While it remains a challenging time for the telecommunications industry, advances are being made to reduce costs and improve service levels. New advanced management capabilities for the optical layer are key to network and business improvement. Features such as wavelength path trace, per-wavelength power monitoring, and power alarm thresholds help network operators improve their network operations while decreasing operational costs and increasing customer satisfaction.

The need for sophisticated optical monitoring has arisen as the metropolitan-area network has evolved. In first-generation MANs, optical links are generally node-to-node, with wavelength-division multiplexing (WDM) providing fiber relief but no topology changes. Thus, frame-based monitoring techniques are generally sufficient for fault sectionalization and isolation. Optical layer monitoring, if provided, is focused on power measurement at a wavelength's source and destination end points only.

With increased bandwidth, shared point-to-point architectures result in excessive processing of through traffic at points such as add-drop multiplexers (ADMs). To alleviate this, second-generation metro networks reduce the electronic switch/processing costs through an increase in optical layer complexity, thus permitting wavelength or waveband routing through optical ADMs (OADMs).

Third-generation systems take this one step further by allowing the wavelength connectivity to be dynamically reconfigurable. Naturally, to manage the capital cost savings allowed by this electronic-to-optical processing shift, a commensurate shift to optical-layer monitoring is required. End-to-end frame-based monitoring is no longer sufficient. It can provide transponder-to-transponder link integrity and link ID verification but provides no fault sectionalization capability. Fault management of a complex static or reconfigurable OADM-based network based on end-point information is impractical; the operational costs involved in implementing and maintaining a metro network would significantly outweigh the capital costs. The need for improved optical-layer performance monitoring (OPM) has been recognized, and several commercial solutions have emerged.

With a few exceptions, most of these solutions focus on detailed spectral analysis, providing high-resolution wavelength, power, and signal-to-noise measurements. However, the detailed measurements provide no information about the course a particular light-path has taken through the network. For such networks, an advanced optical-layer management system that can identify individual services, as opposed to merely wavelengths, is needed. This system will facilitate better and more automated network management and anticipate the migration to reconfigurable networks where real-time management is the key to success.

One of the key requirements for an optical-layer management system is to present relevant information effectively and place it in context so that it is clearly understandable and usable. The trade-offs here involve the amount and type of information to be gathered and delivered-too much can be as bad as too little-and the associated costs. These translate into four requirements for an advanced optical-layer management system.

First, OPM needs to be ubiquitous within a network, and architectures that focus on precise optical spectral analysis are too expensive for this. Such solutions tend to be deployed only at key locations within the network or as temporary test cards for troubleshooting. But a lack of deployment would mean a loss of optical layer sectionalization and fault isolation capability.

An alternative approach that complements detailed spectral analysis is to have power measurement at every node-or, even better, at every optical module within a network. This is coupled with a method of providing a unique ID to every wavelength and the ability to read the ID without expensive optical or electronic frame-based processing. Digital signal-processing technology makes this method possible at low costs, without relying on more-expensive optical signal processing. Such functionality should be fully integrated so that optical management functions can be provided systemwide, not just locally. Having integrated power and ID measurement capability within all optical cards permits network-wide path/power management, independent of the network topology or architecture. With the introduction of reconfigurable or dynamic connectivity, either through reconfigurable OADMs or optical switches, this management capability becomes essential, analogous to the Sonet performance monitors provided to help manage ADM-based connectivity.

Network status
The second requirement is to provide multiple logical views of performance data, creating a much clearer picture of the network's operational status. A spectral (channelized) view on a single fiber is great for visualizing spectral utilization. A spatial view-a power trace along a light path-monitors each wavelength/service each step along the way. A multiple-fiber view provides a spectral power view on the working and protected, forward and reverse paths across the network.

OPM must also provide optical path tracing and ID, independent of assigned wavelength colors-because multiple services with the same optical wavelength (on different fibers) can route through the same multifiber node/hub. This is possible if unique ID tags are assigned to each wavelength. Finally, efficient real-time operational information and notification of faults are required.

For more on this topic and three examples of situations where an advanced optical-layer management system reduces the operational costs and enhances the quality of service, see the complete online article at www.eet.com/in_focus/.

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