The GBS represents the packet switching network that provides GPRS connectivity between the BSS and external packet data networks to support GPRS terminals. The GBS comprises several different types of network elements as well as the interconnecting transmission hardware (e.g. routers. repeaters) and the transmission links between them.
The ETSI standards introduce new functional network elements:
- Serving GPRS Support Node (SGSN)
- Gateway GPRS Support Node (GGSN)
The SGSN provides subscriber management, mobility management, as well as session management for any mobile GPRS user that has been associated with this SGSN. In order to achieve this task, the SGSN holds interfaces to the GSM subscriber databases: HLR, VLR, AUC and EIR. The SGSNs also hold the interfaces to the BSSs, and provides the authentication and encryption services for secure transmission of user data.
The GGSN provides connectivity to external Packet Data Networks (PDNs). The ETSI standards specify the Internet and X.25 networks as external PDNS. The GGSN also provides address translation services. Rate adaptation services between the GBS and external networks may also be included in the GGSN. The Border Gateway provides connectivity to another Operator’s GPRS network.
New interfaces will be required to connect the new entities to the existing GSM network elements. These interfaces will be pre-fixed with the character ‘G’ and will support both traffic and signal connections.
Unlike circuit switched services, packet services allow concurrent usage of the same resources by multiple users. In order to support GPRS in a GSM network, the BSS has to be upgraded to support packet services and a GPRS Backbone System (GBS) has to be added to the basic GSM network to provide packet connection from GPRS capable mobile stations to other packet users, both fixed and mobile.
The GPRS Backbone System (GBS) comprises of the following:
A GPRS operator managed IP domain and Domain Name Server (DNS) to map logical names for each element connected to the GBS domain to IP addresses
Multiple Serving GPRS Support Nodes (SGSN) that provide packet service management for GPRS subscribers
Multiple Gateway GPRS Support Nodes GGSNs that provide subscribers with access to external packet data networks and Public Land Mobile Networks PLMNs
A GBS Management Network Element Manager (NEM) called an Operations and Maintenance Centre for the GBS or OMC-G
A Performance Gateway function that collects Measurement Data from the GSNs and forwards to a Performance Monitoring Centre
A Charging Gateway function that collects Accounting Data from the GSNs and forwards to a Billing Centre
The IP domain may he entirely operator provisioned or part of a larger IP network operated as a Virtual Private Network domain. The Network supporting the IP domain is called the GPRS Backbone Network (GBN).