Toward Terabit Autonomic OpticalNetworks Based on a SoftwareDefined Adaptive/Cognitive

Approach [Invited]Julio Oliveira, Juliano Oliveira, Eduardo Magalhães, João Januário, Marcos Siqueira,Rafael Scaraficci, Marcos Salvador, Leonardo Mariote, Neil Guerrero, Luis Carvalho,

Fabian van’t Hooft, Giovanni Santos, and Miquel Garrich

Abstract—Terabit elastic optical networking (EON) isforeseen as a viable solution to extend the lifetime of a net-work exploiting the available bandwidth in previously de-ployed optical fibers. EON is based on bandwidth-variabletransponders capable of supporting multiple bit rates and/or modulation formats according to traffic requirementsand node architectures that route arbitrary channel band-widths. Thus, EON increases the heterogeneity of thenetwork, which may create the need for autonomic adap-tive and/or cognitive techniques. In this context, thesoftware-defined networking (SDN) paradigm emerges asan opportunity to enable such techniques thanks to thecentralized view of the network by decoupling the controlplane and the data plane. This paper surveys different ac-tivities carried out at the Optical Technologies Division inCentro de Pesquisa e Desenvolvimento em Telecomunica-ções, Brazil. We review an optical transport SDN controllerfor virtual optical networks that supports two adaptive al-gorithms. First, the autonomic flexible transponder recon-figures the transmission modulation format according toa threshold level. Second, the adaptive global spectrumequalization reconfigures the wavelengths’ attenuationprofiles applied at the optical nodes to improve the signals’optical signal-to-noise ratio (OSNR) at reception. Finally,we report experimental results of an in-band OSNR moni-tor for advanced modulation formats.

Index Terms—Coherent communications; Fiber opticsand optical communications; Networks; Optical communi-cations.

I. INTRODUCTION

I nternet traffic is continuously increasing due to the ap-pearance of new high-bandwidth services and applica-

tions [1]. These growing traffic demands pose numerous

constraints on all the segments of the telecom operators’network infrastructure. A clear example is the Spanishnationwide optical backbone network, which requiresnew fiber deployment in 2020 according to current predic-tions [2]. Fortunately, elastic optical networking (EON) is aviable solution to extend the lifetime of the networkexploiting the available bandwidth in previously deployedoptical fibers [3]. For instance, still considering the Spanishnationwide optical backbone network, new fiber deploy-ment can be postponed until 2024 if EON is applied [2].EON is based on the following approaches on the transmis-sion and networking sides.

On the transmission side, EON uses bandwidth-variabletransponders (BVTs) capable of supporting multiple bitrates and/or modulation formats according to trafficrequirements [i.e., bandwidth, path lengths, and requiredbit error rate (BER)performances]. In this context, 100Gbpsoptical transponders are already being deployed while thefirst (to the authors’ best knowledge) 400 Gbps∕1 Tbpstransponders have been demonstrated in field andlaboratorial trials [4–6]. Such approaches increase theheterogeneity of the network since switching requirementscan be very different from node to node.

On the networking side, EON is based on node architec-tures that route arbitrary channel bandwidths. Inparticular, reconfigurable optical add–drop multiplexers(ROADMs) use wavelength-selective switches (WSSs) thatsupport variable channel bandwidths in a flexible manner.However, the attenuation profile applied to the channelwavelengths passing throughWSSs may reduce the opticalsignal-to-noise ratio (OSNR) of the signals at reception.Therefore, WSS-based ROADMs need proper controlmechanisms to guarantee BER reception below theforward-error correction (FEC) limit. Similarly, efficienterbium-doped fiber amplifier (EDFA) control mechanismsare required due to their noise contribution under the am-plification process at the optical layer. Moreover, hybrid op-tical amplification based on Raman/EDFA is starting to beconsidered for network performance and energy efficiencyimprovements [7]. Therefore, previous simple routinessuch as spectrum equalization, optical amplifier gainhttp://dx.doi.org/10.1364/JOCN.7.00A421

Manuscript received July 21, 2014; revised October 25, 2014; acceptedNovember 24, 2014; published February 2, 2015 (Doc. ID 217029).

Julio Oliveira, Juliano Oliveira (e-mail: jrfo@cpqd.com.br), EduardoMagalhães, João Januário, Rafael Scaraficci, Leonardo Mariote, Neil Guer-rero, Luis Carvalho, Fabian van’t Hooft, Giovanni Santos, and Miquel Gar-rich are with the CPqD Foundation, Rua Dr. Ricardo BennetonMartins, s/n,CEP 13086-902, Campinas—SP, Brazil.

Marcos Siqueira is with Padtec S.A., Brazil.Marcos Salvador is with Lenovo Innovation Center, Brazil.

