  We report an inelastic neutron scattering study of the spin waves of the
one-dimensional antiferromagnetic spin ladder compound RbFe$_2$Se$_3$. The
results reveal that the products, $SJ$'s, of the spin $S$ and the magnetic
exchange interactions $J$'s along the antiferromagnetic (leg) direction and the
ferromagnetic (rung) direction are comparable with those for the stripe ordered
phase of the parent compounds of the iron-based superconductors. The
universality of the $SJ$'s implies nearly universal spin wave dynamics and the
irrelevance of the fermiology for the existence of the stripe antiferromagnetic
order among various Fe-based materials.
