Monitoring the Quality of Signal in Packet-Switched Networks Using Optical Correlators

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<ul><li><p>JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 23, DECEMBER 1, 2009 5417</p><p>Monitoring the Quality of Signal in Packet-SwitchedNetworks Using Optical Correlators</p><p>Ruth Vilar Mateo, Jaime Garca, Guillaume Tremblay, Youngjae Kim, Sophie LaRochelle, Member, IEEE,Francisco Ramos, Member, IEEE, and Javier Mart, Member, IEEE</p><p>AbstractThe increasing demand in the Internet network forreal-time multimedia data traffic with high quality of service (QoS)is pushing the limits of existing network structure. Optical packetswitching (OPS) is considered as a possible technology for futuretelecommunication networks due to its compatibility with burstytraffic and efficient use of the network resources. But OPS bringsabout new challenges to the research in optical performance mon-itoring (OPM). In this scenario, each packet follows its own pathalong the network depending on the routing information containedin the label and thereby packets suffer from different signal degra-dations. Therefore, a definitive goal for OPM is to provide compre-hensive signal quality information as part of QoS implementationto keep the level of QoS promised to customers. In this paper, anovel optical SNR (OSNR) monitoring technique based on the useof optical correlation is presented. A fiber Bragg grating-based cor-relator was constructed and used to experimentally demonstratethe successful correlation. Experiments performed on a 40 Gb/ssystem confirm the viability of this approach. By measuring statis-tics from the autocorrelation peak, the monitor is capable of directOSNR monitoring with an error of less than 0.5 dB.</p><p>Index TermsFiber Bragg gratings, optical packet switching,optical packet-switched networks, optical performance moni-toring, optical SNR (OSNR).</p><p>I. INTRODUCTION</p><p>T ELECOMMUNICATION networks are experiencing adramatic increase in demand for capacity, much of itdue to the exponential growth of the Internet and associatedservices. In addition to rapidly increasing capacity demands,new multimedia and data services are driving the needs forhigh-performance and high-utilization networks. Optical packetswitching (OPS) technology is particularly attractive as a pos-sible technology for future telecommunication networks, dueto its compatibility with Internet protocol (IP) and efficient</p><p>Manuscript received February 17, 2009; revised June 18, 2009. First pub-lished July 21, 2009; current version published October 16, 2009This work(at COPL, Universit Laval) was supported by the all-optical packet switchingproject of the Canadian Institute for Photonic Innovations (CIPI). The work ofR. Vilar was supported by the Spanish Government under an FPU grant.</p><p>R. V. Mateo, J. Garca, F. Ramos, and J. Mart are with the NanophotonicsTechnology Centre, Universidad Politecnica de Valencia, Valencia 46022, Spain(e-mail: rutvima@ntc.upv.es).</p><p>G. Tremblay, Y. Kim, and S. LaRochelle are with the Centre doptique, pho-tonique et laser, Dpartement de gnie lectrique et de gnie informatique, Uni-versit Laval, Qubec, Canada.</p><p>Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.</p><p>Digital Object Identifier 10.1109/JLT.2009.2028034</p><p>use of the network resources [1], [2]. Indeed, the benefit ofOPS rises from the higher network utilization at subwavelengthgranularity and from the supporting of more diverse serviceswith bursty traffic patterns. Moreover, OPS technology offers anew capability to process packets directly at the optical layer.At this point, the concept of all-optical label switching (AOLS)appears to be a solution to avoid the bottleneck imposed bythe nodes based on electronic processing [3]. In such an AOLSscenario, all packet-by-packet routing and forwarding functionsof multiprotocol label switching are implemented directly inthe optical domain. By using optical labels, the IP packets aredirected through the core optical networks without requiringO/E/O conversions whenever a routing decision is necessary.The main advantage of this approach is the ability to routepackets/burst independently of bit rate, packet format, andpacket length. In addition to packet routing and forwarding,the processing of optical labels brings about new challengesto the research in optical performance monitoring (OPM).OPM is expected to play an important role in next-generationoptical networks performing some network crucial functionssuch as signal quality characterization for quality of service(QoS) assurance and service level agreement fulfillment [4].QoS refers to the capability of a network to provide betterservice or guarantee a certain level of performance to selectednetwork packets. Therefore, new techniques to monitor thesignal quality directly at the physical layer must be deployedas part of QoS, to insure that networks are performing at thedesired level and thereby realizing a reliable, high-performanceand service-differentiation enabled all-optical network [5].