  NGC 5548 was recently monitored intensively from NIR to X-rays as part of the
STORM campaign. Its disc emission was found to lag behind the observed X-rays,
while the measured time lag was increasing with wavelength. These results are
consistent with the assumption that short term variability in AGN emission is
driven by the X-ray illumination of the accretion disc. In this work, we
studied the power spectrum of UV/optical and X-ray emission of NGC 5548, using
the data of the STORM campaign as well as previous Swift data, in order to
investigate the relation between the UV/optical and X-ray variability and to
examine its consistency with the above picture. We demonstrate that even the
power spectrum results are compatible with a standard disc being illuminated by
X-rays, with low accretion rates, but the details are not entirely consistent
with the results from the modelling of the "{\tau} vs {\lambda}" relation. The
differences indicate that the inner disc might be covered by a "warm corona"
which does not allow the detection of UV/optical emission from the inner disc.
Finally, we found strong evidence that the UV emission of NGC 5548 is not
stationary.
