Sridhar Iyer

Next-Generation Variable-Line-Rate Optical WDM Networks: Issues and Challenges

Abstract With the ever-increasing traffic demands, the infrastructure of current 10 Gbit/s optical network needs to be enhanced. The legacy infrastructure can be enhanced not only by increasing the capacity, but also by adapting advance modulation format, having increased spectral efficiency at higher data rate. In all-optical mixed line rate (MLR) network, feasibility of a lightpath is determined by physical layer impairment (PLI) accumulation. Contrary to physical layer impairment-aware routing and wavelength assignment (PLIA-RWA) algorithm applicable for a 10 Gbit/s WDM network, that selects a feasible route-keeping the signal to noise ratio value at the receiver above the threshold limit, a new Routing, Wavelength, Modulation format assignment (RWMFA) algorithm is required for a MLR optical network. This article reviews the major PLIs present in an optical fiber, emphasizing those that result in performance degradation of an MLR system. The article also surveys the advance modulation formats that are spectrally efficient and noise resistant. The article further presents a survey of different RWMFA (PLIA-RWA) algorithms for MLR networks and finally identifies several open problems for future research.

Study on Mitigation of Transmission Impairments and Issues and Challenges with PLIA-RWA in Optical WDM Networks

Abstract In all-optical WDM networks, feasibility of a lightpath are determined by physical layer impairment accumulation. Physical layer impairment-aware routing and wavelength assignment (PLIA-RWA) algorithms select a feasible route keeping signal to noise ratio value at the receiver above the threshold limit. This paper reviews the impairments present in optical fiber emphasizing those that result in performance degradation of WDM system. Different types of channel allocation schemes to reduce the number of four wave mixing components generated and performance of the WDM optical network under these channel allocation schemes have also been reviewed. The performance of WDM network due to collective effect of FWM-SRS and FWM-XPM has been discussed. Limitations of amplifier and its optimal location have also been discussed. The article further examines and surveys the role of wavelength converters/regenerators in regard to efficient network design. The paper also presents a survey of different PLIA-RWA algorithms and identifies several open problems for future research.

A novel hybrid node architecture for reducing time delay in Wavelength Division Multiplexed (WDM) translucent network

The practical deployment of widely favoured transparent optical networks is prohibitive as transmission reach of optical signals in such networks is limited by accumulation of physical layer impairments (PLIs). Translucent optical networks have thus become a viable solution and are seen as technically feasible in the near future. In translucent networks, the optical-electrical-optical (OEO) conversion process involved for both, regeneration and/or wavelength conversion leads to significant time delay in data and voice transmission, which becomes a performance bottleneck. In this paper, we propose a hybrid translucent node architecture which performs OEO conversion when 1) only regeneration or 2) simultaneous regeneration and wavelength conversion is required but uses an all-optical wavelength converter when only wavelength contention is to be resolved, thereby reducing the delay involved due to OEO conversion. Performance of the hybrid node is compared to translucent and transparent node architectures present in literature and results clearly show that the use of hybrid node gives the same blocking performance compared to nodes which use OEO conversion for both, regeneration and/or wavelength conversion but incurs less time delay. The results also indicate that there exists a definitive trade-off between cost of the network and OEO conversion time delay.

A novel offline PLI-RWA, regenerator placement and wavelength converter placement algorithm for translucent optical WDM networks

The emergence of translucency in WDM networks as a trade-off between the low cost of full transparency and high signal quality provided by full opacity has led to immense interest in translucent network design by the research community in recent years. Efficient translucent network design necessitates the judicious choice of regeneration sites and number of regenerators in order to maintain a predefined quality of transmission and to reduce the overall network cost. In this paper, we explicate a framework that corroborates the offline version of Physical Layer Impairment Aware Routing and Wavelength Assignment (PLI-RWA) problem in translucent networks. The originality of the proposed PLI-Signal Quality Aware RWA (PLI-SQARWA) algorithm lies in finding the routes which use fewest amounts of regenerators and constituting an efficient Regenerator Placement (RP) algorithm to place regenerators before wavelengths are assigned leading to maximal use of regenerators as wavelength converters. Subsequently, in the WA phase, for any further wavelength contention resolutions; we resort to optical wavelength converters. A relevant difference between existing studies and the proposed algorithm is that PLI-SQARWA provisions both, regeneration and wavelength conversion which guarantees zero signal and wavelength blocking. We proceed to the performance comparison of PLI-SQARWA with a heuristic for RWA and RP called COR2P (Cross-Optimization for RWA and RP) and the comparison results reveal that PLI-SQARWA outperforms COR2P in terms of number of network regenerators and time delay while demonstrating similar blocking performance to COR2P at various traffic loads. The results also suggest that to minimize time delay due to Optical-Electrical-Optical (OEO) conversions, using optical wavelength converters for only wavelength contention resolution is a judicious choice rather than resorting to regenerators. Finally, it can be inferred from the study that there exists a trade-off between the overall network cost and the maximum tolerable delay.

