  We report the results from analysis of six observations of Cygnus X-1 by
Large Area X-ray Proportional Counters (LAXPC) and Soft X-ray Telescope (SXT)
on-board AstroSat, when the source was in the hard spectral state as revealed
by the broad band spectra. The spectra obtained from all the observations can
be described by a single temperature Comptonizing region with disk and
reflection components. The event mode data from LAXPC provides unprecedented
energy dependent fractional root mean square (rms) and time-lag at different
frequencies which we fit with empirical functions. We invoke a fluctuation
propagation model for a simple geometry of a truncated disk with a hot inner
region. Unlike other propagation models, the hard X-ray emission (> 4 keV) is
assumed to be from the hot inner disk by a single temperature thermal
Comptonization process. The fluctuations first cause a variation in the
temperature of the truncated disk and then the temperature of the inner disk
after a frequency dependent time delay. We find that the model can explain the
energy dependent rms and time-lag at different frequencies.
