Inspired by the pioneering work of Gilles Kahn on concurrent systems, we propose to model timed systems as a network of software components (implemented as real-time processes or tasks), each of which is specified to compute a collection of functions according to given timing constraints. We present a fixed-point semantics for this model which shows that each system function of such a network computes for a given set of (timed) input streams, a deterministic (timed) output stream. As a desired feature, such a network model can be modified by integrating new components for adding new system functions without changing the existing ones. Additionally, existing components may be replaced also by new ones fulfilling given requirements. Thanks to the deterministic semantics, a model-based approach is enabled for not only building systems but also updating them after deployment, allowing for efficient analysis techniques such as model-in-the-loop simulation to verify the complete behaviour of the updated system.