Filippo Cugini

Challenges and requirements for introducing impairment-awareness into the management and control planes of ASON/GMPLS WDM networks

The absence of electrical regenerators in transparent WDM networks significantly contributes to reduce the overall network cost. In transparent WDM networks, a proper resource allocation requires that the presence of physical impairments in routing and wavelength assignment (RWA) and lightpath provisioning be taken into account. In this article a centralized, a hybrid centralized-distributed and two distributed approaches that integrate information about most relevant physical impairments in RWA and lightpath provisioning are presented and assessed. Both centralized and hybrid approaches perform a centralized path computation at the management-plane level, utilizing physical impairment information, while the lightpath provisioning is done by the management plane or the control plane, respectively. The distributed approaches fall entirely within the scope of the ASON/GMPLS control plane. For these two approaches, we provide functional requirements, architectural functional blocks, and protocol extensions for implementing either an impairment-aware real-time RWA, or a lightpath provisioning based on impairment-aware signaling.

Push-Pull Defragmentation Without Traffic Disruption in Flexible Grid Optical Networks

In flexi-grid optical networks, fragmentation of spectrum resources may significantly affect the overall network efficiency. Effective techniques for defragmentation (i.e., re-optimization) are then required to limit the wasting of spectrum resources. However, current defragmentation techniques can only be implemented thanks to the presence of additional resources, such as spare expensive transponders. In this study, we propose, discuss and evaluate a novel defragmentation technique called push-pull. The technique is based on dynamic lightpath frequency retuning upon proper reconfiguration of allocated spectrum resources. It does not require additional transponders and does not determine traffic disruption. All the relevant technological limitations that may affect the push-pull applicability are discussed in the context of both optically-amplified direct and coherent detection systems. The technique is then successfully demonstrated in two different flexi-grid network testbeds, reproducing the two aforementioned scenarios. In particular, the reoptimization of a 10 Gb/s OOK lightpath is safely completed in few seconds (mainly due just to node configuration latencies) without experiencing any traffic disruption. Similarly, the push-pull is successfully performed on a 100 Gb/s PM-QPSK lightpath, providing no traffic disruption.

OpenFlow-based segment protection in Ethernet networks

Metro and carrier-grade Ethernet networks, as well as industrial area networks and specific local area networks (LANs), have to guarantee fast resiliency upon network failure. However, the current OpenFlow architecture, originally designed for LANs, does not include effective mechanisms for fast resiliency. In this paper, the OpenFlow architecture is enhanced to support segment protection in Ethernet-based networks. Novel mechanisms have been specifically introduced to maintain working and backup flows at different priorities and to guarantee effective network resource utilization when the failed link is recovered. Emulation and experimental demonstration implementation results show that the proposed architecture avoids both the utilization of a full-state controller and the intervention of the controller upon failure, thus guaranteeing a recovery time only due to the failure detection time, i.e., a few tens of milliseconds within the considered scenario.

Demonstration of Flexible Optical Network Based on Path Computation Element

Flexible optical networks, based on bandwidth-variable optical cross-connects (BV-OXCs) and novel flexible transponders, are expected to significantly improve the overall spectrum efficiency with respect to traditional networks where fixed frequency spacing is applied. Flexible optical networks will exploit the BV-OXC capability to dynamically configure the reserved bandwidth as a set of frequency slots. In addition, flexible transponders will be employed to dynamically configure transmission parameters, such as bit-rate and modulation format. To enable these new configuration capabilities, network operation enhancements need to be efficiently introduced and investigated. In this study, we focus for the first time on the Path Computation Element (PCE) architecture for flexible optical networks. PCE architecture and PCE communication protocol are enhanced to maximize the spectral efficiency and to provide indications also on the specific transmission parameters to configure. Experimental demonstration is provided through two different experiments, successfully showing the PCE capability to trigger dynamic rerouting with bit-rate or modulation format adaptation. In particular, the experiments demonstrate, in a real testbed, dynamic frequency slot assignment and format adaptation from DP-16QAM to DP-QPSK at 100 Gb/s, and bit-rate adaptation at DP-16QAM from 200 Gb/s to 100 Gb/s.

