Wavelength Division Multiplexing Networks Research Articles

Benefits of Elastic Spectrum Allocation in Optical Networks with Dynamic Traffic

Traditional optical core networks based on Wavelength Division Multiplexing (WDM) provide optical channels (wavelengths) rigidly allocated over the optical spectrum and separated by 50 or 100 GHz. The novel concept of Elastic Optical Network (EON) can help to improve network flexibility by allo-cating multiple sub-channels to incoming connection requests with finer bandwidth granularity (hence the term elastic) over a flexible spectrum grid. In this paper, we propose an algorithm for connection resource provisioning in EONs. We compare the per-formance of flexible-grid EONs to that of fixed-grid WDM net-works under dynamic traffic, with and without grooming capa-bility, in terms of blocking probability and network resource (i.e. overall spectrum) occupation, thus showing quantitatively the advantage of EON compared to grooming-based WDM networks.

Performance and Complexity of Digital Clock Recovery for Nyquist UDWDM-PON in Real Time

We analyze the performance and the complexity of a real-time digital signal processing (DSP)-based optical network unit architecture for the digital detection of Nyquist-shaped signals in ultra-dense wavelength-division multiplexing passive optical network (UDWDM-PON) scenarios. Using field-programmable gate array-based 8-bit DSP, we experimentally demonstrate in a real-time mode operation the receiver performance in terms of Nyquist filter roll-off factor for $8\times 2.5$ Gb/s quadrature phase-shift keying (QPSK) with 2.5 GHz of channel spacing. We analyze the performance of the linear and cubic interpolation for the clock recovery as well as the Gardner algorithm for the timing-error estimation, considering the amplitude and power formulations. The experimental evaluation shows that the linear interpolation along with the power formulation is an attractive solution for the QPSK-based UDWDM-PON, considering the tradeoff between the performance and the complexity.

Schemes for inter-domain lightpath establishment based on PCE architecture

Despite the research advances in intra-domain lightpath provisioning in Wavelength Division Multiplexing networks, efficient and practical schemes for path computation and resource advertisement in multi-domain mesh networks still need to be developed. Most of the solutions proposed in the literature lacks the ability to convey optical network-specific Traffic Engineering information and are based on a periodic message flooding technique. This paper proposes three novel solutions for inter-domain lightpath provisioning in WDM circuit switched mesh networks. Two routing advertisement schemes and two path and wavelength selection criteria for the PCE architecture are proposed. The solutions provide a complete inter-domain routing and wavelength solution. Simulation experiments show the effectiveness of the proposed solutions which significantly reduce the total amount of message exchanged as well as overall call blocking.

Asymmetrical TWDM-PON with 4 × 25-Gb/s downstream DSB OFDM and 4 × 10-Gb/s upstream OOK modulations

We propose and experimentally demonstrate an asymmetrical time wavelength division multiplexing passive optical network (TWDM-PON) scheme with 100-Gb/s downstream and 40-Gb/s upstream capacity using four pairs of wavelengths. Double sideband (DSB) orthogonal frequency division multiplexing (OFDM) with 16-QAM mapping is used for downstream transmission, while on–off keying (OOK) modulation is used for upstream transmission. The experimental results show that the power budget for bidirectional transmission is more than 35.5-dB. This scheme can be a promising solution for 100-Gb/s TWDM-PON with high power budget.

RSOA based WDM-PON delivering 10-Gbps Downstream/5-Gbps Upstream Data

We demonstrate bidirectional wavelength division multiplexing passive optical network (WDM-PON) based on soliton pulse using two channels. The system uses 1550 nm and 1550.04 nm wavelength for each channel, which is capable of delivering 5-Gbps downstream data over 203 km and 2.5-Gbps upstream data over 153 km optical fiber with 0.2dB/km attenuation loss. The optical source for downstream transmission is sech pulse generator at central office and refractive semiconductor optical amplifier at each optical network unit for upstream transmission. The maximum optical to signal noise ratio for upstream and downstream transmission is obtained at input power 7.56dBm. We investigate backscattered optical power, quality factor and bit error rate (BER). The BER performance shows that our proposed scheme is a practical solution to meet the data rate and cost-efficient of the optical links simultaneously in tomorrow’s WDM-PON access networks.

