It also performs replication of the user traffic in case of lawful interception. The Packet Data Network Gateway (PDN Gateway, also PGW) provides connectivity from the User Equipment (UE) to external packet data networks (PDNs) by being its the point of exit and entry of traffic. A piece of User Equipment may have simultaneous connectivity with more than one Packet Data Network Gateway for accessing multiple packet data networks. The PDN Gateway performs policy enforcement, packet filtering for each user, charging support, lawful interception and packet screening. Another key role of the Packet Data Network Gateway is to act as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2 (CDMA 1X and EvDO). The Home Subscriber Server is a central database that contains user-related and subscription-related information. The functions of the HSS include functionalities such as mobility management, call and session establishment support, user authentication and access authorization. The HSS is based on pre-Rel-4 Home Location Register (HLR) and Authentication Center (AuC). The ANDSF provides information to the UE about connectivity to 3GPP and non-3GPP access networks (such as Wi-Fi). The purpose of the ANDSF is to assist the UE to discover the access networks in their vicinity and to provide rules (policies) to prioritize and manage connections to these networks. The main function of the ePDG is to secure the data transmission with a UE connected to the EPC over untrusted non-3GPP access, e.g. VoWi-Fi. For this purpose, the ePDG acts as a termination node of IPsec tunnels established with the UE. The Non-Access Stratum (NAS) protocols form the highest stratum of the control plane between the user equipment (UE) and MME. NAS protocols support the mobility of the UE and the session management procedures to establish and maintain IP connectivity between the UE and a PDN GW. They define the rules for a mapping between parameters during inter-system mobility with 3G networks or non-3GPP access networks. They also provide the NAS security by integrity protection and ciphering of NAS signaling messages. EPS provides the subscriber with a "ready-to-use" IP connectivity and an "always-on" experience by linking between mobility management and session management procedures during the UE attach procedure. Complete NAS transactions consist of specific sequences of elementary procedures with EPS Mobility Management (EMM) and EPS Session Management (ESM) protocols.