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Publié le 31 août 2011

Live Fiber Monitoring in CWDM Networks—Part 3

Part 2 of this article examined out-of-band 1650 nm OTDR testing as well as in-band use of a CWDM channel for OTDR testing, in addition to taking an in-depth look at testing through a broadband tap coupler of CWDM optics.

Practical Examples of In-Band Remote Testing and Proactive Monitoring

While out-of-band testing, which uses 1650 nm, is used more and more in coarse wavelength-division multiplexing (CWDM) networks, the in-band approach, which uses one dedicated CWDM channel, is not used very much, although it provides network maintenance teams with additional benefits such as:

  • Test path(s) can be established directly from the CWDM passive optics installed for transmission, if at least one channel is reserved for testing purposes;
  • The loss budget remains the same with or without testing capabilities added;
  • The attenuation measured at the test channel is the same or very close to the other channels, which reduces the difficulty to analyze if a degradation actually affects transmission;
  • Selecting lower wavelengths (e.g., 1470 nm) can reduce the effect of Raman noise compared to using 1625 or 1650 nm, although larger effective mode area fibers available today tend to reduce these non-linearities.

A network test setup is presented below (on a reduced scale), and includes a test unit located at a ring major hub. This can be extended to a larger network by multiplying this schema. Each fiber pair is tested from the central location using a 1470 nm channel. Provision for no point of failure can be added at the opposite side of the link, enabling bidirectional testing. Test units are connected over TCP/IP in a ring topology, this creates a no-point-of-failure testing topology. When using a different channel, such as 1490 nm, each fiber can be tested from both sides at the same time. It can also help diagnose and locate specific fiber degradation that could otherwise be missed or reported with lower precision, if tested from one side only.
CWDM simple network with OTDR monitoring on each live fiber: unidirectional or optional bidirectional on each strand.

Figure 1. CWDM simple network with OTDR monitoring on each live fiber: unidirectional or optional bidirectional on each strand.

In the above topology, each link extends up to 20 dB fiber attenuation, which is typically an attenuation that EXFO’s CWDM optical time-domain reflectometer (OTDR) can measure in total from one end, while detecting and locating small degradations. A single fiber carrying CWDM in both directions closes the two long east/west paths—this fiber can be tested with one additional unit since the hub test unit cannot measure well enough beyond the attenuation created by the fibers connected to it. Again, even for this fiber, surveillance and remote testing can be performed at 1470 nm or bidirectionally with 1470/1490 nm channels.

Assuming that each CWDM filter adds a 2.5 dB loss, this requires that an OTDR be able to measure up to at least 25 dB optical attenuation, in which we include the optical switch unit as well as the jumper(s) insertion loss. This can be achieved by using a 10 μs pulse on the OTDR and requires no more than an acquisition time of 15 seconds. The dynamic range, as per standard OTDR definition, is 40 dB for this module.

10 μs pulse width; 15-second OTDR acquisition on a live CWDM fiber with 20 dB attenuation

Figure 2. 10 μs pulse width; 15-second OTDR acquisition on a live CWDM fiber with 20 dB attenuation.

The above trace was taken with 10 dBm CWDM TX injection power in live-fiber conditions. The OTDR detects back-scattered signals but also detects the diffused or scattered power from the other signals, which cannot be totally eliminated using extra band-pass filters. Below, we compare the same link tested dark and live, as per above conditions (in red is the dark fiber):

Comparison between a dark (red) and live (black) fiber

Figure 3. Comparison between a dark (red) and live (black) fiber; live is 10 dBm injected power into the fiber being tested. Extra isolation was added around the OTDR channel for cases where transmitters are at the opposite side of the link or when an OTDR at opposite side is testing at same time

The figure below illustrates an implementation example of cable and fiber monitoring from central location to two remote premises connected optically from end to end. Here, we assume that the distances and attenuation are consistent with CWDM standards from the central to a remote location. The CWDM OTDR is used at 1470 nm; it monitors the two live fibers in the main loop in co- and counter-propagation and at the passive nodes, the OTDR signal from one of the fibers is re-directed to a maintenance dark fiber of the access line.

