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.
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.
The Carrier-enabled FortiGate unit only provides protection on the Gn, Gp, and Gi interfaces.
GPRS network interfaces, their roles, and billing
Name Device connections that use this interface
Traffic Protocol used
Its role or how it affects billing
Ga CDR and GSN (SGSNs and GGSNs)
GTP‘ – GTP mod- ified 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 net- works (PDNs)
Gn SGSN and external SGSNs and internal GGSNs
IP based This is the connection to the Internet. If the GTP tunnel is deleted without noti- fying 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.
GTP When the GTP tunnel is deleted, need to inform other interfaces immediately to
prevent misuse of connections remaining
Gp Internal SGSN and external
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.
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.
Packet flow through the GPRS network
To better understand the GPRS network, we will follow the path data takes for a normal connection. For this example a call placed from a mobile phone involves accessing services on the Internet.
Sample GPRS network topology
1. A mobile phone places a call using a mobile station (MS). This connection between the mobile phone and the MS is a radio connection using one of the radio access technologies. See Radio Access Technology (RAT) type.
2. The MS connects to a GPRS System Node (GSN) specifically a Sending GSN. This connection uses the Gb interface and typically uses IP address or Frame Relay.
3. The SGSN checks the mobile phone information located in the home location register (HLR) or visitor location register (VLR) to ensure there is subscriber information for that phone. If this mobile phone is from another network, the SGSN uses the VLR and updates its home carrier’s information with its current location and information. This connection involves the Ga or Gz interfaces, and uses the GTP’ protocol for communication.
4. The SGSN checks to make sure the phone did not transfer this connection from a different MS. If it did, the connection has already been established (along with the billing) and is handed off to this SGSN. If the call is being handed over from another SGSN, it will use the Gn interface between the two SGSNs.
5. The SGSN sends GTP messages to the local external Gateway GSN (GGSN) to create a GTP tunnel for this PDP context to access the Internet. It is possible that a remote GGSN has access to a service, such as a WAP gateway, that the local GGSN is missing. In this situation, the local SGSN uses the Gp interface to connect to the remote GGSN. Both the Gn and Gp interfaces use GTP.
6. The both the local and remote GGSNs connect to external services outside the GPRS network. These services can include a WAP gateway, a corporate IP network directly connected to the GPRS network, or the Internet. The connection from the GGSN to the external services uses the Gi interface.
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