scriber 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 signalling Figure 14.14 SGSN control functions, example Control Network 14.6.2 GGSN The GGSN provides functions such as routing, packet session establishment and termi- nation, message screening, service measurements and billing. Tunnels are dynamically established between an SGSN and GGSN using the GPRS tunnelling protocol (GTP), and are deactivated when no longer needed. The GGSN functions include assignment of an IP address to the user equipment, main- taining packet data protocol (PDP) context, and collection of information for billing. GGSN controls the interface for IP bearer path to other networks, such as the public Internet, a VPN, or another wireless service provider's network. The GGSN handles rout- ing of wireless IP multimedia subscriber packets and supports the QoS classes. It supports QoS interworking with external networks to support end-to-end QoS requests. 14.7 IMS DOMAIN = IP MULTIMEDIA SUBSYSTEM 14.7.1 MCS The goals of the MCS (multimedia core system) are to: Allow telephony service providers to extend their services portfolio with more advanced, IP-based, mul