Analysis of Spectrum Defragmentation in QoT-Constrained Elastic Optical Networks

Paper in proceeding, 2025

Spectrum fragmentation and physical-layer impairments pose significant challenges to the efficient operation of elastic optical networks (EONs). While defragmentation techniques can consolidate idle spectrum and reduce blocking, most existing studies neglect the dynamic interplay between amplified spontaneous emission (ASE) noise and nonlinear interference (NLI). In this work, we implement a defragmentation algorithm and conduct a comprehensive performance evaluation of the routing, modulation format, and spectrum assignment (RMSA) problem on the European topology under dynamic network traffic. Three quality-of-transmission (QoT) constraints to the modulation format selection are compared: distance-based reach limits, ASE-only impairment, and ASE+NLI impairment. Our results show that, although defragmentation reduces blocking rates in all cases, relative gains under realistic ASE+NLI conditions are smaller, up to 40% lower than those reported using the simplified distance-based model. At higher loads, the proposed approach triggers fewer defragmentation cycles, although each cycle defragments more lightpaths, which tend to increase control plane signaling overhead. These findings underscore the need for integrating realistic physical-layer impairment models into resource allocation and defragmentation strategies for next-generation EON deployments.