Wavelength-tracking technology looks into the fiber, providing real-time information on a wavelength-by-wavelength basis, simplifies provisioning and troubleshooting and reduces costs in optical networks.
By Robert M. Lane
Tropic Networks Inc.
Network management has traditionally been less expensive and relatively easy to incorporate in networks that use electronic gear to break data down to its simplest form -- ones and zeros. All-optical networks (AONs), on the other hand, pose significant and costly challenges when it comes to peering into the actual fiber plant.
While the operational state of the physical fiber, or even a group of channels within a fiber, is possible to manage, attaining information for monitoring specific optical channels has been very costly -- until now. Today, new wavelength-tracking technology is providing a cost-effective method for moving beyond the aggregate WDM signal and providing clear visibility to individual wavelength activity -- and network architects are "seeing the light."
Engineers face many challenges in achieving SONET/SDH-like levels of performance monitoring and fault detection within an all-optical DWDM architecture. Forward error correction (FEC) and loss of signal (LOS) accommodation are achievable at the data link layer -- but only when supported by particular network protocols. Any protocol-independent scheme currently requires the presence of facilities via the optical carrier in the physical layer.
For true quality-of-service (QOS) within the physical layer, each and every optical channel must be monitored. Monitoring options include optical signal-to-noise ratio (OSNR), signal power, noise power, dispersion, and channel wavelength drift (via an optical spectrum analyzer, or OSA). An OSA can detect optical channels and optical channel dithering can detect and identify specific optical channels.
Continual real-time optical power measurement is essential to minimize crosstalk and prevent erbium-doped fiber amplifier (EDFA) saturation. These measurements are possible within 10 msec to 1 sec using an OSA. Total average optical power measurements within 10 msec can be made using a photodiode.
The ability to identify a specific optical channel in the optical network is vital for automatic topology recovery. Optical path tracing, or detection of an identification tag of each optical channel, is a viable technique that allows misconnections and/or misrouting to be traced and corrected.
Seeing the light
A new technique has recently been developed that enables monitoring of individual optical channels at any point in the metro DWDM network. This wavelength-tracking technology attaches an optical tag to each optical channel as the signal is introduced into the network. This optical tag enables real-time monitoring of each channel anywhere in the network and is used to trace individual wavelength paths and simplify the troubleshooting of optical networks.
Inexpensive decoders, integrated on cards and nodes, are located at multiple points in the network. They monitor the information contained in the optical tag, eliminating the need for expensive optical test equipment and the optical engineers required to interpret test equipment displays. The optical tag decoders can also be interpreted remotely, eliminating truck rolls.
Wavelength tracking applies a unique optical signature to each input signal to monitor and report on properties of the light traveling through the optical metro network. This optical variant of the SONET/SDH path trace feature provides complete visibility of each optical path, regardless of how many additional circuits are passed through, added, or dropped. Unlike similar methods of optical channel monitoring, this unique wavelength-tracking signature can differentiate between multiple instances of the same wavelength passing through different parts of the network. Optical channel tagging combined with real-time optical power measurement is a cost-effective solution that reduces provisioning and troubleshooting costs in optical networks.
Fast and accurate
Wavelength-tracking technology is integrated directly into the network management system (NMS), which enables quick detection of problems generated by network extension activities as well as those that occur during normal network operation.
When network extension activities are in progress, wavelength tracking can immediately detect associated problems, such as misconnections or accidental damage to a fiber connector during installation. This allows the problem to be corrected before the engineer leaves the site -- saving costly truck rolls necessary to fix the problem later. During normal network operation, the NMS uses wavelength tracking to warn of both potential network problems and network failures, enabling the operator to quickly identify the problem area.
The wavelength-tracking technology's optical power-monitoring facility includes warning notifications when pre-determined power thresholds are crossed. These thresholds are set to warn of potential problems before bit-error-rate alarm situations occur. This enables the operator to take preventative measures before a loss of light condition affects customer service. Electronically variable optical attenuators integrated into the metro DWDM transport platform are remotely controllable from the NMS, enabling quick reaction to both unexpected and planned changes in the network.
The same optical power-monitoring facilities enable the NMS to accurately pinpoint a fault in the optical network, isolating any problem right down to the port level. When a fault occurs in the network, such as a misconnected fiber during installation, the operator easily and quickly notices that the optical signature expected at one port has been detected on another port -- saving costly hours of diagnostic time. Should a fiber break occur, the NMS immediately notifies the operator of the location by quickly detecting a discontinuity of the optical path trace signature.
A complete range of symptoms -- misconnected fibers, loose fiber connections, fiber bend, component degradation, card failure, laser drift, dirty patch cords, and even engineer-induced effects such as fiber mishandling -- can all be detected quickly and accurately by wavelength detection technology. It is the most versatile optical DWDM network monitoring and management tool available today -- and it's also less costly.
A recent case study modeled operational expenditures associated with adding a new site to a metro DWDM ring for a major U.S. carrier. Wavelength-tracking technology was estimated to reduce network provisioning costs by up to 35%, service provisioning costs by up to 80% and troubleshooting costs by up to 68%. Additional savings that would be realized from eliminating expensive OSAs and personnel training were not included in the study.
Wavelength-tracking technology provides the ability to peer directly into individual optical channels for easy, cost-effective network management and control. It simplifies network build, operations, and maintenance, enabling field engineers to focus on fixing, rather than locating, network problems. In short, it enables network managers to clearly "see the light."