The purpose of this expose is to explain the generic design of a customized communication subsystem. The expose addresses both functional and non-functional aspects. Starting point is a real-time requirement from the application area building automation. We show how this application requirement and some background information about the application area lead to a system architecture, a communication service, a protocol architecture and to the selection, adaptation, and composition of protocol functionalities. The reader will probably be surprised how much effort is necessary in order to implement the innocuous, innocent, inconspicuous looking application requirement. Formal description techniques (FDTs) will be used in all design phases.