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1943-0620/15/03A421-11$15.00/0 © 2015 Optical Society of America

control, channel instantiation, and even the generalizedmultiprotocol label switching (GMPLS) control plane haveto be revised in order to achieve EON benefits. In thischallenging scenario, adaptive and/or cognitive networksare highly desirable to account for the heterogeneity atboth the transmission and network sides.

• Cognitive network. From the European Commission(EC)-funded CHRON project [8], a cognitive networkhas a process that can perceive current network condi-tions, and then plan, decide, and act on those conditions.The network can learn from these adaptations and usethem to make future decisions, all while taking into ac-count end-to-end goals [9,10]. Moreover, these networksrely on the application of machine learning techniques,i.e., algorithms that improve its performance throughexperience gained over a period of time without completeinformation about the environment in which itoperates [11].

• Adaptive network. In this paper, we consider that anadaptive network can perceive current network condi-tions, and then decide and act on those conditions. In par-ticular, we consider a subset of the cognitive capabilitiessince learning and planning are not required for adaptivenetworks.

Software-defined networking (SDN) [12] is based on de-coupling the data plane from the control plane and is a keyenabler for adaptive and/or cognitive networks due toits promising benefits that include overall network simpli-fication, virtualization, and automation capabilities viaprogrammable interfaces.

This paper reports different activities carried out inthe Optical Technologies Division of the Centro de Pesquisae Desenvolvimento em Telecomunicações (CPqD)1 for devi-ces, network elements, and SDN applications, in order toenable optical networks to support new services and virtu-alization with the required flexibility and scalability.

First, we review our transport SDN controller, whichenables optical network virtualization and autonomicoperation [13].

Second, two different applications are reviewed in thecontext of the SDN controller. On the one hand, a flexibletransponder application identifies the threshold level prox-imity and reconfigures the transmission modulation for-mat. In particular, the application automatically changesfrom multicarrier 448 Gb∕s dual-polarization (DP)-16quadrature amplitude modulation (QAM) to 448 Gb∕sDP-quadrature phase-shift keying (QPSK), reducing thespectrum efficiency to maintain the quality of transmission

(QoT). On the other hand, an autonomic global spectrumequalization application reconfigures the attenuation ap-plied at the WSS ROADMs. By doing so, channel OSNRimprovements up to 5 dB are experienced for 80 DP-QPSKchannels at 112 Gb/s, transmitted along four spans.

Third, related to devices, this paper reviews an in-bandOSNR measurement device able to monitor 112 Gb∕s DP-QPSK and 224 Gb∕s DP-16 QAM signals, from 5 to 20 dB ofOSNR with low error (less than 2 dB). This device wasfirst developed with discrete components, and sub-sequently was integrated into silicon-on-insulator (SOI)photonics technology.

The remainder of the paper is organized as follows:Section II describes related works regarding SDN control-lers, flexible transponders, and OSNRmonitors. Section IIIreviews CPqD’s SDN controller. Section IV details the auto-nomic flexible transponder and the adaptive global spec-trum equalization applications that run on top of theSDN controller. Section V reviews our developed OSNRmonitor. Section VI highlights different future activitiesand research lines. Section VII concludes the paper.

A preliminary brief survey on these activities waspresented in [14].

II. RELATED WORKS

In this section, we describe related works on SDN con-trollers, flexible (i.e., bandwidth-variable) transponders,and OSNR monitors. No related works were found in theliterature regarding the global spectrum equalizationconcept.

A. SDN Controllers

Different network controllers have been recently pro-posed [15] to provide control platforms for programmablenetworks mainly based on OpenFlow (OF) [16]. For in-stance, OF extensions are usually proposed for domainor specific scenario purposes [17].

SDN implementations have been focused initially onEthernet switching, primarily for data center resource op-timization. However, their applicability is now being ex-tended to metro, wide area, and access networks. On theone hand, the optical transport network provides the foun-dation of the networking layer and in order to properly par-ticipate in an SDN framework, it needs to be capable ofproviding resource and topology information and be ableto fulfill requests for network services on demand usingstandard application programming interfaces (APIs). Thebenefits and challenges of extending SDN concepts to vari-ous transport network architectures that include opticalwavelength and fiber switches, circuit switches, and sub-wavelength optical burst switches are reviewed in [18].Recent works on SDN applied to metropolitan networkshave been reported in [19] on the design, implementation,and evaluation of a multitechnology, multirate, and adapt-able network architecture for metropolitan/edge areas.