A Survey on the Path Computation Element (PCE) Architecture

Quality of Service-enabled applications and services rely on Traffic Engineering-based (TE) Label Switched Paths (LSP) established in core networks and controlled by the GMPLS control plane. Path computation process is crucial to achieve the desired TE objective. Its actual effectiveness depends on a number of factors. Mechanisms utilized to update topology and TE information, as well as the latency between path computation and resource reservation, which is typically distributed, may affect path computation efficiency. Moreover, TE visibility is limited in many network scenarios, such as multi-layer, multi-domain and multi-carrier networks, and it may negatively impact resource utilization. The Internet Engineering Task Force (IETF) has promoted the Path Computation Element (PCE) architecture, proposing a dedicated network entity devoted to path computation process. The PCE represents a flexible instrument to overcome visibility and distributed provisioning inefficiencies. Communications between path computation clients (PCC) and PCEs, realized through the PCE Protocol (PCEP), also enable inter-PCE communications offering an attractive way to perform TE-based path computation among cooperating PCEs in multi-layer/domain scenarios, while preserving scalability and confidentiality. This survey presents the state-of-the-art on the PCE architecture for GMPLS-controlled networks carried out by research and standardization community. In this work, packet (i.e., MPLS-TE and MPLS-TP) and wavelength/spectrum (i.e., WSON and SSON) switching capabilities are the considered technological platforms, in which the PCE is shown to achieve a number of evident benefits.

Modeling and Distributed Provisioning in 10–40–100-Gb/s Multirate Wavelength Switched Optical Networks

In wavelength-switched optical networks (WSONs), quality of transmission (QoT) has to be guaranteed during lightpath provisioning. In multibit-rate WSONs, this task is complicated by the coexistence of optical connections operating at different bit-rates and modulation formats. The major issue consists in accounting for the severe impairments due to cross-phase modulation (XPM) induced by 10 Gb/s lightpaths on neighbor 40 or 100 Gb/s lightpaths. In this paper, QoT modeling is first reviewed for 10, 40, and 100 Gb/s transmission according to the adopted modulation format and detection type. In addition, a Gaussian approximation to compute the bit error rate of differential quadrature phase-shift keying (DQPSK) and QPSK signals is proposed, as well as closed formulas to compute the nonlinear phase noise variance due to XPM. Also, discussions about the XPM cumulation over spans in a WSON and how XPM can be considered in a dynamic network are provided. Then, four lightpath provisioning schemes are proposed to effectively account for QoT and, in particular, for XPM. The schemes differently exploit: 1) augmented spectral separation among lightpaths at different bit rates; 2) XPM worst-case scenario; and 3) current and novel generalized multiprotocol label switching extensions. The performance of the proposed schemes is evaluated through simulations in several multibit-rate scenarios. Results show that the proposed schemes provide effective network resource utilization while guaranteeing the adequate QoT to lightpaths at any bit rate.

Toward high-rate and flexible optical networks

The evolution of optical technologies is driving the introduction of multirate optical networks exploiting advanced transmission techniques and efficient switching devices. In the short term, optical connections operating at 10 and 100 Gb/s will coexist in the same multi-rate network infrastructure. This, however, might introduce significant issues due to detrimental inter-channels effects, which need to be considered during network planning or connection provisioning. In the long term, connections at higher bit-rates (e.g., 400 Gb/s) and based on complex modulation formats (e.g., quadrature amplitude modulation - QAM) are expected, together with the adoption of innovative and flexible bandwidth-variable optical cross-connects (BV-OXCs). BV-OXCs have the potential to significantly improve the overall network spectrum efficiency. However, critical issues might arise in the dynamic control of network operations. This article discusses the enhancements required during operation and control of future optical networks with quality of transmission guaranteed. A first network evolution scenario is considered, where 100 Gb/s lightpaths are introduced in a native 10 Gb/s network. In such a scenario, inter-channel effects between 10 and 100 Gb/s lightpaths are highlighted. Relevant methods to account for these effects are discussed and evaluated. Then, a second network evolution scenario is assumed, in which traditional OXCs are replaced with BV-OXCs, and even higher bit-rates (e.g., 400 Gb/s 16-QAM) are introduced in the network. In particular, the problem of scalability when advertising and storing spectrum resource (i.e., frequency slices) availability is presented for flex-grid optical networks (i.e., optical networks exploiting BVOXCs). Consequently, a method to efficiently handle availability information is proposed and evaluated, showing the capability to overcome scalability issues without impacting the overall network resource utilization.

