This Technical Specification specifies the general rules for an on-board data communication system between the different systems that may be used within public transport vehicles. This includes operational support systems, passenger information systems, fare collection systems, etc. This Technical Specification describes: - the requirements for an on board IP network; - the overview architecture and components for an IP based on-board network; - the modular structure of the network architecture; - the Service Oriented Architecture (SOA) approach, and approach to defining services. Systems directly related to the safe operation of the vehicle (including propulsion management, brake systems, door opening systems) are excluded from the scope of this Technical Specification and are dealt with in other standardization bodies. However, the architecture described in this Technical Specification may be used for support services such as safety information messages. Interfaces to safety-critical systems should be provided through dedicated gateways with appropriate security provisions; for the purposes of this Technical Specification, these are regarded as simply external information sources. This Technical Specification is designed primarily for vehicles with a fixed primary structure, where networks can be installed on a permanent basis and the system configuration task consists largely of the integration, adjustment or removal of the functional end systems that produce and/or consume data. Public transport vehicles consisting of units linked temporarily for operational purposes (specifically, trains in which individual engines, cars or consists are routinely connected and disconnected) require additional mechanisms to enable the communications network itself to reconfigure. Such mechanisms are provided through other standards, notably the IEC 61375 series.