R stores information about the SGSN serving the subscriber. The AuC is basically a subset of the HLR that holds the security information including the keys for mutual authentication, integrity protection, and the ciphering of the radio interface for a given subscriber. 2.7.2 Home Subscriber Server The Home Subscriber Server (HSS) 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. However, as stated above, the specifications and the literature are not always consistent in the use of terms. However this book uses HLR for GPRS, and HSS for EPS, as this is in line with most specifications and literature. Anyway, the difference is just a terminology issue as the HSS is a superset of the HLR, and the difference is only in the IMS functionality. Hence, in the case of GPRS and EPS network service only the HLR functionality applies. 2.7.3 Equipment Identity Register The Equipment Identity Register (EIR) is a database that stores International Mobile Equipment Identitys (IMEIs) information. IMEI is an identity hard coded into the UE that uniquely and globally identifies the equipment. The EIR contains three lists, white, gray, and black, for IMEI [57]. The white list contains UE that are permitted to use the network, and the black list contains equipment that is barred from the network. The gray list can contain UE that are tracked by the network for some reason. 2.7.4 Other Backend Systems Other backend systems used in the 3GPP network deployments include Authentication, Authorization and Accounting (AAA) servers, and Dynamic Host Configuration Protocol (DHCP) servers [58, 59] for subscriber and address management. The AAA technologies used are Remote Authentication Dial In User Service (RADIUS) [60] and Diameter [61]. RADIUS is a technology standardized and designed by the IETF originally for dial- up networking, but is currently a very widely used AAA technology in many differe