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 [6]). 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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