1The CPqD (Research and Development Center in Telecommunications) isan independent institution whose main objectives are to increase Brazil’scompetitiveness and to further the digital inclusion of the country’s society,based on innovative information and communication technologies (ICTs).Its extensive research and development program, the largest of its kindin Latin America, has produced ICT solutions for private and public corpo-rations in the communications, multimedia, financial, utilities, industrial,defense, and security sectors. Its optical technologies division is in charge ofnumerous projects devoted to the research and development of new opticalsystems, subsystems, and networks for future high-capacity EON.

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The proposed hybrid network architecture in [19] is evalu-ated through simulations across a broad range of trafficprofiles with a scalable bandwidth request. On the otherhand, in the access optical network, SDN allows flow-basedtraffic steering in a proactive or reactive manner. It facil-itates application-specific traffic optimization and can spanmultiple technology and protocol domains. For instance, in[20] some architectures and use cases for software-definedaccess networks are explained.

B. Flexible Transponders

The next generations of coherent optical transmittersshould be able to operate in dynamic and diverse networkscenarios [3]. This capability is required in order to explorelevels of optimal adaptability considering an availablelink’s resources (spectrum, power, OSNR, and so on) andflexible service requirements (data rate, power consump-tion, spectral efficiency, reach, and so on) [21]. In this con-text, different approaches of flexible transponders havebeen recently proposed. In particular, in [22] a band-width-scalable coherent transmitter based on the parallelsynthesis of multiple spectral slices using optical arbitrarywaveform generation is proposed. On the other hand, rate-adaptive modulation and coding approaches can be used toincrease flexibility and adaptability at the transmissionside [23]. Finally, software-defined transponders that aresuitable candidates for the EON paradigm are proposedin [24].

C. OSNR Monitors

Recall that in-band OSNR estimation techniques try toseparate signal and noise power through their intrinsic dif-ferent characteristics. Depending on this separation,OSNR estimation techniques can be separated into twogroups: 1) direct measurement techniques and 2) monitor-ing techniques. The following is a literature review of themain in-band OSNR estimate techniques that were pro-posed to be employed along an optical path in a nonintru-sive way.

The first direct measurement technique for OSNR esti-mation was based on polarization diversity between opticalsignal and noise [25]. Therefore, this approach assumesthat a signal is fully polarized while noise is not fully po-larized. Although it is simple, two main drawbacks are theinaccuracy when polarization mode dispersion (PMD) ispresent and that the OSNR estimation is only possiblefor signals whose modulation is in only one polarization.Concerning the first drawback, several techniques havebeen raised to solve it using digital signal processing[26,27] and optical filtering [28]. The technique proposedin [29] shows how to solve the second drawback with ahigh-speed Stokes polarimeter in a manner that is fasterthan the signal speed. However, while this increases sys-tem complexity, the OSNR cannot be estimated in caseany dispersive effect appears. A relevant direct measure-ment technique for OSNR estimation relies on beat noise

[30]. Indeed, it shows benefits in terms of simplicity androbustness for dispersive effects. However, due to the highprecision needed for the optical filters, it is difficult toensure the accuracy in an experimental setup. To solve thisissue, [31] replaces the optical filters by using an interfer-ometer with a 1 bit delay. However, it is not possible to usebeat noise properties for OSNR estimation when in-phase(I) and quadrature (Q) components are used to define themodulation format of the signal (i.e., most cases in currenttransmission systems). Indeed, since I–Q components donot have a high correlation between them, it is not possibleto obtain with accuracy the beat noise power required bythe techniques previously mentioned.

OSNR monitoring techniques use a previous calibrationto guarantee a proper estimation. This calibration is re-lated to the transmission circuit, so for each input signala new calibration procedure is required. To the authors’best knowledge, the first monitoring method for OSNR es-timation is [32], which relates the radio frequency (RF)spectrum in cases for which only signal is present or incases for which both signal and noise are in the samechannel. An initial drawback of this technique was itsimpossibility to work for on–off keying (OOK) signals. How-ever, [33] solved this issue with a set of new calibrationsrelated to the transmission signal. Another type of monitor-ing technique uses the coherence diversity between the sig-nal and noise, which in the optical domain can be obtainedby an interferometer. The main advantages of these tech-niques are the possibility of application for any kind ofsignal, i.e., they are robust to modulation formats andsignal rates, and they are independent to dispersive effects[34,35]. To the authors’ best knowledge, the first workbased on this property was [34] and the evolution of thiswork led to the methods proposed in [36,37]. Additionally,this kind of technique has the advantage of enabling inte-gration of the interferometer with other solutions andbuilding it with different materials. In this way, [38] showsan OSNR monitor based on an interferometer made with aliquid-crystal-on-silicon (LCoS)-based WSS. Moreover, [39]showed for the first time (to the authors’ best knowledge)an interferometer for OSNR estimation built on a SOI plat-form. However, despite the mentioned advantages, themain drawback of all the interferometric techniques men-tioned previously is the necessity to calibrate the system intwo situations: in the absence of noise and later in theabsence of signal, which makes it impractical for a real sce-nario. More recently, [40] proposed an in-band OSNR mon-itor based on an optical bandpass filter but only forsuperchannel signals.

