se the SGSN and GGSN node become the main core nodes. GGSN will then partly be controlled by IMS CSCF. Understanding Changing Telecommunications - Building a Successful Telecom Business 14.6.1 SGSN The SGSN server, which is located in the packet-switched domain, handles control layer functions related to packet-mode communication services at the border between the access network and the core network. The SGSN server may host the following main functions: Session management. This means the establishment, maintenance and release of end- user packet data protocol (PDP) contexts. This includes interworking with the GGSN for IP addresses. Session management also includes functionality for establishing WCDMA radio access bearers for end-user IP data transportation, as well as functionality for end-user QoS negotiation. Mobility management. This means the functionality that supports roaming within and between GSM and UMTS mobile networks. Integrated VLR and subscriber data management. The SGSN server supports the stan- dardized interface to the HLR in order to manage end-user subscriber data, such as the international mobile subscriber identity (IMSI), QoS profile, access point names, and SO on. GGSN interface-the gateway tunnelling protocol-control (GTP-C) supports control sig- nalling between SGSN servers and the GGSN. GTP-C is transported by UDP/IP and contains functionality for SGSN-to-GGSN tunnel management and control. MAP and RANAP control signalling. The SGSN server supports the RANAP protocol for control signalling over the RNC-SGSN interface for establishing and releasing radio access bearers. The SGSN server also supports MAP signalling to the HLR, EIR, MSC server and SMS centre (SMS-C). Media gateway control function. The SGSN server supports the GCP protocol for controlling the packet-handling functionality of the media gateway. See Figure 14.15. Functions Session management Mobility management Subscriber data management server Security GGSN interface Charging Admission control MAP and RANAP control signall