  We present a phenomenological jet model which is able to reproduce well the
observed variations of the submillimetre-to-radio emission of the bright quasar
3C 273 during the last 20 years. It is a generalization of the original shock
model of Marscher & Gear (1985), which is now able to describe an accelerating
or decelerating shock wave, in a curved, non-conical and non-adiabatic jet. The
model defines the properties of a synchrotron outburst which is expected to be
emitted by the jet material in a small region just behind the shock front. By a
proper parameterization of the average outburst's evolution and of the
peculiarities of individual outbursts, we are able to decompose simultaneously
thirteen long-term light-curves of 3C 273 in a series of seventeen distinct
outbursts. It is the first time that a model is so closely confronted to the
long-term multi-wavelength variability properties of a quasar. The ability of
the model to reproduce the very different shapes of the submillimetre-to-radio
light curves of 3C 273 gives strong support to the shock model of Marscher &
Gear (1985). Indirectly, it also reinforces the idea that the outbursts seen in
the light-curves are physically linked to the distinct features observed to
move along the jet with apparently superluminal velocities.
