4.4.1 Principles

23.2313GPPRelease 17SIP-I based circuit-switched core networkStage 2TS

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:

– "MGW bypass"

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 Clauses 4.4.2, 4.4.3 and 4.4.5, respectively. When applied these procedures shall be combined with the normal call establishment procedures as described in Clause 6.