values. PDP context shows a stage of QoS handling. 15.8 PDP CONTEXT ACTIVATION FOR CONNECTION TO A DATA NETWORK In order for the user to be able to transfer data, a packet data protocol (PDP) context must be activated in the mobile station, SGSN and GGSN. The user initiates this proce- dure, which is similar to logging on to the required destination network. The process is illustrated and described below. The user initiates the logging on process, using an application on the PC or mobile station. The mobile station requests sufficient radio resources to support the context activa- tion procedure. Once the radio resources are allocated, the mobile station sends the activate PDP context request to the SGSN. This signalling message includes key information about the user's static IP address (if applicable), the QoS requested for this context, the access point name (APN) of the external network to which connectivity is requested, the user's identity and any necessary IP configuration parameters (e.g. for security reasons). After receiving the activate PDP context message, the SGSN checks the user's sub- scription record to establish whether the request is valid. Interconnection If the request is valid, the SGSN sends a query containing the requested APN to the DNS server. The DNS server uses the APN to determine the IP address of at least one GGSN that will provide the required connectivity to the external network. The GGSN IP address is returned to the SGSN. The SGSN uses the GGSN IP address to request a connection tunnel to the GGSN. Upon receiving this request the GGSN completes the establishment of the tunnel and returns an IP address to be conveyed to the mobile station. The GGSN associates the tunnel with the required external network connection. 15.9 SECURITY INTERWORKING A mobile user has only access to, and is charged for, the services that his profile allows. Whenever a user moves into a foreign administrative domain, some kind of AAA sig- nalling between the visited domain and the home domain is required t