Study On Interoperability Of SIP In Softswitch And Implementation Of User Agent

As a key technology of NGN, Softswitch provides an open network architecture, which realizes the integration of network and service, and leads the unification of PSTN, Mobile Network, Cable Television Network and Internet. Network integration is a trend of NGN, of which the interworking problem needs to be resolved urgently. As one of the session control protocols which has simpleness,flexibility and other characteristics, SIP (session initial protocol) defined by IETF, has been widely used in NGN and Mobile Network. This thesis mainly analyses and studies the interoperability of SIP in Softswitch, which includes interoperation,model of interworking and design of SIP user agent. Firstly, according to four-layer structure of Softswitch network, the interworking problem should be divided into four parts of different kinds of terminals in access layer, media formats and carrying technology in transport layer, many signals in control layer and different service resource in application layer. Among them, signal adaptation and conversion is session foundation, the interworking between SIP and H.323 or ISUP is described, in which network framework, signal mapping and call flow are discussed in detail. In order to realize the interworking between SIP and PSTN network, the SIP extension is introduced and the ENUM is used. Then the problem of address analysis is resolved. Secondly, multi-protocol interworking scheme using call model is put forward. Call model of Softswitch in which call and carry are separate, can be realized using supplement of connecting module and evolvement of IN's(intelligent network). The interworking of SIP with other protocols can be resolved through call model of Softswitch, which supports access of many protocols and evolvement of network, and reduces hard work for protocols interworking. In the end of this thesis, with the detail research project and according to the Softswitch call model, the design of SIP UA module in Softswitch is given, which includes functions classification, interface design, finite state machine implementation and basic class structures. As a result of the implementation of this design, SIP UA can realize multi-protocol interworking with ISUP and other protocols, and finish the Offer/Answer model of media negotiation. Now the design with high capability meets the requirement of SIP in Softswitch.