Reconfigurable DP-16QAM/QPSK transponders for the transmission of coherent 200 Gb/s carriers in a flexgrid super-channel arrangement

The next generation of Dense Wavelength Division Multiplexing (DWDM) networks are likely to use Flexgrid arrangement, providing operators with additional flexibility when assigning spectrum compared to traditional DWDM networks using the 50GHz ITU grid. Flexgrid breaks the spectrum up into small (typically 12.5GHz) slots, but its key feature is that contiguous slots can be joined together to form arbitrary sized blocks of spectrum. This additional flexibility will allow faster transponders that utilize high spectral efficiency modulation techniques, like multi-level m-QAM schemes. From the use of these new spectrum efficient modulation formats and finer control over spectrum allocations, a key benefit that Flexgrid offers to the network operators is that their DWDM networks can carry more traffic with optimized spectral efficiency. High speed technology became essential for realizing greater network capacity and enabling network operators to meet the increasing bandwidth challenges from new generation superfast devices, services and applications. Coherent technologies beyond 100G are now focusing on higher level modulation formats and multiple sub-carriers/channels, using super-channels to achieve Terabit transmission.

Reduction of Four Wave Mixing Nonlinearity Effect in WDM Radio over Fiber Systems

The optical networks are fast, robust and error-free, but there are some nonlinearity hindrances which prevent them from being a perfect medium. The performance of wavelength division multiplexing (WDM) in radio over fiber (RoF) systems is found to be intensely influenced by nonlinearity characteristics within the fiber. In this paper, the performance of WDM network is analyzed using optical filter and various external modulation schemes under FWM nonlinearity effect. Here, the effect of input signal power on FWM is studied. Also, the threshold value of input power for different external modulation technique is calculated and hence duobinary modulation is found to be optimal technique for reducing FWM effect in WDM networks.

Energy‐minimized design of IP over flexible optical networks

SummaryEnergy efficiency is an important desirable property for optical networks and a key parameter for the sustainability of the future Internet. In this work, we consider the energy‐minimized design problem for IP over flexible optical networks. We provide an energy‐aware multilayer network‐planning algorithm, which takes as input the network topology, the IP end‐to‐end traffic matrix, the modular model of the IP/MPLS routers, and the feasible configurations of the flexible optical transponders. The algorithm serves the demands for their requested rates by placing router modules, selecting the paths in the IP topology, and the corresponding paths and spectrum slots in the underlying optical topology, together with the flexible optical transponders' transmission configurations. The algorithm's objective is to minimize the network's energy consumption, jointly accounting for the energy consumption of the routers' modules installed, the transponders used, and their configurations. Using realistic energy consumption and network traffic models, we evaluate the energy savings obtained when the proposed algorithm is applied to a flexible network, as opposed to a mixed‐line‐rate wavelength division multiplexing optical network. Moreover, we consider two optimization scenarios, comparing the joint energy consumption optimization of IP and optical layers to a sequential optimization of the energy consumed at these two layers. Copyright © 2015 John Wiley & Sons, Ltd.

Performance of WDM-OFDM for Using Unequalized Optical Frequency Comb

Optical Frequency Comb Generation (OFCG) technique using a Mach Zehnder modulator (MZM) allows to diminish cost of an optical access network based on Wavelength Division Multiplexing (WDM), because it just needs a single source for generating several optical carriers. However, the unequalized comb power is a limitation for the performance of the network. In order to maintain its simplicity and low cost, we propose to combine WDM with Orthogonal Frequency Division Multiplexing (OFDM). This solution allows changing the number of bits per carrier, being adaptive to several link impairments. An algorithm based on bit‒loading strategies maximizes the transmission rate and mitigates the effects of the unequalized channels on a WDM network. The simulation results show the efficiency of the adaptive algorithm to increase the channel capacity up to 35 Gbps over 5 unequalized WDM channels.

Optical power allocation for adaptive transmissions in wavelength-division multiplexing free space optical networks

Attracting increasing attention in recent years, the Free Space Optics (FSO) technology has been recognized as a cost-effective wireless access technology for multi-Gigabit rate wireless networks. Radio on Free Space Optics (RoFSO) provides a new approach to support various bandwidth-intensive wireless services in an optical wireless link. In an RoFSO system using wavelength-division multiplexing (WDM), it is possible to concurrently transmit multiple data streams consisting of various wireless services at very high rate. In this paper, we investigate the problem of optical power allocation under power budget and eye safety constraints for adaptive WDM transmission in RoFSO networks. We develop power allocation schemes for adaptive WDM transmissions to combat the effect of weather turbulence on RoFSO links. Simulation results show that WDM RoFSO can support high data rates even over long distance or under bad weather conditions with an adequate system design.

