  We develop a discrete model to account for the effects of inhomogeneities on
the redshift of photons. Using this model we compute the probability
distribution of the observed redshift respect to the background value,
obtaining that its dispersion around the central value is proportional the
square root of the comoving distance. This implies that data analysis should
include a contribution to the total error budget which depends on the distance
of the source. Assuming large scale inhomogeneities with a power spectrum given
by primordial curvature perturbations the effect is expected to be small at low
red-shift, and to become important only at very high redshift. We then consider
what are the general implications for the estimation of background cosmological
parameters, giving some example for the case of quantities related to the
luminosity distance such as the Hubble parameter and the cosmological constant.
  The model correctly reproduces the expected stochastic properties of the
propagation of photons in inhomogeneous media and due to its computational
simplicity it could be particularly suitable for the numerical estimation of
the effects of inhomogeneities on cosmological observables using Montecarlo
methods.
