  Blazars, a subclass of active galactic nuclei (AGN), are known to be bright
$\gamma$-ray sources, frequently exhibiting active (flaring) periods. The
blazar PKS~2155-304 is a high synchrotron-peaked BL Lac object located at
redshift $z=0.116$. On 2006 July 28, an extremely remarkable outburst of VHE
$\gamma$-ray emission from this blazar was reported by the H.E.S.S. experiment,
with an average flux more than 10 times the low-state level. The variability
timescale of this extraordinary flare was as short as approximately 200~s. In
order to guarantee the transparency of the emission region for TeV photons, the
fast variability demands an extremely high Doppler factor $\delta_{\rm D}>50$
of the jet within the classical one-zone model, leading to the so-called
"Doppler factor crisis". Here we demonstrate that the stochastic dissipation
model, which is a multi-blob scenario for blazars, can self-consistently
explain the giant TeV flares of PKS~2155-304 and the low-state emission before
and after the flares, in terms of both multi-wavelength spectral and
variability characteristics. The required Doppler factor in this model can be
as low as 20, which is a reasonable and typical value for blazar jets. The
obtained model parameters may shed some light on the physical properties of the
relativistic jet.
