The present document studies PS domain Online Charging in a roaming context, when the Gateway node implementing the IP access to the PDN for data traffic, pertains to the VPLMN, whereas the Online Charging System (OCS) pertains to the HPLMN. The study will focus, in a first phase, on high level scenarios for Flow Based Charging, relying on defined TS 23.203 PCC roaming architectures with Gy crossing PLMNs, in order to investigate the following aspects:

• Information availability in both PLMNs
• Inter-PLMN connection mechanisms for Diameter messages
• Impacts on Inter-operator charging
• Whether a "proxy OCS" is needed

The two following configurations will be addressed:

• Gy between OCS residing in HPLMN and PCEF located in PGW VPLMN, for Local Breakout Roaming configuration defined by TS 23.401 and TS 23.203.
• Gy between OCS residing in HPLMN and PCEF located in IP-Edge fixed broadband access owned by a Convergent fixed-mobile VPLMN, for Non Seamless WLAN Offload (NSWO) 3GPP UEs traffic defined by annex S of TS 23.203.

During this first phase, the study will also investigate a basic set of Online Charging features and parameters, to be used in an inter-PLMN situation. Based on conclusions and recommendations for the first phase, the second phase of the study will address the inter-PLMN Gyn Reference Point for the support of Application Based Charging.