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Published on August 31, 2011

Live Fiber Monitoring in CWDM Networks—Part 1

This three-part article addresses the design and implementation of preventive monitoring on live, CWDM traffic-intensive, optical-fiber links.

To cope with the significant increase for storage and on-demand delivery of digital content, enterprises need to boost their optical-pipe capacity. Most turn to coarse wavelength-division multiplexing (CWDM) and dense wavelength-division multiplexing (DWDM) to scale up and avoid installing new fiber. Whether they are planning to lease this extra capacity or use it themselves, someone has to deal with maintenance. One may ask: “Can’t I just go on maintaining my new optical network as I did before?”

CWDM, by design, is a mid-range technology perceived as a cost-wise, first step toward the use of DWDM which, to some extent, can then be deployed over CWDM optical filters. CWDM also favors connectivity in a multiple-operator, multiple-network environment, where using only one fiber on a specific trunk (instead of two) can make the difference in establishing connectivity between two sites at an affordable cost—and not only in terms of fiber utilization cost, but also in terms of operation and maintenance. However, using fewer fibers, or increasing traffic on existing ones, also increases the importance of maintaining their availability at all times. “How can I simultaneously deploy traffic, while testing and monitoring the fibers I lease, if I decide to transmit CWDM signals?” is a question we’ll answer in this article.

OTDR for Network Support and Maintenance Activities

An optical time-domain-reflectometer (OTDR) is a well-known and efficient tool for monitoring and troubleshooting optical fibers in cabled optical networks. OTDR surpasses most other techniques as it enables:

  • Single-ended, one-operator measurement of the fiber and splice quality
  • Distributed testing along the cable sheath length—with proper configuration, a fiber fault can be located and positioned with respect to the OTDR launch position

With a fiber fault at a given location along the cable sheath length, one can further investigate the “what” when combined to a network inventory management system. If the cable span tested is documented and mapped using GIS technology, then it is the “where” which can be added to the fault analysis process. Although this process is described in simple terms, outside-plant documentation that is digitized with high resolution, up-to-date and detailed down to fiber level is essential to benefit fully from the raw information captured by an OTDR.

Fault position with respect to total cable

Figure 1. Fault position with respect to total cable length on the OTDR unit can be transposed to fault geographical location with respect to the piece of the network, cable or splice joint impacted; an in-service, fiber-monitoring solution enables this correlation to take place automatically in fibers carrying traffic.

Once a network is in operation, one can permanently place an OTDR in a rackmount unit  at a centralized location of the network, hub or central office (CO), which can be shared among the network support staff who can, at any time, remotely access the unit, select a specific fiber strand and test it. The OTDR instrument can also be programmed to routinely test some critical fibers, for early detection of degradation that may impair the signals transmitting into those same fibers. This is the main use of EXFO NQMSfiber fiber testing and monitoring solution.

Performing CWDM Live-Fiber Testing

After passive CWDM optical filters are installed, and circuits lighted-up with one or multiple CWDM wavelengths, the first servicing task one might need to perform is a simple OTDR test on one, or multiple, sections of a cable linking two facilities without interrupting traffic; this is also called in-service testing. To perform CWDM live-fiber testing, three methods are possible:

  • Out-of-band, using an additional WDM to MUX the U-band (1650) test wavelength to the line
  • In-band, using one of the CWDM channels as test wavelength
  • Test through tap or monitor port of CWDM optics—when available

Be sure to read Part 2 of this article as it takes an in-depth look at the methods used for performing CWDM live-fiber testing.