This document specifies the choice and the general application's rules of an onboard data transmission bus between the different equipment for service operations and monitoring of the fleet. This applies to equipment installed onboard buses, trolleybuses and tramways only as part of a bus fleet operation. It excludes tramways when they are operated as part of a train, subway or metro operation. This equipment includes operation aid systems, automatic passenger information systems, fare collection systems, etc.The equipment directly related to the safety-related functioning of the vehicle (propulsion management, brake systems, door opening systems, etc.) is excluded from the scope of the present document and are dealt with in other standardisation bodies.For the described application two bus systems are standardised. Part 1 to part 3 describes the WORLDFIP bus system and part 4 to part 6 describes the CANopen bus system. There is no ranking between the two bus systems.This document covers the link between equipments inside a single vehicle. Although it could be applied to multiple vehicles, this application is not explicitly covered by this document.Part 4 of this document (this document) specifies the CANopen-based network. This specification describes the general architecture in terms of hierarchical layers according to the ISO reference model for Open Systems Interconnection (OSI) specified in ISO 7498.Part 5 of this document specifies in detail the connectors and the connector pin assignment and the cabling.Part 6 of this document specifies in detail the application profiles for the virtual devices in public transport