We discuss all the necessary parameters to describe optical racetrack micro resonators fabricated with silicon on insulator (SOI) technology. We focus on some fundamental aspects crucial for a comprehensive and realistic modelling of racetrack resonators as building blocks of complex add-drop filters, like SCISSOR (Side-Coupled Integrated Spaced-Sequences of Resonators) or CROW (Coupled Resonator Optical Waveguides). When the radius of curvature is lower than 5 μm, and/or the separation gaps between waveguide and resonator is small, dispersion law, effective index mismatch, and mode mismatch between bend and straight waveguides are relevant for modelling. A new mode solver, specifically suited for high index contrast small mode area waveguides, is used whose results are compared with the measurement of the optical response of some resonant devices.