4 Main Concepts

23.2053GPPBearer-independent circuit-switched core networkRelease 17Stage 2TS

4.1 General

The circuit switched core network enables the support of different transports (e.g. ATM or IP) in a bearer-independent fashion. For the ATM and IP transport, there is a strict separation between the call control level and the bearer control level. In the case of ATM or IP transport, the passage of compressed speech at variable bit rates is possible through the CS core network.

The CS core network shall employ the MSC server, GMSC server and media gateways. The GMSC server and MSC server shall provide the call control and mobility management functions, and the media gateway shall provide the bearer control and transmission resource functions. The media gateway shall contain the stream manipulating functions.

The GMSC server and MSC servers are connected to the media gateway via the Mc reference point. The MSC servers and GMSC servers are connected with the Nc reference point. There may be a number of call control transit nodes between the MSC server and GMSC server in the Nc reference point. The MGWs are connected with the Nb reference point.

The users connected to the CS core network shall not be aware whether a MSC server – media gateway combination is used, or a monolithic MSC is used.

4.2 Bearer-Independent Call Control

The protocol used on the Nc interface shall be a call control protocol supporting IP and ATM transports in a bearer‑independent manner for the ISDN service set, allowing the physical separation of the call control entities from the bearer control entities.

An exception to this bearer independence concept is if Iu interface is on IP and the IP addresses are to be exchanged via call control plane signalling (known by the MSC due to configuration data). In this case the specific handling is described separately.

Another exception to the bearer independence concept is the case of A interface user plane over IP (AoIP). In that case the IP addresses/UDP port numbers are to be exchanged via call control plane signalling. Specific handling is also described separately for this case.

4.3 H.248/MEGACO

H.248/MEGACO has been jointly developed within the ITU-T and the IETF, and supports a separation of call control entities from bearer control entities, and a separation of bearer control entities from transport entities. H.248 is used on the Mc interface between the (G)MSC servers and the media gateway.