Spectrum fragmentation mitigation in elastic optical networks: a chronologically structured review using a 7-point analytical framework

Elastic optical networks (EONs) play a critical role in enabling scalable, flexible, and high-capacity optical communication to meet rising data demands. However, their dynamic spectrum allocation paradigm leads to spectrum fragmentation, resulting in inefficient resource utilization and degraded network performance. This study presents a chronologically structured review of fragmentation mitigation techniques using a 7-point analysis strategy, encompassing optimization strategies, technical methodologies, simulation topologies, assessment metrics, empirical findings, strengths, constraints and trade-offs. The analysis traces the shift from heuristic and static strategies toward adaptive metaheuristic, multipath-aware, and learning-driven approaches, alongside the emergence of specialized metrics and architectural refinements. Through the visualized performance and deployability timelines, the study reveals how innovation has progressed and where practical implementation remains challenged by centralized assumptions and limited physical-layer modeling. The findings provide a consolidated perspective on the evolution of fragmentation control and highlight future directions toward real-time adaptability, architectural scalability, and physically grounded optimization in EONs.