Spectrally and spatially flexible optical network planning and operations

The advent of spectrally flexible (a.k.a. elastic) optical networking is widely identified as the next generation optical network solution that permits varying bandwidth demands to be dynamically assigned over flexible spectral containers, targeting optimum use of the available network resources. Additionally, the adoption of the space dimension is identified as a promising solution for the capacity expansion of future networks, while novel spatial-spectral switching solutions show that the flexible networking concept can be further expanded over both the spatial and spectral dimensions. This article provides an overview of the latest developments and possible approaches with respect to flexible optical networking and the emerging benefits that spatially flexible networking approaches can offer. The focus is on the network planning and resource optimization functions, the main network operations related to fragmentation and IP/optical layer integration, and the control plane solutions.

Programmable Transponder, Code and Differentiated Filter Configuration in Elastic Optical Networks

Next generation optical networks will require high levels of flexibility both at the data and control planes, being able to fit rate, bandwidth, and optical reach requirements of different connections. Optical transmission should be able to support very high rates (e.g., 1 Tb/s) and to be distance adaptive while optimizing spectral efficiency (i.e., the information rate transmitted over a given bandwidth). Similarly, the control plane should be capable of performing effective routing and spectrum assignment as well as proper selection of the transmission parameters (e.g., modulation format) depending on the required optical reach. In this paper we present and demonstrate a software-defined super-channel transmission based on time frequency packing and on the proposed differentiated filter configuration (DFC). Time frequency packing is a technique able to achieve high spectral efficiency even with low-order modulation formats (e.g., quadrature phase-shift keying). It consists in sending pulses that overlap in time or frequency or both to achieve high spectral efficiency. Coding and detection are properly designed to account for the introduced inter-symbol and inter-carrier interference. We present a software defined network (SDN) controller that sets transmission parameters (e.g., code rate) both at the transmitter and the receiver side. In particular, at the transmitter side, a programmable encoder adding redundancy to the data is controlled by SDN. At the receiver side, the digital signal processing is set by SDN based on the selected transmission parameters (e.g., code rate). Thus, extensions to the OpenFlow architectures are presented to control super-channel transmission based on time frequency packing. Then, the SDN-based DFC is proposed. According to DFC, the passband of the filters traversed by the same connection can be configured to different values. Experiments including data and control planes are shown to demonstrate the feasibility of optical-reach-adaptive super-channel at 1 Tb/s controlled by extended OpenFlow. Then, the effectiveness of the proposed SDN-based DFC is demonstrated in a testbed with both wavelength selective switches and spectrum selective switches, where filters traversed by a connection requires different passband values. Extended OpenFlow messages for time frequency packing and supporting DFC have been captured and shown in the paper.

Sliceable transponder architecture including multiwavelength source

A multiflow transponder in flex-grid optical networks has recently been proposed as a transponder solution to generate multiple optical flows (or subcarriers). Multiflow transponders support high-rate super-channels (i.e., connection composed of multiple corouted subcarriers contiguous in the spectrum) and sliceability; i.e., flows can be flexibly associated to the incoming traffic requests, and, besides composing a super-channel, they can be directed toward different destinations. Transponders supporting sliceability are also called sliceable transponders or sliceable bandwidth variable transponders (SBVTs). Typically, in the literature, SBVTs have been considered composed of multiple laser sources (i.e., one for each subcarrier). In this paper, we propose and evaluate a novel multirate, multimodulation, and code-rate adaptive SBVT architecture. Subcarriers are obtained either through multiple laser sources (i.e., a laser for each subcarrier) or by exploiting a more innovative and cost-effective solution based on a multiwavelength source and micro-ring resonators (MRRs). A multiwavelength source is able to create several optical subcarriers from a single laser source. Then, cascaded MRRs are used to select subcarriers and direct them to the proper modulator. MRRs are designed and analyzed through simulations in this paper. An advanced transmission technique such as time frequency packing is also included. A specific implementation of a SBVT enabling an information rate of 400 Gb¿s is presented considering standard 100 GbE interfaces. A node architecture supporting SBVT is also considered. A simulation analysis is carried out in a flex-grid network. The proposed SBVT architecture with a multiwavelength source permits us to reduce the number of required lasers in the network.