Subscriber Server (HSS) is the database that holds the subscriber profile including the authentication and authorization information. It performs the same functions as the HLR in previous 3GPP network generations. Serving Gateway (SGW) is the mobility anchor for inter-eNodeB mobility. It routes the user traffic between the radio network and the Packet Data Network Gateway (PGW). Practically it performs the same functionality as the 3G-SGSN performs on the user plane. Like the SGSN, SGW can change with the mobility management during a connection. PGW or PDN-GW is the gateway between the EPS and the external IP-networks, and the anchor point for the EPS mobility management. Thus, it does not change during a connection as it is the topological anchor for the UE in the IP network. The PGW performs the same functions as the GGSN in GPRS. The PGW can also be used as a GGSN in a setup where both GPRS and EPS are present. We described in Section 2.2.2, how the SGSN can change during a connection and when the UE is moving about, and how the GGSN as the anchor point does not. Similarly, the PGW stays the same during a connection, and the eNodeB, the MME, and the SGW can change during the connection. The standards allow the SGW and PGW to be implemented in the same physical element. This kind of combined element is called System Architecture Evolution Gateway (SAE-GW). In this configuration, the functionality stays the same as in the separated elements case. Obviously, as the network architecture has changed, SO have the interfaces connecting them. The interfaces listed in Figure 2.10 are the following: S1-MME is the control-plane interface connecting the eNodeB and the MME. S1-U is the user-plane interface connecting the eNodeB and SGW. Basics of the 3GPP Technologies GTP-U GTP-U GTP-U GTP-U UDP/IP UDP/IP UDP/IP UDP/IP S5/S8 LTE-Uu eNodeB Figure 2.11 EPS user-plane protocols. S5/S8 are the interfaces connecting the SGW and the PGW. S5 is used within the home network, and S8 interface is used when the UE is roaming. This is