=== Multilayer Surface Models ===
[[File:PROP26.png|thumb|250px200px|Propagation Module's Penetrable Surface Dialog showing a three-layer wall composition]]
Most of the time, your outdoor propagation scene consists of simple buildings made of single-layer walls with standard material properties (ε<sub>r</sub> and σ). In the case of a single-layer impenetrable surface, the specular interface is an infinite dielectric half-space, which reflects the impinging rays. Single-layer penetrable surfaces, on the other hand, involve finite-thickness dielectric walls, which both reflect and transmit the incident rays. Similarly, most of your indoor propagation scenes involve simple single-layer penetrable walls with the specified material properties ε<sub>r</sub> and σ. A thin wall acts like a finite-thickness dielectric slab that both reflects and transmits incident rays. In the case of the global ground or terrain objects, only ray reflection off the ground surface is considered.