s. The application server can provide service capa- bility interaction manager (SCIM) functions to manage interactions. Control-layer components: Call session controller function-The call session controller function (CSCF) provides call control similar to that of a VoIP softswitch. There are three types of CSCFs: the proxy CSCF (P-CSCF), which is a SIP Service Application Layer Servers Control Call Session Layer Control Function RSTN, Transport Internet, IP, Transport Layer Radio Networks Network Figure 10.2 The IMS layers The Next-Generation Network Infrastructure Application Servers Application or Service Layer S-CSCF HSS, SLF I-CSCF Control Layer P-CSCF Transport Radio Area Internet or Access Networks IP, MPLS Layer Narrowband Broadband Access Figure 10.3 The IMS architecture proxy that provides subscriber access to network-based services and sits in front of the serving CSCF; the serving CSCF (S-CSCF), which is the primary call controller for the operator network; and the interrogating CSCF (I-CSCF), which is a SIP proxy that can be optionally used by a network operator to hide internal configurations. Home subscriber servers (HSSs)-An HSS handles centralized provi- sioning, management, and authentication/authorization. CSCFs consult with the HSS before initiating SIP connections. The HSS includes the home location register (HLR) and the authentication center (AC). The HLR is a database used for storage and management of subscriptions; storage of permanent information about subscribers, including the sub- scriber's service profile; location information; and activity status. The AC Chapter 10 Next-Generation Networks is used to protect subscribers from unauthorized access (by providing authentication and encryption parameters that verify the user's identity) and to ensure the confidentiality of each call. It also protects network operators from different types of fraud found in today's mobile networks. Subscription locator function (SLF)-The SLF locates the database that contains subscriber data in response to