In order to make the interferometric technique indepen-dent of calibration processes, [41] proposes a method tomeasure the OSNR directly, i.e., without calibration. Thisapproach is possible because a set of different interferom-eters are used to obtain an approximation of the inputsignal coherence level. In [42] a simplification of this tech-nique is presented using only one interferometer, butbesides measuring for a single- and dual-polarizationsignal like the first one, it measures only for single-polarization signals. Despite the possibility of directlymeasuring the OSNR without calibration, there are

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drawbacks for both proposed methods. For the case in [41],even when increasing the system performance to get a bet-ter approximation of the signal coherence level, signalswith high bit rates and high OSNR values yield very inac-curate OSNR estimations. Furthermore, the number of fil-ters traversed along the optical path influences the OSNRestimation, since cascaded filters narrow a signal’s band-width, which changes its coherent properties. For the casein [42], the accuracy does not maintain for all states ofpolarization of the signal, thus a polarization controlleris necessary to guarantee a proper estimation.

III. SOFTWARE-DEFINED OPTICAL NETWORKS

In this section we review CPqD’s optical transport SDNcontroller.

By employing model-based development and graph-based algorithms at the component level [WSS, opticalchannel monitoring (OCM), EDFA, multicast switch(MCS), and so on], potent abstractions can be created, hid-ing the complexity of subsystems and allowing these to beeasily integrated into SDN architecture. The graph-basedapproach provides a basis for alarm correlation and ad-vanced path computation. It also provides the informationneeded for multiagent-oriented problem solving, whichwith the currently installed hardware is not feasible.

The proposed transport SDN architecture, shown inFig. 1, includes a transport (T) network operating system(NOS), allowing SDN applications to access topology andmeasurements required for its operation and to perform

required configuration and interactions with transportnetwork elements. Additionally, the T-NOS implements theconcept of network slicing, allowing the implementation ofvirtual optical networks (VONs). As many features pro-vided by GMPLS are not initially supported by pureSDN approaches, such as network discovery, link stateadvertisement, and Optical Internetworking Forum (OIF)user network interface/network-to-network interface(UNI/NNI), we have virtualized our automaticallyswitched optical network (ASON)/GMPLS implementationto run at the SDN controller on top of the T-NOS. TheT-SDN controller provides a network functions API forplugging control applications to perform specific tasks,taking advantage of the network abstraction.

The application is in development and its continuous de-velopment focus aims for cognitive and adaptive controlsfor autonomic optical algorithms, which adapt network el-ements’ operation points, such as global equalization, cog-nitive EDFA gain control, dense wavelength divisionmultiplexing (DWDM) system autoalignment, fault predic-tion, and preventive actions. In [43] we have demonstrateda cognitive gain control mechanism for EDFAs, using aGMPLS control plane to read and control amplifiers ana-lyzing the transponders’ BER in a heterogeneous opticalnetwork scenario. Additionally in the context of this work,a global equalization mechanism for WSS ROADMs wasimplemented aiming at maximizing the OSNR for all net-work wavelengths previously presented in [44] and an ap-plication for flexible autonomic coherent transpondermodulation format adjustment to maintain transmis-sion QoT.

Fig. 1. Transport SDN controller architecture, flexible transponder application OSNR × BER result chart, and signals’ spectra.

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IV. SDN APPLICATIONS

In this section, we review two adaptive applications thatrun on top of the optical transport SDN controller. First, wedetail the autonomic flexible transponder that reconfiguresthe transmission modulation format according to a thresh-old level. Second, we review the adaptive global spectrumequalization that reconfigures the attenuation applied atthe optical nodes to improve the OSNR at the reception.

A. Flexible Transponder

The first adaptive application reported is a software-defined flexible transponder with a fixed bit rate of448 Gb∕s working at two operation modes as describedin Table I. Besides the 448 Gb∕s test optical signal, the net-work was loaded with a legacy wavelength division multi-plexing (WDM) signal aggregate composed of 40 opticalcarriers originated by 500 kHz linewidth distributed feed-back (DFB) lasers, with 100 GHz channel spacing. The op-tical carriers were set to 100 GHz slots in the flexible-gridWSS, while the 448 Gb∕s flexible transponder was set to 75or 150 GHz slots, with spectral efficiencies of 5.6 and2.8 b∕s∕Hz, respectively, depending on the operation mode.The signals were transmitted through four flexible-gridWSS nodes with three 50 km long spans of standard single-mode fiber (SMF). EDFAs were used to compensate linklosses. To emulate link failure, an amplified spontaneousemission (ASE) noise source was built (composed of anextra EDFA with a variable optical attenuator), andcoupled together with a test signal to emulate an OSNRreduction. Subsequently, the 448 Gb∕s channel was re-ceived with an integrated polarization-diverse coherent op-tical receiver. The coherent receiver outputs are sampledusing a 40 GS∕s real-time scope with 20 GHz bandwidth.Data sets with 400 kS were acquired, and the BER for eachcarrier is computed and processed offline by using a set ofalgorithms to recover transmitted information [45].

