  We show that aperiodic and quasiperiodic variability of bright LMXBs - atoll
and Z- sources, on ~sec - msec time scales is caused primarily by variations of
the boundary layer luminosity. The accretion disk emission is less variable on
these time scales and its power density follows 1/f law, contributing to
observed flux variation at low frequencies and low energies only. The kHz QPOs
have the same origin as variability at lower frequencies - independent of the
nature of the "clock", the actual luminosity modulation takes place on the NS
surface. The boundary layer spectrum remains nearly constant during luminosity
variations and can be represented by the Fourier frequency resolved spectrum.
In the range of Mdot~(0.1-1)*Mdot_Edd it depends weakly on the global mass
accretion rate and in the limit Mdot~Mdot_Edd is close to Wien spectrum with
kT~2.4 keV. Its independence on the Mdot lends support to the suggestion by
Inogamov & Sunyaev (1999) that the boundary layer is radiation pressure
supported.
  Based on 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
thickened 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 NS surface.
