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Mobile in Minute papers

 IMS Functions*

There are two major central functions at the core of the IMS control architecture:

• Home Subscriber Server (HSS): The HSS is the central repository of all subscriber-specific authorizations and service profiles and preferences. The HSS integrates several functions, some of which exist already in the functions of the Home Location Register of mobile networks:
  • Subscriber Profile Database
  • Subscriber Service Permissions
  • Subscriber Preference Settings
  • Mobile Authentication Server
  • Home Location Register (HLR) for mobile roaming
  • Subscriber Presence Function
  • Subscriber Location Function
   

• Call Session Control Function (CSCF): The 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:
  • Proxy-CSCF: First point of contact for device and controls authentication
  • Interrogating-CSCF: Entry point of all SIP messages
  • Serving-CSCF: Manages all session control functions

In addition, there the Application Servers and OSS and Billing platform servers that are aggregated and assembled in the Service Plane. This leaves the network nodes, which are a mixture of IP nodes and circuit switched modes in the Network Plane. The result is that all the interfaces for Billing, Provisioning, and Maintenance systems are now aggregated into the Services Plane eliminating a significant amount of duplication and offering significant expense savings for service providers.

Session Initiation Protocol (SIP):
The Glue binding IMS functions and applications.

There are clear interface specifications for the OSS and Billing system interfaces and all call control and application interfaces to the CSCF function are controlled through the Session Initiation Protocol (SIP). SIP, together with the aggregation of all subscriber authentication, service preferences and subscriber details in the HSS comprise the heart of the new IMS architecture.

SIP works in conjunction with the Session Description Protocol (SDP) to initiate multimedia sessions. SDP describes multimedia session functions such as session initiation and session announcement. SIP is used to establish sessions according to the following general steps

• Session Initiation: The user’s device signals the need for a session and the user’s network location is identified and a unique session identifier (SIP URI) is assigned.

• Session Description: Delivers a description of the session to the user device. The SDP protocol is used for this function.

• Session Management: Once the session is accepted by the user device, media streams or other content are then directly exchanged between the end points. Real Time Protocol (RTP) or Real Time Streaming Protocol (RTSP) are commonly used.

• Session Termination: Either party in the session can request session termination once the data or media exchange is complete.