has services related to an unregistered state (e.g., voice mail). Routing incoming requests further to an assigned S-CSCF or the application server. Providing Topology Hiding Inter-network Gateway (THIG) functionality. THIG can act to hide the operator's topology from the peer networks. If the THIG is active, the I-CSCF remains involved in the signaling of the call. 9.3.2.2 Databases There are two main databases in the IMS architecture. Home Subscriber Server (HSS) and Subscription Locator Function (SLF). Both the HSS and the SLF communicate through the DIAMETER protocol. Diameter is a computer networking protocol for authentication, authorization and accounting. HSS maintains all subscriber and service-related data of the IMS including user identities, registration information, access parameters and service triggering information. HSS can be considered a superset that includes subsets of HLR (Home Location Register) and Authentication Center (AuC). HLR as we know is a required element of 2G/3G for both circuit and packet switched domains. To account existing investment in the databases, HSS will provide a consolidated interface to multiple sources of information (HLR, location servers, business repositories, etc.). The HSS can pull the data together under one interface rather then replacing already existing repositories. The SLF is used as a resolution mechanism that enables the I-CSCF, the S-CSCF and the application servers to find the address of the HSS that holds the subscriber data for a given user identity when multiple and separately addressable HSSs have been deployed by the network operator. 9.3.2.3 Other Entities and Functions In addition to the above listed entities IMS CN Subsystem also include several supporting and inter- working functions which will be discussed in this section: AS: The Application Servers (ASs) in the IMS environment are not real applications. The ASs on the other hand provides the application-common functions, such as generation of billing information, management of presence and 