In the framework of the Laplacian transport, describing by a Robin boundary-valued problem in an exterior domain in R^n , we generalize the definition of the Poincaré-Steklov operator to d-set boundaries, n − 1 ≤ d < n , and give its spectral properties to compared to a the spectrum of the interior domain and also to a truncated domain, considered as an approximation of the exterior case. The well-posedness of the Robin boundary-valued problems for the truncated and exterior domains are obtained in the general framework of n-sets.