Initially, EDFA gain control [43] was employed to opti-mize the network power balance per link. Previous resultsshowed that over six cascaded EDFAs using the cognitivecontrol in the test bed ensures BER below a 7% FECthreshold, under a 6 dB attenuation penalty. In case EDFAcontrol actuation does not compensate all the impairments,the T-SDN controller flex-transponder application is trig-gered, reconfiguring the transponders to a new modulationformat with a lower OSNR requirement but maintainingthe data rate in order to guarantee the same bit rate.

As depicted in Fig. 1, the transponder was operatingwith a high OSNR >30 dB, using DP-16 QAM modulation

format, occupying 75 GHz of the spectrum. When theOSNR is degraded and the FEC limit is reached, theSDN controller coordinated the ROADM reconfigurationfor moving the neighbor channel for spectrum defragmen-tation, increasing the channel width to 150 GHz and recon-figuring the transponder modulation format to DP-QPSK.

B. Global WSS-Based Spectrum Equalization

The second adaptive application reported considers aglobal WSS-based equalization to compensate undesiredspectrum tilting due to EDFAs. In particular, when consid-ering an aggregate of DWDM optical signals, the spectralresponse of cascaded EDFAs may cause different gains foreach signal. Indeed, a typical gain-flattening filter of theEDFAs has a residual gain ripple between 0.5 and 1 dBpeak-to-peak. The accumulation of this residual ripplefor long lightpaths may impair network operation. To over-come this limitation, EDFAs in a DWDM system areusually operated in an automatic gain controlled modejointly with local spectral equalization algorithms at eachWSS-ROADM to achieve spectral tilt correction. This ap-proach ensures a flat spectrum (at each node) but at theexpense of end-to-end OSNR degradation. In this context,the concept of a global equalization algorithm arises aim-ing at both spectrum flatness optimization and end-to-endattenuation minimization (per wavelength) leading tohigher OSNRs.

Figure 2(a) shows the global WSS-based spectrumequalization procedure that requires the number ofROADMs N ≥ 2, the number of wavelengths in a DWDMaggregate W, the attenuation at each ROADM, and thespectrum tilt at the receiver. We denote the attenuationat each ROADM i f1;…; N − 1g of the lightpath by a vectorAi � �Ai�1�;…; Ai�w��, where Ai�w� is the optical attenua-tion given at wavelength w. Similarly, we denote the spec-trum tilt at the receiver by a vector T � �T�1�;…; T�w��,where T�w� is the difference between the optical powerof channelw and the minimum channel power of the aggre-gate W at the receiver. Initially, an equalization strategy(described in the following text) is chosen. Then, the threemain steps of the procedure are executed. First, the globalattenuation vector (Atotal) is obtained as the sum of allattenuations applied at each ROADM of the lightpathAf1;…; N − 1g plus the tilt at the receiver T. The lastROADM N is not considered because the broadcast-and-select structure impedes equalization for dropped light-paths. Note that Atotal stores attenuations applied inprevious iterations. Second, the minimum global attenua-tion vector (Γ) is obtained as the difference between the

TABLE ISOFTWARE-DEFINED FLEXIBLE TRANSPONDER DESCRIPTION

OperationMode

Bit Rate(Gb/s)

Baud Rate(GBd)

ModulationFormat

Carrier Spacing(GHz)

Number ofCarriers

Flexible-Grid Slot(GHz)

OSNR at 1 × 10−3 (perCarrier)

Metro-haul 448 28 DP-16QAM 35 2 75 29Long-haul 448 28 DP-QPSK 35 4 150 14

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total attenuation applied to channel w [i.e., Atotal�w�] andthe minimum total attenuation of the aggregate W. Third,the minimum global attenuation vector (Γ) is applied ateach ROADM of the lightpath according to the chosenstrategy. Finally, these three steps are executed recursivelyuntil the minimum allowed spectrum tilt at the receiver isachieved (i.e., the aggregate of channels W is equalized).

