  The central engine causing the production of jets in radio sources may work
intermittently, accelerating shells of plasma with different mass, energy and
velocity. Faster but later shells can then catch up slower earlier ones. In the
resulting collisions shocks develop, converting some of the ordered bulk
kinetic energy into magnetic field and random energy of the electrons which
then radiate. We propose that this internal shock scenario, which is the
scenario generally thought to explain the observed gamma-ray burst radiation,
can work also for radio sources in general, and for blazar in particular. We
investigate in detail this idea, simulating the birth, propagation and
collision of shells, calculating the spectrum produced in each collision, and
summing the locally produced spectra from those regions of the jet which are
simultaneously active in the observer's frame. We can thus construct snapshots
of the overall spectral energy distribution as well as time dependent spectra
and light curves. This allows us to characterize the predicted variability at
any frequency, study correlations among the emission at different frequencies,
specify the contribution of each region of the jet to the total emission, find
correlations between flares at high energies and the birth of superluminal
radio knots and/or radio flares. The model has been applied to qualitatively
reproduce the observed properties of 3C 279. Global agreement in terms of both
spectra and temporal evolution is found. In a forthcoming work, we explore the
constraints which this scenario sets on the initial conditions of the plasma
injected in the jet and the shock dissipation for different classes of blazars.
