This International Standard specifies the architectural communications framework of intelligent transport systems (ITS) for the family of CALM-related International Standards. The architecture is described in an abstract way with several graphical views and examples. The graphical representations partly follow the ISO Open Systems Interconnection (OSI) principles. In addition to the requirements specified within this International Standard, a number of notes and examples are provided to illustrate the CALM concept. Wherever practicable, this International Standard has been developed by reference to suitable extant International Standards, adopted by selection. The architecture provides for regional variations where regulations differ in different countries and regions.