  We present a model in which AGN jets are described by a dense series of
plasma blobs, distributed in masses and velocities. The blobs expand and
collide, and ultimately form a continuous jet flow as observed on larger
scales. When blobs collide, energetic electrons are produced by first order
Fermi acceleration, and emit synchrotron radiation at the radio-optical
frequencies with a double-broken power law spectrum. The difference of this
model to the well-known Marscher&Gear-model is, that in our model shocks are
produced on a range of scales, and exist only temporarily, which causes
differences in the prediction for the spectral evolution of flares. We apply
our model to radio-submm data obtained for 3C454.3, and briefly discuss
implications for gamma-ray production.