 Implementation example in a CWDM all-optical network, connecting remote sites to central location with a redundant path

Figure 4. Implementation example in a CWDM all-optical network, connecting remote sites such as base station or SAN/data-center to central location with a redundant path. OTDR at 1470 nm is used on the fiber pair to test the metro-loop live fibers. One of the two metro fibers would continue and test the access cable integrity using a dark fiber.


When Is Bidirectional Monitoring on the Same Fiber Required?

In the context of establishing fiber surveillance on live CWDM links, one may ask: Should I monitor those links both directions? With the CWDM optical link budget in the 28 to 30 dB range, minus the CWDM filter insertion loss (estimated 5 dB total) and a 3 dB margin, CWDM communication can be established on fibers with up to 20 to 22 dB fiber attenuation at the most. For these levels of fiber attenuation, the ability to detect and locate a large and/or reflective degradation can be precise and easily done with a single-ended, unidirectional OTDR test. The following are cases where it is suggested to establish bidirectional testing:

  • Monitor with higher resolution—using a 10 μs pulse width, as per the above results, leads to inherent difficulty in locating a degradation with high accuracy. Testing with 1 or 2.5 μs pulse width increases this accuracy but reduces the measurement range that can be achieved;
  • If the link is not spliced and reflections exist, the reflecting events measured on the OTDR may hide events that are further down the fiber in the event of attenuation dead-zone. Testing from the opposite side can provide more details about the actual nature and more accurate position of the degradation in this case;
  • The optical network is designed to scale-up using DWDM channels on the 1530 and 1550 nm CWDM channels; this could hike-up the TX power in the fiber due to Raman spontaneous emission scattering (if the measurements get impaired by this undesirable noise, then testing from both sides may become a necessity for adequate coverage);
  • No point of failure is required for monitoring—if the fiber has to be monitored on 100% availability, then testing from both sides becomes a must.

An OTDR scan with 2.5 μs pulse width (in black) and 10 μs pulse width (in red) both with 15 second acquisition time

Figure 5. An OTDR scan with 2.5 μs pulse width (in black) and 10 μs pulse width (in red) both with 15 second acquisition time. With 2.5 μs pulse width, 1 to 2 dB degradation in the last section of the fiber would barely be detected, especially if it is non-reflective (i.e., splice degradation or bend). It may then be required to test from both sides of the fiber, as proposed at two different CWDM wavelengths.


Monitoring with High- or Low-Resolution OTDR Measurement

As indicated above, the ability to precisely locate a fiber fault depends on the test parameters used, mainly OTDR pulse width and averaging time, but also where the event or the attenuation level from the injection point, as well as the nature of the degradation, are located. In some very long links, if the degradation occurs at the far end of the link, it becomes useless to try to test with a shorter pulse width, as averaging up to three minutes time will typically increase measurement range by for 1 to 2 dB only. Then one has no other choice than to test with a longer pulse width. In this case, it is important to remember that the event positioning accuracy is degraded compared to smaller pulse width (e.g., 1 μs or 2.5 μs). In the case of a reflective event, as illustrated below, testing with multiple pulse widths, leads to a noticeable difference in the positioning accuracy:

Testing with multiple pulse widths leads to a noticeable difference in the positioning accuracy Measured drift of a 2 dB connector reflective event position according to pulse width used to characterize it



Figure 6. A connector with 2 dB degradation (typically should be 0.2 to 0.5 dB) tested with various OTDR pulse widths. Inherently, the analysis for positioning the event cannot be made with the same accuracy.


EXFO at the Forefront of CWDM Testing

NQMSfiber is a centralized test and monitoring system solution that provides your maintenance organization with a tool to better manage your precious resources, get early warnings of fiber issues and speed-up your outside-plant restoration process. Once again, EXFO demonstrates its innovation in applying its CWDM OTDR technology to the fiber-monitoring application. This application opens the door to in-band testing without any traffic impairment nor additional budget expenses to implement an out-if-band test route, while also enabling testing at all CWDM channels from one central site, for testing of circuits before service activation.