A real-time window into each wavelength's optical performance can result in more efficiently managed traffic for metro service providers.
WILLIAM L. EMKEY, Lightchip
As demand for bandwidth and enhanced services explodes in the metropolitan-area market, technology advances promise to transform optical networks, but prohibitive costs and inflexibility have slowed progress. Nevertheless, many equipment manufacturers and systems integrators are ready to face the challenge of providing a reliable and service-ready optical-networking platform in the metro environment.
Metropolitan optical networking is challenging because the MAN is not just a scaled-down version of the long-haul network, where DWDM has eased bandwidth bottlenecks. The potential for DWDM in the metro market is equally revolutionary, because it allows optical networks to support growing amounts-and diverse types-of Internet traffic. But in the metro, service providers need to efficiently ramp up services in increasingly congested networks to exploit the demand for bandwidth-intensive broadband services. Service provisioning is difficult due to daunting complexity and quality-of-service (QoS) issues. While long-haul, fixed, point-to-point DWDM technologies can be adopted at a significant cost to expand metro-network capacity, these systems cannot effectively manage diverse network traffic nor flexibly support on-demand provisioning of network services. Brute force solutions, such as pulling additional fiber, are too time-consuming and price-prohibitive for most service providers.
The incorporation of DWDM into metro networks is necessary not only to expand bandwidth availability, but also to provide the high levels of flexibility associated with rapid provisioning, QoS, multiple protocols, and enhanced services. All of that needs to be accomplished while reducing the overall cost of ownership. DWDM systems will enable service providers to meet these challenges, but management of these networks requires optical performance management at the wavelength level.
The complex expansion of the metropolitan-area network (MAN)-more customers demanding more network services-creates the need for DWDM systems that facilitate network management. A DWDM system creates multiple wavelengths on a single fiber; each wavelength is in a sense a "virtual fiber." It is the power of this "virtual fiber network" that provides the bandwidth and flexibility to address the growth and dynamic requirements of the metro environment.
To successfully manage this multiwavelength network, the service provider must be able to nonintrusively monitor the individual wavelengths within any given fiber. Traditionally, monitoring metro-network performance has been expensive, intrusive, and ultimately unmanageable. Today, technology advances have resulted in new solutions.One such solution is an optical performance manager (OPM) that uses measurements provided by a solid-state indium gallium arsenide detector with a simple nonintrusive optical tap (0.5%) to monitor key optical parameters: wavelength, optical power levels, and optical-signal-to-noise ratios (OSNRs) for each channel. Nonintrusive wavelength monitoring means providers can continuously analyze the performance of individual wavelengths without disrupting the signal. The OPM monitoring solution allows service providers to cost-effectively manage individual wave lengths at any point in an optical network, including granular, and bidirectional monitoring (see Figure 1).
With dynamic metro applications, service pro viders need to effectively monitor and manage every wavelength in the network. The OPM provides metro service pro viders with control at the wavelength level. It also allows service pro viders to improve network reliability, provisioning, and management; accelerating delivery of broadband services such as voice over IP, video on demand, and storage-area-network transport. Further, the OPM is transparent to all traffic types and ensures high QoS.
The integration of an OPM solution into a DWDM system results in a flexible service delivery platform. Since the system is monitored at the wavelength level, it is completely transparent to the information protocol and bit rate. A DWDM network link showing the network-management functions is illustrated in Figure 2. Wavelengths are monitored at several locations throughout the optical link-at all inputs/outputs of the primary fibers at a central office, an optical add/drop node, and at key access points. Also shown is an interface to the network operations center, so the system can be configured for reporting, predictive alarming, and overall wavelength management.
Figure 2 also illustrates the use of OPMs for network-element (NE) control. For example, by monitoring all the indicated parameters, it is possible to use a single OPM to lock families of lasers, control channel power levels via erbium-doped fiber-amplifier interfaces, and control or customize power levels from signal sources such as transponders. With monitor points at multiple locations, it is possible to achieve distributed network monitoring and mapping capability. The OPM's graphical user interface shows performance measurements of individual wavelengths, including power levels, and OSNRs (see Figure 3).
By setting appropriate alarm thresholds for wavelength drift, power variation, and OSNR, network degradation can be detected and corrected, thus preventing loss of customer service. Such "advance notice" also enables the provider to be proactive in taking corrective actions (e.g., anticipating change-out of specific NEs).
Because the OPM is monitoring the actual wavelengths transmitted in the fiber-not monitoring via indirect means such as rear facet laser detectors-it can provide rapid and accurate feedback for NE control. By interfacing the OPMs with appropriate NE drivers, it's possible to lock lasers; selectively attenuate, dynamically switch, and add/drop wavelengths; control amplifier gain; and balance transponder outputs. This functionality permits rapid provisioning and custom network links. All contribute to lower costs, greater flexibility, and increased revenue for service providers.
Comparing the inputs from several local monitoring points inherently provides the status of the distributed network, which further increases network reliability by revealing problems in the physical plant, such as fiber degradation, connector failures, and splice failures. By adding a minimal level of control capability within each optical multiplexing element, a higher level of element discovery and network mapping is provided.
Optical wavelength management is an indispensable piece of the metro-network puzzle. Because optical networking is complex and constantly evolving, network builders are turning to suppliers instead of attempting to design and develop their own end-to-end optical management solutions. Lacking the time and expertise to develop complete solutions on their own, savvy builders of next-generation metro networks are enlisting help from manufacturers of wavelength routing and intelligent optical management subsystems.
In addition to offering compelling performance characteristics, the subsystems approach is the best means for providing plug-and-play equipment that can integrate seamlessly into a best-of-breed technology environment. Sub systems are often the most logical choice for service providers, systems integrators, and other equipment manufacturers. By incorporating intelligent, modular subsystems, these companies can streamline deployment, gaining time-to-market advantages through best-of-breed products that are easily plugged into larger networking systems. Service providers are rewarded as well, with superior network performance at a lower cost.
As vendors focus on intelligent optical subsystems, it will balance the metro service equation by accelerating deployment of wavelength-management solutions, thus hastening the promise of all-optical metro networks.
In short, deployment of a cost-effective optical performance management system provides nonintrusive optical monitoring of each individual wavelength carried on a single fiber within an optical network, without disrupting or disturbing the traffic. Wavelength-management systems help metro-network providers control the quality, performance, and efficiency of their networks, enabling carriers to satisfy strict service-level agreements and successfully scale their infrastructures to serve growing numbers of end users. When integrated with a DWDM system, an OPM allows service providers to unleash the power that is inherent in a multiwavelength network.
William L. Emkey is vice president of marketing at Lightchip Inc. (Salem, NH).