  We argue that two-step models, like String and Glasma models, with creation
first of sources extended in rapidity that after locally decay into particles,
lead to long range forward-backward correlations due to fluctuations in the
number or the colour of the sources, and to a similar expression for the {\it
F-B} correlation parameter $b$.
  In the simplest String percolation scenario, $b$ increases with centrality or
the number of participating nucleons. However, asymptotically it decreases with
energy. This is different from the Glasma model where $b$ increases with both,
centrality and energy.
