The SIP-I circuit switched core network supports the IP transport mechanism. 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.
The protocol used on the Nc interface shall be a SIP-I call control profile supporting the IP transport mechanism for the ISDN service set, allowing the physical separation of the call control entities from the media transport entities.
H.248 has been developed within the ITU-T, and supports a separation of call control entities from media transport entities. H.248 is used on the Mc interface between the (G)MSC servers and the media gateway.
A (G)MSC-Server may support one or several of the following optional MGW selection procedures:
"Optimised MGW selection"
A (G)MSC Server may indicate at the initial SIP-I SDP offer the selected MGW identity in a SIP-I message to enable the receiver of the SIP-I message to select the same MGW, if it has a Mc H.248 gateway control protocol interface to this MGW.
"Deferred MGW selection"
The deferred MGW selection procedure provides the opportunity for the receiver of a SIP-I message to select the MGW it prefers and to send back the identity of the MGW to the preceding node in order to enable that node to select the same MGW.
Additionally the procedure allows the offerer to send a "proposed" MGW identity to the next SIP-I node, which may be taken into account when the succeeding node seizes a MGW.
Additionally the procedure allows the offerer to send to the next SIP-I node a list of "additionally proposed" MGW identities that may be proposed in addition to the "proposed" MGW Identity, which may be taken into account when the succeeding node seizes a MGW.
A GMSC-Server or Intermediate Node may support the following optional MGW selection procedures:
For call scenarios where there is no need for the GMSC server to manipulate the bearer, the GMSC Server may perform call control signalling without any associated MGW by transparently relaying bearer related information (e.g. connection addresses) from the preceding/succeeding nodes. If the (G)MSC server applies the MGW bypass this has the effect that if the succeeding node inserts a MGW this MGW performs the task of terminating the external bearer , i.e. it acts as point of ingress of the user plane from the external network.
The general call establishment procedures to allow "Optimised MGW selection"
, "Deferred MGW selection"
, and "MGW bypass"
are described in clause 4.4.2
, clause 4.4.3
and clause 4.4.5
, respectively. When applied these procedures shall be combined with the normal call establishment procedures as described in clause 6