For the design of real-time embedded systems, analysis of performance and resource utilization at an early stage is crucial to evaluate design choices. Network Calculus and its variants provide the tools to perform such analyses for distributed systems processing streams of tasks, based on a max-plus algebra. However, the underlying model employed in Network Calculus cannot capture correlations between the availability of different resources and between the arrivals of tasks, leading to overly conservative performance bounds for some frequently used system topologies. We present a model based on timing constraints relative to pairs of streams, endowed with an analysis technique that can handle such correlations.