In call scenarios without the need for the GMSC or Intermediate Node server to manipulate the bearer, the GMSC or Intermediate Node may perform call control signalling without any associated MGW by not inserting a MGW in the bearer path during the call establishment. In that case, the bearer related information of SDP offers/answers shall be passed transparently through the (G)MSC Server.
Call scenarios where the GMSC or Intermediate Node needs to manipulate the bearer, e.g. scenarios with insertion of tones or announcements, lawful interception, CAMEL services do not allow this optimisation.
MGW removal and MGW re-insertion scenarios once the call is answered are not supported.
shows an example network model for a mobile terminating call with MGW bypass. The "squared"
line represents the call control signalling. The "dotted"
line represents the bearer control signalling (not applicable in A/Gb mode for the A-interface) and the bearer.
shows the corresponding example message sequence. In the example, the GMSC determines that it does not need to include a MGW in the bearer path. SDP offer and SDP answer information is relayed between ingress and egress signalling paths.
The option to support "MGW bypass"
relies on the support of external user plane interworking at the succeeding MGW however the GMSC Server signalling to and from the succeeding 3GPP PLMN node in such a scenario shall comply with the SIP-I on Nc specification.