We evaluated the software-defined autonomic WSS ap-plication with four operation modes. The first one executesthe commonly used local equalization algorithm. Theother operation modes perform our global equalizationalgorithm with three different attenuation limiters.Figures 2(b)–2(d) depict the experimental setup. At thetransmitter [Fig. 2(c)] the optical signal consists of aWDM system composed of 80 optical carriers divided intwo banks originated by 500 kHz linewidth DFB laserswith 50 GHz channel spacing. Each of the lasers’ bankswas modulated using DP-QPSK modulators driven by fourlines of 28 Gb∕s binary signals [pseudorandom bit se-quence (PRBS) 215 − 1] in order to modulate each carrierwith 112 Gb∕s DP-QPSK. The signals were transmittedthrough four WSS ROADMs, followed by three 100 km longstandard SMF spans. EDFAs were used to compensate linklosses. Each link was balanced to operate with 0 dBm as

maximum channel launch power. Next, each 112 Gb∕scarrier was received with an integrated polarization-diverse coherent optical receiver [Fig. 2(d)]. The coherentreceiver is composed of a 40 GS∕s real-time scope. Datasets with 400 kS were acquired, and the BER for eachcarrier is computed and processed offline by using a setof digital signal processing algorithms to recover transmit-ted information.

Figures 3(a) and 3(b) show the measured local equaliza-tion attenuation profile in comparison with the optimizedattenuation profile obtained by the global equalization al-gorithm. It can be seen that the global approach optimizestotal attenuation to obtain the same flat spectrum[Figs. 3(b) and 3(c)]. Additionally, it is worth noting thatfor each loop iteration until steady-state (flatness less than∼0.6 dB), the spectrum attenuation distribution decreasesfor lower wavelength and increases for higher wavelengthleading to OSNR improvements. The measured OSNRs perchannel for each algorithm are depicted in Fig. 3(d). Bycomparing local equalization against the other approachesnote that in some channels the OSNR increased up to 5 dB.These results indicate that the proposed technique couldprovide an attractive solution to realize the equalizationproviding OSNR enhancement for the lowest wavelength.

Fig. 2. (a) Global WSS-based equalization, (b) CPqD’s metropolitan mesh optical network test bed, (c) transmitter, and (d) receiver.

Fig. 3. (a) Per node attenuation profiles, (b) total attenuation profiles for local attenuation and global attenuation schemes, (c) opticalspectra for local and global equalization approaches, (d) OSNR measurements per channel, and (e) BER measurements per channel.

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It is important to emphasize since these channels governthe reach as they have lower OSNR. In future SDN net-works, flexible transponders will be format-agile, beingable to operate with higher-order formats as well. Thus,the higher OSNR required for these formats could leadto a different view of the reach benefit garnered throughimproved OSNR via global adaptive WSS equalization.Furthermore, depending on the attenuation limiter pernode, the system presents a trade-off between power flat-ness versus OSNR flatness. The channels’ BER are shownin Fig. 3(e). Note that all global equalization granted allchannel BER below the FEC threshold (10−3).

V. OSNR MONITOR

In this section, we extensively detail an OSNR monitordeveloped at CPqD. First, we describe the obtained resultsfor the setup composed of discrete components, andsubsequently we show its integration in SOI photonicstechnology.

Regarding the monitoring devices, the utilization of newmodulation formats decreases the accuracy of out-of-bandOSNR measurements since the noise floor becomes moredifficult to detect. The performance monitoring of an opti-cal network aims to support its fast development, providingparameters that allow a high level of reconfigurability andflexibility. One way to guarantee this performance and en-sure a high QoT is through OSNR, which can be used todiagnose a network because the noise level is the majorsource of impairment in an optical link, which is quantifiedthrough the BER. With the legacy system, OSNRmeasure-ment was done from an out-of-band noise level betweenWDM channels. However with the advent of the ROADM,each channel has a vastly different transmission historyand because of that there is not a flatness noise floor touse the traditional out-of-band approach based on an inter-polation method to measure the OSNR. Furthermore, thehigh spectral efficiency reached by new advanced modula-tion formats hampers the out-of-band OSNRmeasurementonce the range of frequencies available to yield the noiselevel is smaller. In this scenario, an in-band technique tomeasure OSNR is essential for guaranteeing the necessarymonitoring performance efficiency, enabling its use formonitoring advanced modulated signals and also the leg-acy system (OOK modulation format), with a high qualityeven in the presence of dispersive effects.

Therefore, we aim to develop a device/mechanism wherecurrent OSNR could be estimated in-band, insensitive tofirst-order dispersive effects for coherent and noncoherentsignals, through the properties of polarization diversityand coherence difference between signal and optical noisefrom an EDFA [46]. Figure 4 depicts the proposed OSNRmonitor schematic.

