three new interfaces, the Nc (network control) interface, the Mc (MGW control) interface and the Nb (network backbone) interface. In order to control MGWs to route user plane traffic through different bearer channels in the backbone, 3GPP release 4 circuit core standards Network Evolution recommend the use of BICC-CS2 as the signalling protocol on the Nc interface. BICC-CS2 is able to establish bearer channels for user plane connections both with an ATM and IP based backbone. Moreover, BICC-CS2 is independent from the signalling message transport layers, which means that BICC-CS2 can be carried on top of TDM (SS7), ATM (SAAL, on signalling ATM adaptation layer) or IP (SIGTRAN). When the backbone network is based on IP transport, the signalling for call control between MSS/GCS elements in the release 4 core circuit network can be based on the SIP protocol. SIP messages are carried on top of the UDP/IP. An MSS/GCS may use domain name server (DNS) functionalities to get IP addresses of the destination MSS/GCS when performing call control related signalling. The use of IP routing for the signalling messages and DNS resolution to resolve domain names to IP addresses is used in the transport of signalling messages on the control plane. There is not much need for higher layer signalling transfer points when SIP is used. It has to be noted that the use of the SIP protocol in the release 4 circuit core is a proprietary solution, not included in 3GPP standards. Media Gateway (MGW) The MGW in the MSS system solution is an independent network element. It is, however, controlled by the MSS through an internal Media Gateway Control (MEGACO). Protocol running on IP. It has two main functions: Trunk switching functions Termination of the A, ATM and IP interfaces Network Backbone The 3G release 4 soft switch solution offers operators the flexibility to choose a suitable network backbone depending on availability and suitability of transmission in the local market environment. The network backbone can be based on any available tra