From Optical Performance Monitoring to Optical Network ...

(Invited)

From Optical Performance Monitoring to Optical Network Management: Research Progress and Challenges Lian-Kuan Chen, Man-Hong Cheung, Chun-Kit Chan Department of Information Engineering, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR Tel: 852-2609-8389, Fax: 852-2609-5032, E-mail: [email protected]

Abstract: Optical performance monitoring (OPM) and optical network management (ONM) are essential in building a reliable, high-capacity, and service-differentiation enabled all-optical network. Early research efforts in OPM focused primarily on achieving the desired monitoring functionality. However, the slow commercial deployment of OPM suggests that several problems including cost-effectiveness and interactions with higher control layers have yet to be resolved. This paper first provides a broad review of some traditional OPM techniques, and then examines the recent trend of multiple-parameter simultaneous monitoring as a means to achieve more cost-effective OPM options. Finally, research efforts that relate OPM to high-layer ONM are reviewed. 1. Introduction Dense wavelength division multiplexing (DWDM) is rapidly gaining prevalence as the enabling technology to realize a high-capacity, national-scale all-optical network. In such high data-rate systems, a short service disruption will affect a colossal amount of data. Compounding the need for advanced monitoring and management is the fact that future optical networks will be (i) transparent, such that optical-electricaloptical (OEO) conversion will be eliminated and bit-rate-, format-, and protocol-independent physical layer monitoring is required, and (ii) dynamic reconfigurable, such that monitoring has to be without prior knowledge of the signal origin and transport history as well as more intelligent management is required. Advanced OPM and ONM will serve a number of crucial network functions in next-generation optical networks including (i) active time-varying equalization and distortion compensation, such as tuning a dynamic gain equalizer (DGE) for amplifiers and a PMD compensator, (ii) intelligent service provisioning and traffic routing, as well as new network element support, (iii) fault forecasting, detection, diagnosis, localization, and resilience mechanism activation, and (iv) signal quality characterization for quality of service (QoS) assurance and service level agreement (SLA) fulfilment. In the following we will review research efforts in both physical layer OPM and its interaction with higher management layers that contribute to realize a reliable, high-performance, and service-differentiation enabled all-optical network. 2. Single Parameter Physical-Layer Monitoring The broad spectrum of OPM includes a plethora of parameters to be monitored which can be classified into three categories as shown in Fig.1: Signal loss monitoring refers to the monitoring of in-line component failures and fiber cuts that cause a change in opacity in optical transmission. It is particularly important to monitor the active components such as erbium-doped fiber amplifier (EDFAs) and optical crossconnects (OXCs) due to their higher failure probability. Signal alignment monitoring, on the other hand, concerns with the alignment of signal wavelength, filter position, and pulse carver to ensure proper operation. Finally, signal quality monitoring pertains to the monitoring of a multitude of disparaging effects that must be minimized or controlled. These impairments include noise, distortion, and time jitter. We will review some of the OPM areas, with particular focus placed on the research done in our research group. Fiber fault monitoring: To avoid the huge data loss due to fiber failure, it is necessary to constantly monitor the fiber status for fault management. Fibre Bragg gratings (FBG) based passive optical surveillance schemes are attractive as they are low-cost and can be easily integrated into the network systems. On the other hand, spectral analysis with Fast Fourier Transform (FFT) is simple and easily scalable. These fiber fault monitoring schemes are centralized and thus facilitate fault management [1]-[3].

OPM

Signal Loss In-line Component Failures

Signal Alignment

Fiber cuts

Wavelength

Signal Quality

Analog Parameters

Digital Parameters

Pulse Carver TX/RX Failures

BER Filter

EDFA failures Other Active/ Passive Component Failures

Optical Power (Average, Peak) OSNR

PMD

Pulse/Bit Shape

Crosstalk

Polarization state

Jitter

Eye diagram

Nonlinear Distortion

Extinction Ratio

Q-factor

CD

Fig. 1 The broad spectrum of OPM

In-line active component monitoring: For proper operation of the networks, it is necessary to monitor EDFAs for failure in pump diodes and possible power fluctuations. It is also crucial to monitor OXCs and optical add-drop multiplexers (ODAMs) for wavelength routing failures due to switch failure or wavelength mismatch due to environmental changes. The use of FBG, pilot tone, and spectral analysis are simple and attractive ways for such in-line component monitoring [2],[4]-[6]. OSNR monitoring: Future dense-wavelength, dynamic reconfigurable networks require accurate in-band OSNR measurement for link provisioning, signal quality characterization, fault localization, and intelligent routing. Polarization-assisted approaches are promising approaches to measure the in-band noise [7]. Simple PMD-insensitive approach based on polarization-nulling with off-center narrowband filtering is a promising candidate for use in high-speed networks for its robustness and simplicity [8]. PMD monitoring: Polarization mode dispersion (PMD) is a critical limitation in high-speed (>10Gb/s) optical networks because it will cause inter-symbol interference (ISI) and network outage. Common monitoring approaches include using degree of polarization (DOP) and the RF notch frequency components [9]. Another method is to use frequency resolved polarization measurement in which self-phase modulation (SPM) can greatly reduce the estimation error and extend the monitoring dynamic range [10]. 3. Multiple Parameter Physical-Layer Monitoring While early OPM options focused on monitoring a particular type of impairment at a time, it is not uncommon that several impairments may affect the monitoring metric simultaneously. In this case, attempts have usually been made to monitor one type of impairment to the exclusion of the others. The current research trend, however, is to use simultaneous monitoring techniques that can quantify multiple signal degradations concurrently, thereby achieving more cost-effective OPM. Optical spectrum analyzer (OSA) is perhaps the most common technique to monitor signal power, wavelength, and OSNR. However, it is bulky and does not provide distortion information. Other research techniques are possible: Simultaneous OSNR and PMD monitoring using polarization-assisted methods: Polarization-assisted methods can monitor OSNR and PMD individually. With suitable modification, simultaneous OSNR and PMD monitoring can be achieved to provide a more comprehensive monitoring picture. Previously, we proposed two schemes based on enhanced RF spectral analysis and DOP assisted with polarization

