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.
Third, we make different assumptions about what parts of the infrastructure are given and fixed. take the locations of optical equipment as an input constraint, while we solve for the optimal places to put tails and regens. This distinction is crucial; don't need any notion of here-and-now decisions about where to place tails and regens separate from wait-and-see decisions about IP link configuration and routing. Other studies demonstrate that, to minimize delay, it is best to set up direct IP links between endpoints exchanging significant amounts of traffic, while relying on packet switching through multiple hops to handle lower demands @cite_3 .
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We develop algorithms for joint IP layer routing and WDM logical topology reconfiguration in IP-over-WDM networks experiencing stochastic traffic. At the WDM layer, we associate a non-negligible tuning latency with WDM reconfiguration, during which time tuned transceivers cannot service backlogged data. The IP layer is modeled as a queueing system. We demonstrate that our algorithms achieve asymptotic throughput optimality by using frame-based maximum weight scheduling decisions. We study both deterministic and random frame durations. In addition to dynamically triggering WDM reconfiguration, our algorithms specify precisely how to route packets over the IP layer during the phases in which the WDM layer remains fixed. Our algorithms remain valid under a variety of optical layer constraints. We provide an analysis of the specific case of WDM networks with multiple ports per node. In order to gauge the delay properties of our algorithms, we conduct a simulation study and demonstrate an important tradeoff between WDM reconfiguration and IP layer routing. We find that multi-hop routing is extremely beneficial at low throughput levels, while single-hop routing achieves improved delay at high throughput levels. For a simple access network, we demonstrate through simulation the benefit of employing multi-hop IP layer routes.