  The drag reduction properties of a turbulent channel flow modified by
spanwise sinusoidal oscillations of the walls are investigated by direct
numerical simulations. The work is based on the linear relation between the
drag reduction and the parameter $S$, function of the maximum wall velocity and
the period of the oscillation. This quantity has been found through physical
arguments pertaining to the action of the oscillating Stokes layer on the
near-wall turbulence dynamics. The predictive potential of the scaling
parameter is exploited to gain insight into the drag-reducing effects of the
oscillating wall technique. The period of oscillation which guarantees the
maximum drag reduction for a given maximum wall displacement is studied for the
first time. The issue of the minimum intensity of wall forcing required to
produce a non-zero drag reduction effect and the dependence of the drag
reduction on the Reynolds number are also addressed. The drag reduction data
available in the literature are compared with the prediction given by the
scaling parameter, thus attaining a comprehensive view of the state of the art.
