The IP Multimedia Subsystem (IMS) Control Plane contains all the call control and authentication functions for call setup, handoffs, and billing recording. The two most important Control Plane functions are the HSS (Home Subscriber Server) and the CSCF (Call Session Control Function). The focus here is on network elements and functions which can authenticate customer access, establish and manage voice and data sessions, record billing data and provide all customer-specific functions.
As with the nodes in the Transport Plane, the Control Plane is also a mixture of pre-IMS signaling protocols such as SS7, IS-41, and TCP/IP. There are further inter-working functions and gateways defined to bridge the gaps between legacy systems and the IMS functions. An example is that traditional PSTN network nodes signal using ISUP over SS7 networks and the IMS architecture uses SIP. Therefore, a SGW (Signaling Gateway) is used to translate the SS7 signaling to SIP. In this way, existing capital investments are maintained while traffic volume efficiently migrates from traditional PSTN modes to IMS based servers.
As the sole location in the network of customer-specific data and functions, the Control Plane is the critical point of authentication and customer billing control. All customer authorizations and preferences on services are housed in the Home Subscriber Server (HSS) which is a database similar to the Home Location Register (HLR) of the mobile GSM networks. This database controls all subscriber access functions for authentication and policy decisions for using network resources. In many networks, multiple HSS’ are used and another function called the Subscriber Location Function (SLF) is needed for tracking which network the subscriber is presently located on. Another possible companion function records Presence information which indicates whether specific subscribers are present somewhere in the network. Taken together, Location and Presence information offers valuable functions for service developers to create new location-based services and community-driven functions among subscriber self-aligned groups such as instant messaging.
Since the HSS contains the HLR functions from mobile networks, roaming across networks, even between wireline and wireless networks, continues. The HSS manages all authorizations and tracking of where the subscriber can be found. Further, the IMS concept defines that the Home HSS retains control of the session for subscribers even as they roam onto other networks.
The Call Session Control Function (CSCF) is the primary SIP signaling function in the network. It may likely be manifest in multiple nodes and redundant and diverse to enhance reliability. The CSCF consists of several types of SIP servers and process all the SIP signaling in the network. The three most common SIP servers in the CSCF are:
- P-CSCF (Proxy-CSCF): This is the first point of contact for the user’s device and can occur either in the Home network or a Roam network. In the case where the roaming network is not yet IMS-compliant, the P-CSCF function will be in the home network. The P-CSCF remains assigned to the IMS terminal throughout the session and inspects every SIP message, authenticates the user and establishes an IPsec security association with the IMS device. Once authenticated by the P-CSCF, other nodes respect the trust and multiple authentications are not required in a session. The P-CSCF also manages the Quality of Service (QoS) for the session and manages network resource allocation. The P-CSCF also controls the charging function for billing.
- I-CSCF (Interrogating-CSCF): The I-CSCF is the entry point for all SIP messages into the administrative domain. It retrieves the subscriber location from the HSS and then routes the SIP request to the appropriate Serving-CSCF.
- S-CSCF (Serving-CSCF): The S-CSCF manages all session control for the Control Plane. As such, it controls Session Initiation, Session Description, Session Management and Session Termination functions.
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