warding user packets. xxxvi Glossary GPRS mobility management and session management Protocol for performing mobil- ity management functions, and connection activation, modification, and deactivation. GPRS Tunneling Protocol (GTP) Responsible for transporting the user's IP packets over the 2G, 3G, and 4G core networks, and controlling the associated tunnels. In the User Plane, the basic functionality of GTP is to uniquely identify the GTP-tunnel to which the user packets belong. The GTP-tunnel maps directly to the PDP Context or EPS bearer. In the control plane GTP is used for session and mobility management within the Core Network. Gr-interface The interface between the SGSN and the HLR. The interface is based on SS7. GSM See 2G and GSM. Home Location Register (HLR) Database on 2G/3G networks containing the subscriber data, which includes the authentication data used to authenticate the user, user profiles including what services the user is subscribed to, and the location where the user currently is. Home Subscriber Server (HSS) The Home Subscriber Server is a combination of the AuC, HLR, and functionality to support the operator service infrastructure, the IMS. HSS replaced the HLR from the 3GPP Release-5 onwards with the introduction of the IMS. Replaces HLR in EPS. Interface The interface is an overloaded term with meaning determined by the context. In the 3GPP context, it means the whole vertical protocol stack between two network elements - sometimes also called a reference point. In the IP context it means the network connection of an IP stack. In the software architecture context it means the defined interactions between two software modules, and sometimes specifically it is a short name for an application programming interface. Interface IDentifier (IID) The host part of an IPv6 address. Typically 64 bits in length. The IID can be based on a hardware address, such as the IEEE 802 MAC, be manu- ally or algorithmically generated. A random IID generation is often used for privacy purposes to prevent tracki