FortiCarrier GPRS network common interfaces

GPRS network common interfaces

There are interfaces for each connection on the GPRS network. An interface is an established standard form of communication between two devices. Consider a TCP/IP network. In addition to the transport protocol (TCP) there are other protocols on that network that describe how devices can expect communications to be organized, just like GPRS interfaces.

GPRS network common interfaces

Interfaces between devices on the network

There are a series of interfaces that define how different devices on the carrier network communicate with each other. There interfaces are called Ga to Gz, and each one defines how a specific pair of devices will communicate. For example Gb is the interface between the base station and the SGSN, and Gn is one possible interface between the SGSN and GGSN.

The SGSN and GGSN keep track of the CDR information and forward it to the Charging Data Function (CDF) using the Gr interface between the SGSN and home location register (HLR), Gs interface between the SGSN and MSC (VLR), Gx interface between the GGSN and the Charging Rules Function (CRF), Gy between the GGSN and online charging system (OCS), and finally Gz which is the off-line (CDR-based) charging interface between the GSN and the CG that uses GTP’.

Each of these interfaces on the GPRS network is has a name in the format of Gx where x is a letter of the alphabet that determines what part of the network the interface is used in. It is common for network diagrams of GPRS networks to include the interface name on connections between devices.

GPRS network interfaces, their roles, and billing

Name Device connections that use Traffic Protocol

this interface                          used

Its role or how it affects billing
Ga CDR and GSN (SGSNs and GGSNs) GTP‘ – GTP modified to include CDR role CDR have the accounting records, that are compiled in the GSN and then sent to the Charging Gateway (CG)
Gb MS and SGSN Frame Relay or IP When an IP address moves to a new MS, the old MS may continue to use and bill that IP address.
Gi GGSN and public data networks (PDNs) IP based This is the connection to the Internet. If the GTP tunnel is deleted without notifying the Gi interface, the connection may remain open incurring additional charges. FortiOS Carrier adds this interface to a firewall. See Anti-overbilling with FortiOS Carrier.
Gn SGSN and external SGSNs and internal GGSNs GTP When the GTP tunnel is deleted, need to inform other interfaces immediately to prevent misuse of connections remaining
Gp Internal SGSN and external


GTP open. FortiOS Carrier adds this interface to a firewall.
Gz GSN (SGSN and GGSN) and the charging gateway (CG) GTP‘ Used for the offline charging interface. Ga is used for online charging.

GPRS network common interfaces

Corporate customers may have a direct connection to the Gi interface for higher security. The Gi interface is normally an IP network, though a tunnelling protocol such as GRE or IPsec may be used instead.


Introduction to GTP

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