Roaming between GSM/UMTS and IMS networks

Operators around the world are planning deployment of IMS Networks. It is therefore expected that GSM, UMTS and IMS Networks will co-exist for several years before full transition to IMS networks happens. There is a need therefore to support roaming of users from the legacy GSM/UMTS to IMS networks if the user device hosts IMS client software. This paper analyzes the problem and identifies the need for a VLR functional entity in the IMS network. It evaluates architectural alternatives for VLR placement and recommends use of one alternative. Finally, it captures the call flows for Registration, Call origination and Call termination to validate the recommended architectural alternative.