  Recently, Internet service providers (ISPs) have gained increased flexibility
in how they configure their in-ground optical fiber into an IP network. This
greater control has been made possible by (i) the maturation of software
defined networking (SDN), and (ii) improvements in optical switching
technology. Whereas traditionally, at network design time, each IP link was
assigned a fixed optical path and bandwidth, modern colorless and directionless
Reconfigurable Optical Add/Drop Multiplexers (CD ROADMs) allow a remote SDN
controller to remap the IP topology to the optical underlay on the fly.
Consequently, ISPs face new opportunities and challenges in the design and
operation of their backbone networks.
  Specifically, ISPs must determine how best to design their networks to take
advantage of the new capabilities; they need an automated way to generate the
least expensive network design that still delivers all offered traffic, even in
the presence of equipment failures. This problem is difficult because of the
physical constraints governing the placement of optical regenerators, a piece
of optical equipment necessary for maintaining an optical signal over long
stretches of fiber. As a solution, we present an integer linear program (ILP)
which (1) solves the equipment-placement network design problem; (2) determines
the optimal mapping of IP links to the optical infrastructure for any given
failure scenario; and (3) determines how best to route the offered traffic over
the IP topology. To scale to larger networks, we also describe an efficient
heuristic that finds nearly optimal network designs in a fraction of the time.
Further, in our experiments our ILP offers cost savings of up to 29% compared
to traditional network design techniques.
