Global IoT services (GIoTS) are combining locally available IoT resources with Cloud-based services. They are targeting world-wide services. GIoTS require interoperability between the locally installed heterogeneous IoT systems. Semantic processing is an important technology to enable data mediation as well as knowledge-based processing. This paper explains a system architecture for achieving world-wide semantic interoperability using international standards like oneM2M and the OMA NGSI-9/10 context interfaces (as used in the European Future Internet Platform FIWARE). Semantics also enables the use of Knowledge-based Semantic Processing Agents. Furthermore, we explain how semantic verification enables the testing of such complex systems.