The SGSN (Serving GPRS Support Node) is a core network node that enables packet-switched mobile data (internet) in 2G GSM and 3G UMTS networks. It works with the GGSN (Gateway GPRS Support Node) to provide the connection between the user equipment (UE) and the external data networks.
The SGSN is responsible for the following functions and procedures:
• Authentication and authorization of the UE, based on the information obtained from the HLR (Home Location Register) via the Gr interface.
• Location management and mobility management of the UE, by updating the HLR with the current location of the UE and performing the necessary routing and handover procedures.
• Session management and QoS management of the UE, by establishing and maintaining the PDP (Packet Data Protocol) contexts and the dedicated bearers for the UE, and applying the QoS policies and parameters.
• Charging and billing of the UE, by generating and sending the CDRs (Call Detail Records) to the CG (Charging Gateway) via the Ga interface.
• NAS (Non-Access Stratum) transport of the UE, by relaying the NAS messages between the UE and the MME (Mobility Management Entity) via the S3 interface.
The SGSN communicates with the following network nodes via different interfaces:
• The eNodeB (Evolved Node B) and the RNC (Radio Network Controller) via the S1-U and the Iu-PS interfaces, respectively, to transfer the user plane data between the UE and the GGSN.
• The GGSN via the Gn interface, to establish and maintain the IP connectivity between the UE and the external data networks.
• The HLR via the Gr interface, to obtain and update the subscription and location information of the UE.
• The CG via the Ga interface, to send the CDRs for charging and billing purposes.
• The MME via the S3 interface, to support the interworking between the EPS (Evolved Packet System) and the CS (Circuit Switched) domains.
The SGSN is an essential node for mobile packet core networks, as it enables the users to access and enjoy the packet-switched data services in 2G and 3G networks.