Fragmentation Risk-Aware Spectrum Allocation for Protection Path in Elastic Optical Networks

Elastic optical networks (EONs) face critical challenges in balancing dynamic service provisioning with long-term spectral efficiency, particularly for protecting critical paths against failures. Traditional spectrum allocation strategies for protection paths prioritize short-term objectives like spectrum continuity or local fragmentation minimization, often leading to fragmentation accumulation and underutilized protection resources. This decision-making exacerbates long-term inefficiencies, increasing blocking probabilities for future services in space-division multiplexing (SDM) environments with multi-core fibers (MCFs). To address this, this paper proposes a fragmentation risk index (FRI)-based approach that quantifies the propagation risk of spectrum fragmentation caused by protection path allocations. Unlike existing methods, FRI evaluates candidate spectrum blocks by modeling their impact on future reusability across spatial and spectral dimensions, enabling risk-aware allocation decisions. We integrate FRI into a dynamic spectrum assignment framework, prioritizing allocations that preserve contiguous spectrum regions even at marginal short-term costs. Simulation results demonstrate that FRI reduces blocking probabilities and delays fragmentation accumulation by preserving more usable spectrum in multi-core networks.