Data, big or small, in social networks, evolutionary dynamics, the
world-wide-web, or citation networks are indexed by social agents, individuals
of a population, web sites, or authors all very different from time marks or
image pixels. The relations among these data are captured by a graph and not as
simple as with data samples in traditional time series, nor pixels in images. We
extend traditional discrete signal processing (DSP) tools and concepts to
signals defined in graphs.