28 Gb/s duobinary signal transmission over 40 km based on 10 GHz DML and PIN for 100 Gb/s PON.

In this paper, we demonstrate the direct modulation and direct detection of 28-Gb/s duobinary signal for the future downstream capacity upgrade in next generation passive optical network (PON). Commercial 10-GHz directly modulated laser (DML) and PIN with a combined modulation bandwidth of ~7 GHz are used as transmitter and receiver respectively. In order to mitigate the chromatic dispersion induced signal distortion, an optical delay interferometer (DI) is employed to narrow down the signal spectrum, thereby realizing 40-km single mode fiber (SMF) transmission in C-band. Besides, the chirp-induced spectral broadening of the directly modulated signal enables a higher launch power than external modulation schemes, which increases the loss budget of the system. As a result, 31-dB loss budget is achieved, supporting 64 users with 40-km reach. Also, as the transceivers in both optical line terminal (OLT) and optical network unit (ONU) are commercial l0-GHz devices, the proposed scheme is compatible with 40-Gb/s time and wavelength division multiplexing passive optical network (TWDM-PON) systems, providing a cost-efficient alternative for the development of 100G PON.

A Review of Various Algorithms Used toOptimize RWA Problem in WDM Network

Wavelength Division Multiplexing (WDM) is expected to be a popular technique for constructing large optical networks interconnecting a large number of nodes. With suitable optical cross connects (OXC) at the nodes, such a network will allow very flexible switching of the various wavelengths and light-paths between the active sourcedestination pairs. This flexibility in routing will make these networks easy to configure and operate and will also improve the reliability of the network by providing easy-to-set-up alternate paths in case of node and link failures. This paper discuss various nature inspired algorithms to optimize the RWA problem WDM network where traffic conditions are static or may change dynamically as a wavelength assignment problem. Aim of the algorithms explained in this paper is to reduce blocking probability, increase throughput and make the network more efficient for communication.

Coherent ultra dense wavelength division multiplexing passive optical networks

In this paper, we firstly review the progress in ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON), by making use of the key attributes of this technology in the context of optical access and metro networks. Besides the inherit properties of coherent technology, we explore different modulation formats and pulse shaping. The performance is experimentally demonstrated through a 12×10Gb/s bidirectional UDWDM-PON over hybrid 80km standard single mode fiber (SSMF) and optical wireless link. High density, 6.25GHz grid, Nyquist shaped 16-ary quadrature amplitude modulation (16QAM) and digital frequency shifting are some of the properties exploited together in the tests. Also, bidirectional transmission in fiber, relevant in the context, is analyzed in terms of nonlinear and back-reflection effects on receiver sensitivity. In addition, as a basis for the discussion on market readiness, we experimentally demonstrate real-time detection of a Nyquist-shaped quaternary phase-shift keying (QPSK) signal using simple 8-bit digital signal processing (DSP) on a field-programmable gate array (FPGA).

In-band simultaneous transmission of baseband and broadcast signals in wavelength reused bidirectional passive optical network

Reflective semiconductor optical amplifier (RSOA) based bidirectional wavelength division multiplexing passive optical network (WDM-PON) is proposed and analyzed for broadcasting services along with conventional baseband signal. The downstream baseband signal is spectrally shaped for in-band transmission of broadcast signal, which ensures effective utilization of transmission bandwidth. The modulated downstream optical signal is further reused for upstream data modulation and transmitted over the same fiber with suppressed re-modulation and rayleigh backscattering noises. The proposed WDM-PON is successfully demonstrated for 10Gb/s downstream baseband signal, 100Mb/s 16-QAM (quadrature amplitude modulation) in-band broadcast signal and wavelength reused 1.25Gb/s upstream signal. Error free operation for both the baseband signals as well as low value of error vector magnitude (EVM) for broadcast data are simultaneously achieved in acceptable receiver sensitivity.

Multi-service RoF links with colorless upstream transmission based on orthogonal phase-correlated modulation.

We propose and experimentally demonstrate a full-duplex radio-over-fiber (RoF) system with colorless upstream transmission based on orthogonal phase-correlated modulation (OPM). This new OPM scheme, which realized by a polarization rotator (PR) and a single-driver Mach-Zahnder modulator (MZM) at the central office (CO), achieves polarization-orthogonality between the optical carrier (OC) and subcarriers generated by radio frequency (RF) signals. By adjusting a polarization controller (PC) in the remote access units (RAU), different modulation schemes can be flexibly implemented, e.g. double-sideband (DSB) modulation for low RF service and optical carrier suppression (OCS) modulation for millimeter-wave (mm-wave) service. In the meantime, the OC can be reused for the upstream transmission without any filtering and additional PC. A proof-of-concept experiment is conducted to demonstrate the feasibility of proposed scheme, where downstream 800-Mb/s orthogonal frequency division multiplexing (OFDM) signal at 58 GHz as an mm-wave service and 800-Mb/s OFDM signal at 0.3 GHz as a low frequency wireless service, as well as an upstream 1-Gb/s on-off keying (OOK) are simultaneously delivered in a shared architecture. By providing heterogeneous services and colorless upstream transmission, the proposed architecture can be seamlessly integrated in wavelength division multiplexing passive optical network (WDM-PON).

