  Spatially extended stationary and traveling states in the strongly nonlinear
regime of convection in layers of binary fluid mixtures heated from below are
described by a few-mode-model. It is derived from the proper hydrodynamic
balance equations including experimentally relevant boundary conditions with a
non-standard Galerkin approximation that uses numerically obtained, symmetry
decomposed modes. Properties of the model are elucidated and compared with full
numerical solutions of the field equations.