Investigation of cost,power,and spectral efficiency in fixed-and flexi-grid networks

With the steady growth of traffic volume in core networks, it is predicted that future optical network communication will be constrained mainly by cost and power consumption. Thus, for Internet sustainability, it will be necessary to ensure cost and power efficiency in optical networks. The aims of this study are (i) to identify the main sources of cost and power consumption in fixed-grid (SLR and MLR) and flexi-grid (OFDM) optical networks, and (ii) to compare techniques for improving cost and power efficiency in SLR/MLR- and OFDM-based networks. To this end, we conducted a comparative analysis of cost and power efficiency for the OFDM- and MLR/SLR-based networks, and considering realistic networks, evaluated the cost and power consumed by different components in the optical layer. Our results show that (i) OFDM-based networks outperform those based on MLR/SLR in terms of both cost and power-efficiency, (ii) the extra equipment cost incurred due to under-utilization of spectrum is reduced by switching to a flexi-grid network, (iii) lower power consumption per bit is obtained when the networking solution ensures a finer bit-rate granularity, and (iv) there exists a power and spectrum minimization trade-off that is network characteristic dependent.

A Novel Signal and Delay Aware, Zero Blocking PLI-RWA and Translucent Node Architecture for Optical WDM Networks

equipped with the novel hybrid translucent node, optimum delay value for obtaining the desired QoT is found to be lower compared to when the network is equipped with node architectures existing in literature.

Effect of SRS and FWM on performance of optical star WDM networks: A deterministic approach

In all optical WDM network the signal remains in optical domain between source and destination without optical to electrical (O-E) conversion at intermediate nodes. As the signal propagates through the network the fiber nonlinearities like four wave mixing (FWM), and stimulated Raman scattering (SRS) degrades the signal quality. This paper investigates the combined effect of FWM and SRS on the performance of WDM all optical networks. Performance comparison in terms of signal to noise ratio of the worst affected channel has been carried out when (a) thermal, shot, FWM and (b) thermal, shot, FWM and SRS noise are present. Numerical results are presented in the graphical forms for the practical values of parameters.

Comparison of Cost, Power Consumption, and Spectrum Utilization in Protected Fixed- and Flexi-Grid Optical Networks

ABSTRACT In the current work, with a view on network resilience, we investigate gains which result from utilization of the flexible elastic optical networks (EONs). We also focus on an important capability of the EON, viz. the squeezed protection which, in cases of failure(s), minimizes the need for resource(s). We conduct in-depth simulations considering realistic (1) assumptions and (2) topologies, which are characterized by different node(s) numbers, link distances, and corresponding realistic traffic matrices. Through simulations, for the various protection schemes, we compare the fixed-grid wavelength-switched optical networks (WSONs) and the EONs, and the obtained simulation results demonstrate that EONs significantly outperform WSONs. Also, for the considered performance metrics, viz. CapEx/OpEx, power consumption, and spectrum utilization, the WSONs are observed to require more amounts of resources compared to the EONs. Finally, on the one hand, the squeezed protection approach used in EONs is seen to allow for a significant minimization of resources; on the other hand, it only protects a fraction of the traffic.

Solutions to large cases of RSA with transceiver and regeneration allocation (RSAwTCRA) problem in elastic optical networks

In view of determining the optimal solutions to large problem cases of the routing and spectrum allocation with transceiver and regeneration allocation problem in elastic optical networks, the optimization proves to be challenging in terms of the (i) size of the network, (ii) demand(s) amount, and (ii) width of the spectrum. In the current work, we develop an optimization approach that is a combination of advanced scalable (heuristics) and large-scale optimization approaches. The routing and spectrum allocation with transceiver and regeneration allocation problem is formulated as a mixed integer program, and to solve it efficiently, the branch and price algorithm is upgraded with the following methods: (i) relaxation(s) of the problem and cuts to improve the optimal objective value’s lower bounds and (ii) formulation of a heuristic to improve the upper bounds, and finally, the aforementioned elements are mixed to form an effective optimization method. The simulation conducted for (i) realistic topologies characterized by different node(s) numbers and link distances, and (ii) large sets of demands demonstrate that the developed algorithm’s performance is efficient, and it is also applicable to problem cases for which determining the solution(s) proves to be difficult by using commercially available solvers (e.g. CPLEX) in the market.

Launch power determination algorithm for dynamic traffic provisioning in mixed-line-rate optical wavelength division multiplexed networks

In transparent mixed-line-rate MLR optical networks, different line rates, on different wavelengths, can coexist on the same fibre. However, along the path, signal experiences various physical layer impairments PLIs, and its quality also degrades. A major factor that affects transmission quality is launch power of the optical signal. On one hand, power must be high enough to ensure less noise at receiver; on the other hand, it must be lower than the limit where PLIs start to distort the signal. Further, high launch power is disruptive to both, the actual lightpath and its neighbours. In this study, we investigate the problem of determining appropriate launch power for provisioning dynamic connection requests in transparent MLR networks. We propose a heuristic that determines the appropriate launch power of a lightpath. The PLI-average PLI-A approach is based on the optical reach of signals, is practical, and can adapt to the needs of network operators. Results show that performances of the proposed approach are better than the existing schemes.