A routing and wavelength assignment scheme supporting multiple light-source nodes in multi-carrier-distributed optical mesh networks with wavelength reuse

This paper proposes a routing and wavelength assignment (RWA) scheme that supports multiple light-source nodes to minimize the number of required wavelengths for wavelength-reusable multi-carrier-distributed (WRMD) mesh networks. These networks use a light source, called the multi-carrier light source (MCLS), to ease the difficulty of controlling many light source devices. Moreover, they are able to reuse optical carriers for improving the effectiveness of wavelength usage. Since the WRMD network suffers constraints different from those of the conventional wavelength division multiplexing (WDM) network, an RWA scheme for the WRMD network with a single light source was recently developed. To use the resources of large-scale networks efficiently, there may be a need to have more than one MCLS node. In this paper, we first formulate the RWA problem as the integer linear programming (ILP) problem of obtaining the minimum number of required wavelengths to satisfy the given lightpath setup requests. A heuristic RWA scheme is then introduced to solve the RWA problem in practical times. Simulation results show that the proposed heuristic RWA scheme for the WRMD network achieves a near-optimum number of wavelengths. In addition, optimizing MCLS node location is shown to reduce the number of required wavelengths for lightpath establishment.

Impact of Wavelength and Modulation Conversion on Translucent Elastic Optical Networks Using MILP

Compared to legacy wavelength division multiplexing networks, elastic optical networks (EONs) have added flexibility to network deployment and management. EONs can include previously available functionality, such as signal regeneration and wavelength conversion, as well as new features such as finer-granularity spectrum assignment and modulation conversion. Yet each added feature adds to the cost of the network. In order to quantify the potential benefit of each functionality, we present a link-based mixed-integer linear programming (MILP) formulation to solve the optimal resource allocation problem. We then propose a recursive model in order to either augment existing network deployments (spectrum and regenerators) or speed up the resource allocation computation time for larger networks with higher traffic demand requirements than can be solved using an MILP. We show through simulation that systems equipped with signal regenerators or wavelength converters require a notably smaller total bandwidth, depending on the topology of the network. We also show that the suboptimal recursive solution speeds up the calculation and makes the running time more predictable, compared to the optimal MILP.

Virtual topology reconfiguration for mixed-line-rate optical WDM networks under dynamic traffic

For the mixed-line-rate (MLR) wavelength-division multiplexing (WDM) networks, each wavelength in a fiber can provide different transmitting rates (in 10/40/100 Gbps) by using different modulation formats. Since the MLR-WDM becomes a cost-efficient technique for the network upgrading, the problem of supporting virtual topology reconfiguration (VTR) becomes an important issue. In this paper, the VTR problem is studied for MLR-WDM networks under dynamic traffic demand. By monitoring traffic of the lightpaths, a reconfiguration method is proposed to follow the changes in traffic without a priori knowledge of the future traffic pattern. The proposed algorithm can optimize resource utilization and network traffic performance by either adjusting (increasing or decreasing), adding or deleting one or more lightpaths at a time. Simulations reveal the effects of the various system parameters. Specifically, we find that the proposed method adapts very well to the changes in the offered traffic on MLR-WDM networks.

Architecture to Integrate a Large-Scale DOPC-Based Optical Switching System on a Chip

In this paper, we propose a four-level architecture to integrate an entire N × M × K optical switching system on a chip using wavelength selective optical switches based on digital optical phase conjugation (DOPC), where N is the number of input fibers, M the number of output fibers and K the number of wavelengths carried within each input and output fiber. We show that a strictly non-blocking 128 × 128 × 48 system might be built on a 131 mm × 67 mm chip. With a 0.8 nm channel space, such optical switching systems might be widely used for dense wavelength division multiplexing networks.

Dynamic bandwidth allocation in heterogeneous WDM EPONs

Several dynamic bandwidth allocation algorithms have been introduced to schedule upstream wavelength channels in wavelength division multiplexing Ethernet passive optical networks (WDM EPONs), but mostly for homogenous WDM EPON networks with the same distance between each optical network unit (ONU) and the optical line terminal. For WDM EPON with heterogeneous round trip times (RTTs), we propose two algorithms for ONU scheduling, called nearest first and early allocation (EA); and a wavelength assignment algorithm, called best fit (BF). Both ONU scheduling algorithms take RTT dissimilarities into account, thus minimizing packet delay and packet drop ratio at ONUs. Additionally, EA can relive the common drawback of offline scheduling, i.e., channel idle time. On the other hand, the BF wavelength assignment is proposed that assigns the best wavelength to each ONU in order to improve network performances in terms of packet queuing delay and packet drop ratio at ONU sides.