Realization of Interworking in LTE Roaming Using a Diameter-Based Interworking Function

While operators around the world are onboarding or planning to onboard the LTE-based Evolved Packet System (EPS), some troubling questions linger on. The prospects of LTE are enticing. However, would the LTE and Legacy networks interwork and co-exist? Would inbound/outbound roamers face any service disruption? For a green-field operator, possibly with a tight budget, is adopting LTE a risky proposition when over 90% of the world's network is still based on Legacy Signaling System No.7 (SS7)? Various standards bodies have provided guidelines and specifications to identify and address some of the interworking and co-existence scenarios, but their implementation is complex and requires a detailed knowledge of the disparate worlds of EPS and Legacy protocols in finding an acceptable intersection between the two. This paper describes the important interworking issues between Diameter and TCAP (SS7)-based protocols and some of the practical aspects that transcend information that is disseminated through the standards bodies. Some specific scenarios such as a 2G/3G subscriber roaming into an LTE network using home-routed applications and S8 (EPS) - Gp (Legacy) interworking are not covered here and shall be discussed in a subsequent paper. Similarly, this paper has not focused on Diameter-RADIUS interworking needed to support RADIUS-based AAA Server. However, most other scenarios have been addressed and it is hoped that the solution presented here will alleviate the roaming, co-existence and interworking concerns of the reader.