) context. It might be a logical association between a mobile station and a public data network running across a GPRS network. The context may define aspects such as routing, QoS, security, billing, type of network, network address, access point name and radio priority. Largely, we are again back to the fundamental plans.' (See also the centre of Figure 14.4). A control layer for multimedia traffic requires a number of central functions, as opposed to the original Internet, often considered be a stupid network. A key future component is the call state/session control function/server, CSCF. The CSCF has similarities with the traffic control subsystem in ordinary tele- phone exchanges. A corresponding parallel can be found for other main traffic-handling functions. Let us look at a few FP oriented tasks performed by the CSCF: Numbering plan: Address Handling (AH). The Address Handling (AH) function handles analysis, translation, modification if required and address portability Routing plan: Routing of incoming calls Routing and interconnection plan: Cooperation with BGCF (border gateway control function) for calls to the PSTN to determine the MGCF (media gateway control func- tion) to use Transmission/QoS plan: Analysis of need for a transcoder for mobile to mobile calls Switching plan: Controlling the MRF in order to support need for multi-party calls Billing Telecom business Service Service Service, security, QoS Service layer management Dimensioning Numbering Charging Routing Signalling Control layer Dependability Mobility management Switching Frequency Transmission Traffic resources Organization Synchronization in connectivity layer Figure 14.4 Security and QoS are also part of the control network and its tasks Control Network Charging: (and security) plan: Call monitoring and logging for billing, verification, auditing, intercept or other purpose Security plan: Is the requested outgoing communication allowed given the current subscription? QoS: Authorize the bearer resources for the appropriate QoS level. Dete