s traffic per subscriber (R99)

Carry up to about 7.2 mbit/s traffic downlink & 2.0 mbit/s traffic uplink (HSPA)

Tunnel/detunnel downlink/uplink packets toward the RNC

Carry out mobility management to the level of an RNC for connected mode mobiles.

These differences in functionality have led some manufacturers to

créate specialist SGSNs for éach of WCDMA and GSM which do not support

the other networks, whilst other manufacturers have succeeded in

créating both together, but with a performance cost due to the compromises

Main article  Access Point Name

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

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 PDP(Packet Data Protocol, e.g. IP, X.25, FrameRelay) context is a data structure present on both the SGSN and

the 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 subscribers

access point.   The data recorded includes.

Tunnel ID (TEID) at the GGSN

Tunnel ID (TEID) at the SGSN

The tunnel ID (TEID) is a number allocated by the GSN which identifies

the tunnelled data related to a particular PDP context.

There are two kinds of PDP contexts.

Has a unique IP address associated with it

Shares an IP address with another PDP context

Is créated based on an existing PDP context (to share the IP address)

Secondary PDP contexts may have different Quality Of Service settings

A total of 11 PDP contexts (with any combination of Primary and Secondary) can co-exist.

Within the GPRS core network standards there are a number of

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

Gb - Interface between the Base Station Subsystem and the SGSN the transmission protocol could be Frame Relay or IP.

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.

Ga - The interface servers the CDRs (Accounting records) which are written in the GSN and sent to the CG (Charging Gateway).  This interface uses an GTP Protocol, with extensions that supports CDRs (Called GTP' or GTP prime).

Gr - Interface between the SGSN and the HLR. Messages going through this interface uses the MAP3 Protocol.

Gd - Interface between the SGSN and the SMS Gateway. Can use MAP1, MAP2 or MAP3.

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 RA (Routing Aréa) the station is attached to. An RA is a part of a larger LA (Location Aréa). 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.

Gi - The interface between the GGSN and a PDN (Public Data Network) either directly to the Internet or through a WAP gateway. Uses the IP protocol.

Ge - The interface between the SGSN and the SCP (Service Control Point). Uses the CAP Protocol.

Gx - The on-line policy interface between the GGSN and the CRF (Charging Rules Function). It is used for provisioning service data flow based charging rules. Uses the Diameter Protocol.

Gy - The on-line charging interface between the GGSN and the OCS (Online Charging System). Uses the Diameter Protocol (DCCA application).

Gz - The off-line charging interface between the GSN and the CG (Charging Gateway). Uses the CDRs (Accounting records).

Gmb - The interface between the GGSN and the BM-SC (Broadcast-Multicast Service Center), used for controlling MBMS béarers.