scrambling [11]-[12]. These schemes differ in their monitoring dynamic range, sensitivity, robustness, and complexity. We will review these schemes and present their respective pros and cons in the presentation. Simultaneous PMD and GVD monitoring using enhanced RF power analysis or eye pattern analysis: RF power analysis of clock tones or different notch-frequency components is a common way to monitor PMD or Group Velocity Dispersion (GVD) individually. Transmitter-side polarization scrambling and receiverside RF power analysis was employed together for the first time to achieve simultaneous first-order PMD and GVD monitoring [13]. A novel eye pattern analysis was also proposed in [14] to monitor PMD and CD simultaneously. These schemes differ in their speed, complexity, monitoring dynamic range, and sensitivity. Simultaneous wavelength, power, and path monitoring using frequency-modulated pilot tones: Pilot tones are attractive candidates for monitoring because they follow the same optical path as the optical signal and they are cost-effective [15]. Using FFT and electrical processing technique, frequency-modulated pilot tones can provide useful and easy measurement of wavelength, power, and path of WDM channels without suffering from any ghost tones problem or receiver sensitivity degradation [16]. Clearly, the research trend is to use a simple scheme to characterize multiple parameters concurrently in a cost-effective manner. It is, however, a great research challenge to decouple one type of impairment from a host of others, especially when the number of impairments goes up, as the difficulty scales rapidly. 4. Higher-Layer Optical Network Management Another issue closely related to OPM is its communication with higher control layers for ONM, which is responsible for network topology discovery, resource management, connection provisioning, fault management, and performance management [17]. There is considerable confusion on whether OPM should be used to control a network element or as higher-level device to report the monitoring metrics to the higher control layers [18]. In fact, for some OPM functions, like PMD monitoring for tuning a PMD compensator and power monitoring for dynamic gain equalization (DGE), the OPM metrics may remain locally at the network node level (NEL) to control the particular network element directly. For other functions, like fault monitoring and network element status monitoring, the OPM metrics have to be disseminated to the higherlayer element management system (EMS) or network management system (NMS) to engineer optical paths and initiate administrative actions. There are several worthy research directions to enable the most effective interactions between OPM and ONM. The first one is the management hierarchy design problem, such as how to design the EMS and NMS and whether to collect the monitoring information in a centralized or distributed way. The second one is the protocol design to disseminate the monitoring metrics to higher layers. This problem has been addressed in [19], in which an extension to the existing CCS7 protocol is proposed to include the surveillance information in the signaling network. The third one is the optimization of placement or amount of performance monitors in the optical networks. It is apparent that one-monitor-per-link/component is not the most cost-effective and efficient solution as it does not take into account the failure probability of the link/component nor does it scale well with the network size. Moreover, multiple alarms may be generated for the same failure which may complicate and heavily stress the management system. Many research endeavors are directed to the last problem. A number of heuristic algorithms proposed for optimum monitor placement solutions are channel-based, in which the placement of monitors is related to the light paths already setup, under the assumption that the network is static [20]-[21]. These algorithms may help reduce the number of monitors and redundant alarms but are not practically usable in dynamic reconfigurable networks. Another stream of algorithms relies on using the network topology to design the optimum placement solution. These schemes use separate wavelength channels as supervisory channels to probe the network and the network health is deduced from the probe results. In [22], for example, a meshed network is broken down into “monitoring cycles” and a monitoring unit is assigned to each cycle. A loopback dedicated supervisory channel is set up and all OPM information, such as optical power and OSNR, is collected by this channel to deduce any fault in link or node in the cycle. The aim is to reduce the number of monitoring cycles and hence the average number of supervisory channels required. While this kind of network-based scheme is efficient for fault localization and diagnosis, it uses a different channel for monitoring, thus does not directly measure the in-service channel quality. A valuable next research step may be to find an optimum monitor placement solution that takes into account of network topology for not only the efficient fault diagnosis but also the in-service channel health for performance management.

Another possibility is to optimize the amount of monitoring information that needs to be collected so that more cost-effective monitoring module with less functionality may be deployed. In summary, this paper reviews some projects on the performance monitoring schemes as well as their interactions with higher-layer optical network management. A comprehensive list of physical layer monitoring techniques ranging from component fault to signal quality monitoring is presented. Especially, the current trend towards using simultaneous monitoring techniques as more cost-effective OPM options is highlighted. Major issues for communications between OPM and higher control layers, like management hierarchy, protocol design, and monitor placement are also discussed. These research endeavors are essential to realize a reliable, high-capacity, QoS enabled next-generation all-optical network. References: [1] C. K. Chan, F. Tong, L. K. Chen, J. Song, D. 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