Design and analysis of spectrum-efficient next-generation optical networks

Abstract

Optical fibers form the backbone of the Internet and carry about 99% of the global Internet traffic currently. The traffic generated through the access and metro wireless/wired networks is aggregated to the terrestrial backbone/core optical net works for long-distance communication. As per the Cisco visual networking index (VNI) yearly forecasts, the global internet traffic is continuously increasing at a rate of about 3-fold every 5 years. Until recently, the line rates supported by the stan dard single mode fiber (SSMF) equipped wavelength division multiplexing (WDM) networks were sufficient to satisfy the bandwidth demands in the core network. The incessant growth in the global Internet traffic has caused ‘capacity crunch’ in the core optical network, and it is a general consensus that the currently deployed WDM networks will not be able to support line rates above 100 Gbps, which is insufficient in this Zettabyte era. Elastic optical network (EON) has been widely accepted by both the industry and academia as a promising technology alternative to WDM networks to satisfy the bandwidth demands in near-future, whereas space division multiplexing (SDM) is envisioned as a long-term solution to the increasing bandwidth demands. EON supports increased dynamicity enabled by distance adaptive modulation (DAM), bit-rate variable transceivers, subcarrier multiplexing, among others, thus improving the network resource utilization. SDM exploits the spatial domain using multicore fiber (MCF) and/or multimode fiber (MMF). The optical network supported by both the EON and SDM is commonly known as spectrally-spatially flexible optical network (SS-FON) or SDM-EON. EON requires technology migration at the nodes only, while utilizing the existing SSMFs, whereas, SS-FON equipped with MCF will require the existing fibers to be replaced. The routing and wavelength assignment (RWA) schemes in the existing WDM networks are rather simple and straightforward as compared to the EON and SS FON, where fixed wavelength grid is assigned to a lightpath. With the advance ments in optical network technology, the RWA problem of WDM networks becomes complex in EON, and SS-FON due to spectral, and spectral-spatial flexibility, re spectively. In EON, it is known as routing, spectrum, and modulation assignment (RSMA), and in SS-FON equipped with MCF, it is generally termed as routing, spectrum, modulation, and core assignment (RSMCA). The RSMA and RSMCA problems are subject to new types of constraints introduced by the technology, which affect the complexity of the RSMA and RSMCA algorithms. In optical net works, complex and computationally time-consuming algorithms affect the network latency, which ultimately reduces the advantage of ‘speed of light’ offered by optical fiber communication. Thus, spectrum-efficient RSMA and RSMCA schemes while being simple and fast is one of the main objectives of EON and SS-FON, which is the main focus of this thesis. In this thesis, initially, spectrum efficiency of WDM and EON is compared under an SLA-aware differentiated service (DiffServ) scenario. Here, exhaustive multilevel differentiation at the levels of routing, spectrum allocation, and survivability is done to provision the network resources to the lightpath demands while satisfying the SLA requirements of three different classes of service (CoSs). Worst-case improvement of EON as compared to the WDM networks are obtained under an SLA-aware DiffServ scenario, and the advantages of the flexibility provided by EONs is highlighted. An SLA aware differentiated QoS (SADQ) scheme is proposed to perform resource provisioning, and to compare the performance of EON and WDM networks. Further, the proposed SADQ is compared with other conventional methods in EONs, and the advantages of SADQ are highlighted in terms of better spectrum utilization and reduced fragmentation. DAM is one of the most promising feature of EON. Leveraging the DAM ca pability of EON, a k-distance adaptive paths (KDAP) scheme is proposed. It is demonstrated that the spectrum consumption of the commonly used routing schemes varies in EON, depending on the source-destination pair. Moreover, all these con ventional routing scheme do not utilize the EON spectrum efficiently. Using the proposed KDAP, offline route calculation and prioritization is done utilizing the DAM capability of EON. The proposed KDAP is compared with the conventional routing schemes, namely, hop-count (HC), k-shortest path (KSP), and link-disjoint (LD) routing for three different sized network topologies. Simulations results indi cate that the proposed KDAP can significantly improve the spectrum efficiency of EONs. Survivability against single-link failures can be achieved by restoring the lost lightpath through the LD routes. However, in case of multi-link failures caused by natural disasters such as earthquakes, the commonly used survivability schemes cannot be used. Thus, a multi-link failure scenario in backbone optical network is considered further. A stochastic network model is developed to estimate the sur vivability of optical networks under earthquake induced multi-link failures using graph theory, stochastic geometry, and real seismic data. Further, a physical node relocation scheme is proposed for earthquake risk aware node placement. The pro posed node relocation scheme is compared with the conventional dedicated path protection (DPP) and shared path protection (SPP) schemes under the developed stochastic network model as well as using the real seismic data, hazard-maps, and network topologies from India and the U.S. The advantages of physical topology modification as compared to the conventional survivability methods are highlighted considering EON technology. MCF equipped SS-FON can significantly increase the optical network capac ity. SS-FONs have been considered further and it is observed that the routing in SS-FONs is not merely the selection of links in the network. Instead, it is two dimensional (2D), where each 2D route offers a different crosstalk (XT)-margin. Leveraging the spatial flexibility, a low-crosstalk-margin (LCM) routing scheme is proposed for SS-FONs that performs better than all the existing proactive XT management methods in MCFs. Moreover, it is shown that heterogeneous MCFs of specific core-designs can achieve zero-XT-margin in SS-FONs. The proposed LCM routing obtains near-optimal performance without performing complex and dynamic calculations that are required by the optimal method. It has been observed that the capabilities of the next-generation optical network technologies such as spectral/spatial flexibility, DAM, MCF structure and types, core switching, margins, among others have not been explored completely in the literature, which can aid in efficient network planning and in improving the per formance of optical networks. The schemes developed in this thesis improve the spectrum efficiency of optical networks leveraging the advanced capabilities of opti cal communication and network technologies, and through pre-calculation and offline prioritization based on known network information and parameters.