WDM Optical Networks Planning Using Greedy Algorithms

Abstract

Optical networks have been established as the enabling technology for today’s high-speed communication networks. Wavelength Division Multiplexing (WDM) enables the efficient utilization of optical fibers by dividing its tremendous bandwidth into a set of disjoint wavelength bands, referred to as wavelengths. Each wavelength supports one communication channel which corresponds to an end user operating at an arbitrary speed, e.g. peak electronic speed. This helps to overcome the opto-electronic mismatch between the multiple terabit-per-second bandwidth of optical fibers and the gigabit-per-second electronic processing speeds at end users. In wavelength-routed WDM networks, all-optical directed channels, called lightpaths, can be established between pairs of nodes which are not necessarily neighboring in the physical topology. A set of lightpaths creates a so-called virtual topology over the physical interconnection of fibers. Packet-switched traffic is then routed over this virtual topology, independent of the physical topology. Traffic send via a lightpath is transmitted in the optical domain with no opto-electronic conversion at intermediate nodes. Establishing a lightpath requires a transmitter and receiver at the source and destination nodes, respectively, and includes routing it over the physical topology and assigning to it a wavelength. One of the main challenges in wavelength-routed WDM networks is to successfully solve the Virtual Topology Design (VTD) problem. This problem is usually divided into the following four sub-problems. The first is to determine the set of lightpaths which is to form the virtual topology. This set of lightpaths can be static, scheduled or dynamic. Static lightpaths are established semi-permanently and chosen on the basis of a traffic matrix representing the estimated average traffic floes between node pairs. Scheduled lightpaths, on the other hand, try to exploit the periodic nature of traffic by defining a schedule for establishing and tearing down lightpaths based on periodic traffic trends. Lastly, dynamic lightpaths are established as connection requests arrive with no a priori information regarding traffic demands. Unless specified otherwise, the VTD problem usually refers to the static case which we will be discussing in the remainder of this chapter. Thus, we use these terms interchangeably. The second sub-problem in VTD is to find for each lightpath a corresponding route in the physical topology, while the third is to assign to each a wavelength subject to certain constraints. Lightpaths routed over the same physical links at the same time cannot be O pe n A cc es s D at ab as e w w w .in te ch w eb .o rg