  In this work, we consider a novel distributed data storage/caching scenario
in a cellular setting where multiple nodes may fail/depart at the same time. In
order to maintain the target reliability, we allow cooperative regeneration of
lost nodes with the help of base stations allocated in a set of hierarchical
layers. Due to this layered structure, a symbol download from each base station
has a different cost, while the link capacities connecting the nodes of the
cellular system and the base stations are also limited. In this more practical
and general scenario, we present the fundamental trade-off between repair
bandwidth cost and the storage space per node. Particularly interesting
operating points are the minimum storage as well as bandwidth cost points in
this trade-off curve. We provide closed-form expressions for the corresponding
bandwidth (cost) and storage space per node for these operating points.
Finally, we provide an explicit optimal code construction for the minimum
storage regeneration point for a given set of system parameters.