This technique should be cost-effective, nonintrusive,and should be easy to integrate with other components thatdepend on OSNR measurement to add value on its solu-tion. There have been several in-band OSNR monitoringmethods; nevertheless, the interferometric techniquesare the closest ones to achieve all sets of specifications

about this problem, once they are able to be used for signalsdefined by only one polarization and polarization multi-plexed signals, guaranteeing a monitoring quality overdispersive effects [47].

In order to meet all requirements defined to performancemonitoring in optical networks a new OSNR measurementmethod was proposed [14,47]. This new technique is basedon polarization diversity and the coherent difference be-tween the signal and optical noise and uses a calibrationprocedure to ensure the desired performance. Figure 4depicts a schematic diagram of this new method.

As shown in Fig. 4, the proposed method has a PBS toensure that each orthogonal polarization component frominput signal passes through an interferometer with aspecific delay line, τd. Moreover, an external driver setsthe inference pattern for each polarization component.

The calibration procedure is related to a specifictransmitted signal, so when after this calibration thereis a significant temporal distortion at the signal due todispersive effects, the OSNR measurement loses accuracybecause the signal qualities are not the same as at themoment when the calibration was done. Considering theeffects of polarization mode dispersion, in order to mini-mize the loss of accuracy from that, the calibration pro-cedure needs to be based on each orthogonal componentof polarization from an input signal. To validate this ap-proach, a simulation was run in which the schematic shownin Fig. 4 was considered. For that, the signal non-return-to-zero (NRZ)-DP-QPSK at 112 Gb∕s and an interferometerdefined by a delay line at 20 ps was used. In this scenario,the calibration was performed in two cases. The first oneconsidered both orthogonal polarizations from the inputsignal, and in the second one the PBS was take out andthe calibration was made just considering an input signal.Figure 5 shows the OSNRmonitoring quality for both casesin the presence of 45 ps of differential group delay (DGD),although the calibration was made in the absence of thiseffect.

As illustrated in Fig. 5, even when the DGD is consid-ered, the evaluation of each polarization component en-sures a measurement error below 0.5 dB, whereas thiserror goes up to 12 dB in the other case in which the cal-ibration was made just considering the input signal.

Fig. 4. Schematic diagram of the proposed in-band OSNRmethod: PBS, polarization beam splitter; PD, photodetector;DSP, digital signal processing.

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The experimental evaluation of this new approach wasperformed with an interferometer with a delay line at20 ps and considering two different input signals: NRZ-DP-QPSK at 112 Gb∕s and NRZ-16 QAM at 224 Gb∕s.The calibration procedure for all these signals was madein the absence of dispersive effects; however, all OSNRmeasurements were obtained from a system with 45 psof DGD. Figure 6 shows all these experimental results.

Figure 6 depicts an OSNR measurement error below3.0 dB for an OSNR range from 5 to 25 dB for bothevaluated signals (112 Gb∕s DP-QPSK and 224 Gb∕sDP-16 QAM).

Despite ensuring experimentally the monitoring perfor-mance with the proposed OSNR method, this solutionneeds to evolve in order to decrease the calibration depend-ence. The best case will be achieved when this approachdoes not need a calibration procedure at all. Anotherpoint that can be emphasized is the choice of an interfero-metric method to monitoring the OSNR, once it is easy tobe implemented in SOI technology, which means a greatcapability to aggregate the value to a set of other solutionsthat can be integrated on the same platform. Beyondthat, other features with the choice of an interferometricapproach are the low cost and the high efficiency reachedby a customized project. Hence, to take advantage of aproject designed on an SOI platform, a set of interferome-ters was made on a silicon substrate with differentvalues of delay lines. To couple light inside the chip, a

coupling grid was used, which also functions like a PBS,splitting two orthogonal components of polarization fromthe input signal and adding a polarization rotation at oneof its output branches, to align it with the state of polari-zation defined by the waveguide designed on silicon.Figure 7 shows the set of interferometers made on anSOI platform to measure OSNR through the proposedmethod.

There are eight interferometers depicted in Fig. 7. Eachof them has a length of 700 μm but different values of delayline that go from 5 to 45 ps.

Fig. 5. OSNR monitoring performance with and without a PBS:(a) measured OSNR versus real OSNR, (b) measured OSNR versusreal OSNR.

Fig. 6. Experimental demonstration of OSNR monitoring perfor-mance in the presence of 45 ps of DGD: (a) measured OSNR versusreal OSNR, (b) measured OSNR versus real OSNR.

Fig. 7. Set of interferometers made on the SOI platform.

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For future work on this project, the proposed OSNR ap-proachwill be evaluated using the set of interferometers de-picted previously. Initially, it will be able to choose the bestinterferometer for this measurement based on its delay linevalue.After that, theresultsshowninFig.6willbecomparedwith the results achieved with this interferometer.

