Over the last decade the electronic content in cars has increased dramatically. Today's cars employ a large number of control units that are networked to provide advanced functionality. One of the most difficult issues is the seamless integration of heterogeneous electronic subsystems-typically developed by different suppliers-into an integrated computer system. Consequently, system integrators have a pressing need for a composable system architecture and tool support that facilitates the painless integration of subsystems.During the system design phase this requires a two level design approach. At the cluster or overall system design level the system integrator defines the properties and interactions of the overall systems precisely in the value and time domain. The cluster design serves as a requirements definition for the node level design where node local details can be defined independently. This two level design approach is ideally complemented by the TTP/C communication protocol that provides hard real-time communication, fault-tolerance, clock-synchronization, and a membership service.This paper describes a tool set (TTPplan and TTPbuild) supporting the two level design approach and the TTP® communication protocol.