Larger systems built from multiple stacks in higher topologies. 1D wormhole routing. User-mode accessible with full protection. Local buffer to or from remote buffer. Small MPI kernel on computational nodes.
PDSS network is very wide with multiple virtual channels in each direction.
For the given packaging constraints, best use of wires is a 1D network. 1D networks are not fault tolerant. If one node fails, stack fails. Most node failures will not be in router. Also, stacking gets rid of connectors, another common source of failure.