</p><p>With the introduction of new-generation multimedia services,OPS networks transport different types of traffic with differentquality requirements so that performance monitoring is espe-cially important to ensure that packets receive appropriate treat-ment as they travel through the network. Packets entering thenetwork must be analyzed to determine their QoS requirementsdepending on the kind of network service, and then, the signalquality must be monitored in each intermediate node to guar-antee certain level of performance. In this scenario, each opticalsignal may traverse different paths and different optical compo-nents; thereby having its own history and quality. This is shownin Fig. 1, where packets coming from different services and withdistinct QoS requirements, and , are generated from sev-eral sources and go through different paths along the network.The three packets pass diverse optical links and switches, thenexit from the same node (node A). The bottom of Fig. 1 de-picts the optical power of the output packets from node A. Wecan see that the ASE noise level in each packet is different and</p><p>0733-8724/$26.00 2009 IEEE</p></li><li><p>5418 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 23, DECEMBER 1, 2009</p><p>Fig. 1. Packets with different quality in an OPS network.</p><p>thereby the packets have different level of performance. There-fore, in order to determine the health of optical signals in OPSnetworks and to guarantee certain QoS level, the optical packetsin each intermediate node must be monitored.</p><p>The traditional monitoring techniques based on BER estima-tion [6], [7] or Q-factor [8], [9] are not appropriate for the futureultrahigh-speed optical networks due to the O/E conversionsthat limit the bit rate unless it is preceded by optical time de-multiplexing. In fact, new performance monitoring techniquesshould be defined in order to overcome this limitation. Recentworks have demonstrated all-OPM and selective packet drop-ping by monitoring the error of the optical header field to inferthe error of the data payload [10][12]. The technique proposedin [10] uses labels at lower bit rates than the payload, so thequality is not monitored at the packet bit rate. With respect to[11] and [12], the basis of the monitoring techniques is different.Both of them are based on time-to-live (TTL) field concept, sothey do not measure the real value of the optical SNR (OSNR),but they estimate it as function of the TTL value.</p><p>In this paper, we propose and demonstrate a novel techniquebased on the use of optical correlation to assess the signal qualityat the optical domain with relaxed speed requirements. A spe-cific data word, with information associated with QoS or kindof service, is inserted into the packet header and is processedby means of an optical correlator based on fiber Bragg gratings(FBGs). Therefore, the technique uses serial optical labels trav-eling at the same bit rate as the payload allowing performancemonitoring at high speeds as the correlation is performed in theoptical domain. In particular, the OSNR of the incoming packetsis estimated from the noise statistics of the autocorrelation pulsepeak power to ensure that they fulfill the QoS requirements.Using a 10-ps pulsewidth 40 Gb/s return-to-zero ONOFF keying</p><p>(RZ-OOK) system, we show that OSNR can be monitored withan error 0.5 dB.</p><p>The paper is structured as follows. In Section II, the principleof operation of the novel monitoring technique is described.Section III provides an overview of different digital correlationimplementations and explains the basis of an FBG-based corre-lator. In Section IV, the monitoring technique is demonstratedexperimentally. Finally, the paper is summarized in Section V.</p><p>II. PRINCIPLE OF OPERATION</p><p>The signal quality monitoring method is based on sendinga specific data word (monitoring field) inserted into the packetheader, which specifies the QoS requirement of the incomingpackets. This QoS field fixes the quality of signal requirementsfor each packet flow coming from a specific service. Then op-tical headers of packets with the same QoS requirements, i.e.,the same monitoring field associated with the same kind of ser-vice, are processed in each intermediate node to check if theestimated quality fulfills the terms of QoS and to guarantee therequired level of performance. Fig. 2 shows the block diagramof the proposed monitor for optical packets [13]. The monitor iscomposed of correlators, each of which is configured to pro-duce a match signal for a specific QoS. The number of corre-lators, , defines the number of type of services or QoS levelsprovided by the carriers. Three basic levels are normally definedin the network: best-effort service, differentiated service, andguaranteed service [14].</p><p>When the packet arrives at the node, the monitoring infor-mation field, with or 1 and</p><p>, is extracted with a pulse peak power of .The circuit responsible for this task is composed of two blocks:</p></li><li><p>VILAR MATEO et al.: MONITORING THE QUALITY OF SIGNAL IN PACKET-SWITCHED NETWORKS USING OPTICAL CORRELATORS 5419</p><p>Fig. 2. Block diagram of the proposed OSNR monitor.