  We demonstrate that Fourier-frequency resolved spectra of atoll and Z-
sources are identical, despite significant difference in their average spectra
and luminosity (by a factor of ~10-20). This result fits in the picture we
suggested earlier, namely that the f> 1 Hz variability in luminous LMXBs is
primarily due to variations of the boundary layer luminosity. In this picture
the frequency resolved spectrum equals the boundary layer spectrum, which
therefore can be straightforwardly determnined from the data. The obtained so
boundary layer spectrum is well approximated by the saturated Comptonization
model, its high energy cut-off follows kT~2.4 keV black body. Its independence
on the global mass accretion rate lends support to the theoretical suggestion
by Inogamov &Sunyaev (1999) that the boundary layer is radiation pressure
supported. With this assumption we constrain the gravity on the neutron star
surface and its mass and radius. Equipped with the knowledge of the boundary
layer spectrum we attempt to relate the motion along the Z-track to changes of
physically meaningful parameters. Our results suggest that the contribution of
the boundary layer to the observed emission decreases along the Z-track from
conventional ~50% on the horizontal branch to a rather small number on the
normal branch. This decrease can be caused, for example, by obscuration of the
boundary layer by the geometrically thick accretion disk at Mdot ~ Mdot_Edd.
Alternatively, this can indicate significant change of the structure of the
accretion flow at Mdot ~ Mdot_ Edd and disappearance of the boundary layer as a
distinct region of the significant energy release associated with the neutron
star surface.
