  This paper proposes a new model for wireless relay networks referred to as
"1-2-1 network", where two nodes can communicate only if they point "beams" at
each other, while if they do not point beams at each other, no signal can be
exchanged or interference can be generated. This model is motivated by
millimeter wave communications where, due to the high path loss, a link between
two nodes can exist only if beamforming gain at both sides is established,
while in the absence of beamforming gain the signal is received well below the
thermal noise floor. The main result in this paper is that the 1-2-1 network
capacity can be approximated by routing information along at most $2N+2$ paths,
where $N$ is the number of relays connecting a source and a destination through
an arbitrary topology.
