This part of ISO 15765 specifies requirements for controller area networks (CAN) where one or more controllers comply with on-board diagnostics (OBD) or world-wide harmonized on-board diagnostics (WWH-OBD) regulations. The network presumes the use of an external test equipment for inspection and repair diagnostics, as defined by the regulations. The CAN network requirements for the vehicle and the external test equipment are based on the specifications of ISO 15765-2, ISO 11898-1 and ISO 11898-2. This part of ISO 15765 places restrictions on those International Standards for the fulfilment of the regulations. It does not specify in-vehicle CAN bus architecture, but seeks to ensure that the vehicle's regulated CAN communications comply with external test equipment requirements. This part of ISO 15765 defines the requirements to successfully establish, maintain and terminate communication with a vehicle that implements the requirements of the OBD/WWH-OBD regulations. Plug-and-play communication capabilities among vehicles and test equipment are defined to assure the interoperation of external test equipment and vehicles. This part of ISO 15765 details all of the OSI layer requirements to achieve this goal. This part of ISO 15765 is the entry point for DoCAN (Diagnostic communication over CAN). Based on the results of the initialization, the external test equipment determines which protocol and diagnostic services are supported by the vehicle's emissions-related system: — legislated OBD: ISO 15031 (all parts), — legislated WWH-OBD: ISO 27145 (all parts).