On the Optimal Design of a Spectrum-Switched Optical Network With Multiple Modulation Formats and Rates

Abstract

Spectrum-switched optical networks (SSONs) are an attractive solution for core networks. With respect to wavelength-switched optical networks (WSONs), they achieve a higher spectrum efficiency thanks to the use of a flexible grid, instead of a fixed one. In SSONs, the wellknown wavelength assignment (WA) problem of WSONs becomes a spectrum assignment (SA) problem. The SA problem aims at assigning a portion of the spectrum (called a frequency slot) to lightpaths. The width of the frequency slot depends on the requested bit rate and the modulation format adopted for the lightpath, and it is a multiple of the minimum bandwidth granularity, referred to as frequency slice. Thus, differently from WA, which assigns a single wavelength (or color) to each lightpath, SA assigns a set of colors (i.e., a set of frequency slices) to the lightpath. The shift from WA to SA introduces an additional constraint, which is related to the spectral adjacency of such frequency slices. This paper proves that the adjacency constraint in SA is not required and that by solving the WA problem (or the coloring problem) it is possible to derive a solution with spectrally adjacent slices in polynomial time. Based on such results, an integer linear programming formulation (ILP) for the optimal SA in a SSON with multi modulation formats and multi line rates (MMF/MLR) is presented for minimizing the network cost. The total cost of the network comprises the spectrum cost and the transponder card cost. Optimal results are presented for a MMF/MLR-SSON ring and show the amount of total occupied bandwidth and the network costs for different loads and ring lengths. Optimal selection of the modulation format and line rate is driven by the slice cost and the transponder card cost. Result comparison indicates that support of MMF and MLR is especially effective for improving spectrum utilization and allows spectrum saving up to 20% with respect to an SSON ring with single modulation format and line rate.

Improving Link Spectrum Utilization in Flexgrid Optical Networks

“…If a 2 (or 4) THz spectrum is considered for a given link, the number of available FS is 160 (or 320). Each FS can carry some amount of traffic mainly depending on the modulation format used and the required reach [13]. For the sake of simplicity, this work just focus on the spectrum (number of FS) used, avoiding the spectral efficiency problem.…”

“…Furthermore, different modulation formats lately proposed give an additional degree of freedom to accommodate different bit rates on a given bandwidth. For example, (see [3,13] for detailed analysis) a 2 FS connection can convey capacities from <40 Gbit/s (when binary modulation formats are used) to >100 Gbit/s (if complex multi-level modulation formats and coherent detection are present). On the other hand, if the Odd case is considered, elastic to multi-rate connections mapping schemes have been proposed [17], which allow combining different connections to attain even FS values.…”

Combining heuristic and exact approaches for solving the routing and spectrum assignment problem

“…The channels were pre-calculated beforehand in a simple procedure and incorporated in the formulation, thus significantly reducing the number of variables and constraints associated with the spectrum contiguity constraint. The research effort from [15] provided an efficient scheme for enforcing the contiguity constraint in polynomial time by mapping the RSA problem to weighted graphcolouring one where the slots assigned to each vertex are also contiguous. While different ILP versions have dealt with variants of the RSA problem, these ILP models have nevertheless the complexity of non-deterministic polynomial-time hardness (NPhard) [10,13] and therefore, the running time is prohibitively large for practical cases.…”

Spectrum assignment in rings with shortest-path routing: Complexity and approximation algorithms

“…Most of these studies employ heuristics, although some interesting theoretical results do exist. For instance, using results from graph coloring theory, it was shown in [13] that there exists a (4+2 )-approximation algorithm for the spectrum assignment problem in rings; whereas the work in [9] proves that the contiguity (i.e., adjacency) constraint in spectrum assignment can always be satisfied starting from an optimal solution to a corresponding coloring problem.…”