Open IMS Call Session Control Functions - CSCFs

Even though IMS builds upon IETF specifications like the SIP standard, the SIP protocol requires certain extensions for being used in an IMS environment. The primary requirement for the Open IMS Core project therefore was to provide a set of IMS compliant core components that will enable the development of the other layers around them. The main goal currently is to obtain compliant CSCFs to the functionality required by 3GPP in for its Release 6 specifications.

The Open IMS CSCFs are built upon the SIP Express Router (SER) which can act as SIP registrar, proxy or redirect server and is capable of handling many thousands of calls per second. It has a modular structure that permits to make functionality additions. Each CSCF entity of the Open IMS Core is implemented as a SER dynamically loadable module that adds the required operations to the SER so that it can act according to the specific 3GPP technical specifications. The modules are capable of parallel processing and can keep supplementary state information. There is a special focus towards scalability for both load distribution and data quantity.

Another requirement to the CSCFs was to maintain as much as possible SER’s performance. As SER obtained a wide adoption in the SIP world, becoming almost a standard for performance, it can be safely assumed that the Open IMS CSCFs, sharing such a large base with SIP routers, would have similar performance standards. The Open Source IMS Core maintains SER’s user scripting facilities, which enables full flexibility in exploitation. The IMS features are to be exported but their orchestration had to be easily configurable.