ing 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, multimedia services. Provide support for a number of successful multimedia services to ensure that these systems will continue to focus on multimedia, rather than telephony over IP. The MCS provides complete solutions for multimedia access, communication and ser- vices with transparency between fixed and mobile communication services. The architecture and specifications are based on the IM subsystem defined by the 3GPP for UMTS. SIP is used for call/session control signalling, and MEGACO architecture is used for edge nodes/media gateways. It has an open service creation environment. By using standardized APIs such as Parlay, the customer can develop its own services or bring third-party services into the network. MCS includes the elements necessary to support IP multimedia services in UMTS as defined by the 3GPP. MCS complements, but does not replace, MSC-based mobile telephony systems that will continue to provide voice-centric services. 14.7.2 MU