Multiplexing within motorcars, that means using invehicle- controller-networks, is a new trend to push car developement to more economics, to less pollution, to more security and to more convenience. More and more functions will be integrated into the networks which will grow larger and larger. That leads to so called structured or hierarchical networks.One main difference of controller-nets in comparison with other local area networks is, that they have to meet realtime requirements. Messages, crossing the communication lines, have to do that within a certain time to fullfill certain control functions.To support all phases of network developement, beginning with planning and ending with test and maintenance, an according toolset family - comprising Simulator, Emulator and Analyzer - has been introduced by I + ME-Company , , , 19].This paper results from a joint venture between Fachhochschule and I + ME-Company in Wolfenbüttel, Germany (FRG). It will start with a short overview about structured networks, about its developement phases and its appropriate developement tools and then concentrate on planning those networks by the support of an appropriate simulator.