GTP’ (GTP prime)
GTP is used to handle tunnels of user traffic between SGSNs and GGSNs. However for billing purposes, other devices that are not supported by GTP are required. GTP’ (GTP prime) is a modified form of GTP and is used to communicate with these devices such as the Charging Data Function (CDF) that communicates billing information to the Charging Gateway Function (CGF). In most cases, GTP‘ transports user records from many individual network elements, such as the GGSNs, to a centralised computer which then delivers the charging data more conveniently to the network operator’s billing center, often through the CGF. 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.
GPRS network common interfaces
GTP’ is used by the Ga and Gz interfaces to transfer billing information. GTP’ uses registered UDP/TCP port 3386. GTP’ defines a different header, additional messages, field values, as well as a synchronisation protocol to avoid losing or duplicating CDRs on CGF or SGSN/GGSN failure. Transferred CDRs are encoded in ASN.1.
HLR
The Home Location Register (HLR) is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network. There can be several logical, and physical, HLRs per public land mobile network (PLMN), though one international mobile subscriber identity (IMSI)/MSISDN pair can be associated with only one logical HLR (which can span several physical nodes) at a time. The HLRs store details of every SIM card issued by the mobile phone operator. Each SIM has a unique identifier called an IMSI which is the primary key to each HLR record.
VLR
The Visitor Location Register (VLR) is a database which stores information about all the mobile devices that are currently under the jurisdiction of the Mobile Switching Center which it serves. Of all the information the VLR stores about each Mobile Station, the most important is the current Location Area Identity (LAI). This information is vital in the call setup process.
Whenever an MSC detects a new MS in its network, in addition to creating a new record in the VLR, it also updates the HLR of the mobile subscriber, informing it of the new location of that MS.
For more information on GTP‘, see GTP-U and Charging Management Messages.
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.
Packet flow through the GPRS network
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 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
GGSNs |
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. |
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.
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