For crosstalk noise calculation, computing switching windows of a net helps us identify noise sources accurately. Traditional approaches use a single continuous switching window for a net. Under this model, signal switching is assumed to happen any time within the window. Although conservative and sound, this model can result in too much pessimism since in reality the exact timing of signal switching is determined by a path delay up to the net, i.e. the underlying circuit structure does not always allow signal switching at arbitrary time within the continuous switching window. To address this inherent inaccuracy of the continuous switching window, we propose a refinement of the traditional approaches, where signal switching is characterized by a set of discontinuous switching windows instead of a single continuous window. Each continuous switching window is divided into multiple windows, called time slots, and the signal switching activity of each slot is analyzed separately to calculate the maximum noise with more accuracy. By controlling the size of a time slot we can trade off accuracy and runtime, which makes this approach highly scalable. We have confirmed by experiments on industrial circuits that up to 90% of the noise violations detected by the traditional approach can be unreal.