  We consider directed acyclic networks with multiple sources and multiple
terminals where each source generates one i.i.d. random process over an abelian
group and all the terminals want to recover the sum of these random processes.
The different source processes are assumed to be independent. The solvability
of such networks has been considered in some previous works. In this paper we
investigate on the capacity of such networks, referred as {\it sum-networks},
and present some bounds in terms of min-cut, and the numbers of sources and
terminals.