VI. FUTURE WORKS AND RESEARCH LINES

The optical technologies division in CPqD is continuallydevelopingstate-of-the-artnetworkelements for reconfigur-able DWDM optical networks such as optical transponders,WSS ROADMs, and MCS add–drops for colorless anddirectionless (CD) and colorless, directionless, and conten-tionless (CDC) add–drop, respectively. In this context, next-generation optical fiber transmission technologies with bitrates up to 400 Gb∕s and 1 Tb∕s are being intensivelyinvestigated [4–6]. Those systems may employ high-ordermodulation formats, spectral shaping, and densely packedmulticarrier transmitters (superchannels) in order to in-crease overall system capacity in comparison to current100 G technology, as demonstrated in our recent work in[4–6]. Regarding amplifier technologies, our current workin [48,49] investigates a hybrid optical amplifier obtaininga counter-propagating distributed Raman/EDFA hybridtopology and develops its respective hybrid automatic gaincontrol loop, achieving lower noise figure levels togetherwith spectral gain flatness for an all-amplifier dynamicoperation region.

In this context, the optical technologies division inCPqD is considering the following future works. First, ex-perimental demonstrations on a simultaneous flexibletransponder and WSS-based global equalization applica-tions will be carried out. By doing so, global (i.e., end-to-end) improvements on OSNR may allow us to allocatehigher modulation formats maintaining the bit rate, andthus increasing the capacity of fiber links. More generally,the triplet composed of 1) an EDFA adaptive setpoint gaincontrol, 2) global WSS-based adaptive equalization, and3) a flexible transponder provides a complex heterogenicscenario at the physical layer suitable for the migrationof current adaptive approaches to cognitive techniques.Second, the inclusion of the OSNR monitor in CPqD’s met-ropolitan mesh optical network test bed will be carried outto properly perceive current network status and provide afeedback and past measurement history aiming to developcognitive algorithms. Therefore, such capability will clearlybenefit the development of future cognitive applications ontop of the SDN controller. Third, an evolution of the currentoptical transport SDN controller will be carried out to in-clude support for the development of applications. In par-ticular, statistical functionalities, routing computation,protection schemes, and failure recovering policies arehighly desirable to be implemented as support for applica-tions that run on top of the SDN controller.

Future research lines include improving transmission,systems, and control areas to achieve network-wide perfor-mance improvements. Indeed, despite the flexibilityachieved at this point with the introduction of CDC-

ROADMs, flexible coherent transponders, and SDN con-trol, there are still many issues to be addressed.

Regarding optical systems, there are challenges in space,power consumption, and cost regarding the currentROADM-CDC architectures, as well as switching time ofthe order of hundreds of ms is still a major issue for re-routing traffic in the case of failures. Additionally, in orderto increase the optical systems’ capacity by orders of magni-tude, optical amplifier technologiesmust evolve fromsingle-core EDFA to multipumped multicore fiber (MCF)-EDFA(using low-loss fan-in to couple the signal and single-modepump for each core inside the EDF-MCF) in a short periodof time, evolving to innovative EDF-MCF designs (multiplesingle coresandamultimodeor claddingEDFfor thepump),andmultimode EDF (fewmodes) to build amplifiers using asingle multimode pump instead of multiple pumps. In thetransmissionarea,newparadigmshavebeenproposed,suchas multiflow transponders, for increasing flexibility andgranularity. Moreover, new challenges in transmission in-clude the development of technology for multicarrier recep-tion, joint digital signal processing (DSP) for superchanneldetection, and nonlinear compensation. Finally, we remarkthat SDNand optical network virtualization has to improveinmanyaspects, including interface standardization,multi-ple layer integration, as well as resource isolation.

VII. CONCLUSIONS

Thispaper reviewedaproposal, implementation, andval-idation of a SDN controller for transport optical networks,supporting, virtualization, and autonomic operation viaspecific SDN applications implementing adaptive algo-rithms for allowing the optical network to be self-adaptable,according to service requirements and network conditions.First, we highlighted several related works on the reportedactuation areas of the optical technologies division inCPqD.Then we reviewed our optical transport SDN controller.Then we highlighted a flexible-transponder that adaptsthemodulation format and spectrum utilization in a coordi-natedway, according to network conditions. Subsequently, aglobal spectrum equalization application has been demon-strated, which adapts attenuation applied at the ROADMsto improve the OSNR of the signals at the reception. We de-tailed an in-band OSNR measurement device capable ofmonitoring high-modulation signals first with discrete com-ponents, and subsequently integrated them into SOI pho-tonics technology. Finally, we detailed several futureworks to be carried out as well as different future researchlines of high interest.

ACKNOWLEDGMENTS

The authors wish to thank the financial support of FUNT-TEL, FINEP, and the CPqD Foundation under the project100GETH.

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