</p><p>a packet clock recovery circuit and a high-speed SOA-MZI op-tical gate configured to perform a Boolean AND. These sub-systems were demonstrated in the IST-LASAGNE project [15].Once the monitoring field is extracted, the optical correlatorallows signal quality monitoring with high fidelity and in realtime. In the next section, several typical implementations of anoptical correlator are explained.</p><p>In optical correlation, the received signal is correlated inthe optical domain with the sent signal so that the correla-tion function is a measure of how similar and are. Thecorrelation can be implemented in a discrete system as</p><p>(1)</p><p>where is the number of bits in the correlation sequence,is the bit period, represents the reference signal as theweights that multiply each of the delayed received signals, and</p><p>is the received signal delayed by bit times. Let ussuppose that the optical correlation is configured to match with areference signal composed of bits with . If the two sig-nals, and , are identical, (1) becomes an autocorrelationand the output correlation pulse appears as a single sharp peakin the center with an amplitude equal to . If the signals areless well matched, then the peak decreases and the informationon either side of the peak increases. The autocorrelation func-tion is considered in power basis as it will be explained in thefollowing sections. Therefore, from the amplitude of the cor-relators output different types of services (QoS requirements)may be separated (Fig. 2). As an example, the output signal fromthe optical correlator corresponding to is shownin Fig. 3.</p><p>The OSNR of the incoming packet can be calculated byusing the statistics of the autocorrelation pulse peak power. TheOSNR is defined as</p><p>(2)</p><p>where mean is the mean value and is the standard deviationof the autocorrelation pulse peak power. Equation (2) shows thatthe OSNR monitoring depends on the shape of the correlatorsoutput as it has been commented before.</p><p>Fig. 3. Autocorrelation pulse.</p><p>Fig. 4. Simulation results of OSNR for different bit rates.</p><p>At high speeds (above 40 Gb/s), the correct evaluation of BERrequires optical demultiplexing. However, the proposed tech-nique alleviates this speed requirement, as it only measures theautocorrelation pulse, so the system works well at the packetrate. Moreover, the correlation is performed all optically, thusreducing the hardware requirements.</p><p>Fig. 4 shows the simulation results of the OSNR estimationusing the autocorrelation pulse for different bit rates. As it can beseen, the estimated OSNR adjusts perfectly to the real OSNRvalue and it is independent of the bit rate. Now that the proposedtechnique has been validated at high speeds by means of simu-lations, in the next section we demonstrate it experimentally at40 Gb/s.</p><p>III. OPTICAL CORRELATORSNow that the optical correlation as an OSNR monitor has been</p><p>introduced, this section presents several implementations at theoptical domain. A common implementation of an optical corre-lator is the tapped delay line [16]. In this implementation, thereceived signal is sent to a tapped delay line, which requiresone tap for each bit in the desired sequence that are weighted bythe factors 1 or 0 depending on the value of the bit of thesequence. The weights can either be phase shifter or amplitudeweights or both (amplitude weights of 1 s or 0 s are implementedby placing a switch in each path that is closed forand opened for ). The received signal is equally</p></li><li><p>5420 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 23, DECEMBER 1, 2009</p><p>split among the delay lines. Each successive delay line adds oneadditional bit of delay to the received signal before the com-biner, where the signals are added to yield the correlation outputfunction. The problem of this implementation is that the opticaltapped-delay-line structure requires a separate fiber branch andan optical switch for each bit in the desired bit pattern, making itimpractical to construct bank of correlators of many bits. More-over, the length of each fiber branch must be cut precisely to pro-vide the requisite 1-bit delay between successive branches andthat it is extremely difficult at high speeds. Another implementa-tion is based on planar lightwave circuits [17]. But this solutionis disadvantageous in cost and packaging losses (input/outputcoupling losses). A simpler, easily manufactured, and manage-able correlator may be constructed by writing a series of FBGmirrors into a single length of fiber [18]. In this paper, we designan FBG-based correlator in order to validate the OSNR moni-toring technique.</p><p>A. FBG-Based CorrelatorIn this type of correlator, a series of FBGs is written into a</p><p>single length of fiber. In these applications, the array of grat-ings reflects back part of the signal at different times, resultingin multiple replicas of the input signal spaced in time with thedelay increment . The reflectivities of the FBGs provide thesame weighting function as the optical correlati...</p></li></ul>

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