the Primary and Secondary PDP Contexts after the activation. As we have now learned, the UE starts the PDP Context activation process. 3GPP has also specified a network requested PDP Context activation procedure. In this procedure, the subscriber has to have a statically configured address. When the GGSN that has the given address receives an IP packet destined to this address, it will query the HLR for the location of the UE and send Protocol Data Unit (PDU) Notification Request to the SGSN that is handling the UE. The SGSN will then send a Request PDP Context activation mes- sage to the UE, which would trigger the normal PDP Context activation procedure. (A careful reader might now be a bit confused - neither Figure 2.4 nor Figure 2.7 shows an interface between the GGSN and the HLR. This has been left out of the drawing for sim- plicity. The interface between the GGSN and the HLR is the Gc-interface and it is specified only for the Network Requested Primary PDP Context Activation.) However, this net- work requested Primary PDP Context activation procedure has, to the authors' knowledge, never been implemented in any network. In addition to Primary PDP Context activation, a Network Requested Secondary PDP Context Request activation is also specified. In addition to being activated, the PDP Contexts can be also be modified and deac- tivated. The modification and deactivation procedures can be activated by the UE, the SGSN, or the GGSN. The modifications usually relate to changing the QoS parameters of the PDP Context. As described earlier, the Primary and Secondary PDP Contexts are not distinguished after activation. This can be seen in the deactivation process. The IP address related to a bundle of PDP Contexts is deallocated from the UE when the last PDP Context related to that address is deactivated. The deactivation of the last PDP Context does not automatically detach the UE from the network. Obviously, the UE is then no Basics of the 3GPP Technologies eNodeB 1. Attach Request 2. Authentication and Authoriza