In the context of support of roaming for voice over IMS, the GSM Association has decided that local breakout in the visited network will be needed (see GSMA PRD IR.65 ). This is because local breakout can be regarded as one cornerstone to allow the replication of the charging principles on which CS roaming is based on. However there are several other aspects that need to be considered e.g. the split/bundling of user and control plane, capturing of SDP information needed to generate charging records, to make such principles applicable to voice over IMS roaming.
This study item will therefore investigate solutions for the provision of voice over IMS in roaming scenarios that facilitate the realization of a charging model that replicates the principles of CS model.
It will be studied whether changes to the core 3GPP specifications are needed and if so which ones. The study will focus on IMS layer and not EPC aspects.
As some of the aspects of the roaming are outside the scope of 3GPP (e.g. decision to anchor media or control plane, interaction with carrier networks, roaming agreements) this study will be performed in close cooperation with the GSM Association which may provide additional requirements and guidance during its development.
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Ensure that the service control is performed by the HPLMN (e.g. to cater for the case where the destination address is modified). The service control is retained by the HPLMN also in the case where the destination network is the same as the VPLMN where the roaming user is.
Allow to reach a user whose number has been ported to a different PLMN using the same resolution mechanism adopted by the VPLMN for their own users.
It shall be possible, based on roaming agreements, to provide the signalling breakout of the IMS voice call from the VPLMN.
The solution will be such as to allow the VPLMN to support outgoing voice over IMS calls from inbound roamers:
towards any kind of destination network, i.e. CS (e.g. GSM, UMTS, ISDN, POTS) or PS (e.g. TISPAN, 3GPP IMS) network;
using any kind of interconnection it might require, i.e. legacy CS (e.g. ISUP, SIP-I) or PS (e.g. IPX, direct connection) interconnection.
The solution will be such as to allow supporting incoming voice over IMS calls to outbound roamers:
from any kind of originating network, i.e. CS (e.g. GSM, UMTS, ISDN, POTS) or PS (e.g. TISPAN, 3GPP IMS) network;
using service aware interconnection.
The solution will be such to allow the VPLMN to provide the anchor functionality close to where the call is originated from, and the VPLMN shall be able to steer the selection of anchor function by providing the HPLMN with a reference to the preferred anchor function. If the VPLMN does not provide the anchor functionality address the HPLMN shall use the default address for the VPLMN.
The solution shall be compatible with the use of SRVCC as specified in TS 23.216 and TS 23.237, including when ATCF enhancements are used.
The application of a Single Radio VCC procedure to the roaming UE shall not result in changes in the user charging.
The solution should consider the case where the HPLMN requests the user plane to be anchored in the HPLMN.
In order not to contravene the existing interconnection model, the media plane shall not be routed without the associated signalling through intermediate carrier networks.
The solution should consider the cases where voice over IMS is provided with CS media, using one of the following procedures:
"Origination using I2 reference point" (see TS 23.292);
"Termination using I2 reference point" (see TS 23.292);
"Termination to non ICS UE not registered in IMS" (see TS 23.292);
The solution shall be such as to permit the application of Optimal Media Routing as specified in TS 23.002 and TS 23.228. This is in line with GSM where the support of optimal routing (TS 23.079) is optional.
For those solutions which rely on routing the session set up signalling back to the originating VPLMN once the session has been processed by the HPLMN, the VPLMN needs to be able to produce easily a complete Call record. Correlation is needed between the two signalling flows, namely VPLMN to HPLMN and HPLMN to VPLMN.
The VPLMN-provided charging record may indicate whether a signalling loopback has been executed (indicating that such CDR was produced as a result of returning the session set up signalling only back to the originated VPLMN) or not.