Part 1 of this article addressed the design and implementation of preventive monitoring on line coarse wavelength-division multiplexing (CWDM) traffic-intensive optical fiber links.
Out-of-band testing for coarse wavelength-division multiplexing (CWDM) requires the use of a 1650 nm optical time-domain reflectometer (OTDR) instead of 1625 nm because the last CWDM channel at 1611 nm is too close to 1625 nm, which is traditionally used for live fiber testing of 1310/1550 nm circuits. For this, a three-port wavelength-division multiplexer (WDM) needs to be added on the line to inject the OTDR signal to the fiber carrying CWDM channels. If it is not inserted prior to CWDM commissioning, it will not be possible to test live without interrupting traffic – interruption to last long enough for device connection and test. Out-of-band testing adds a minimum of 1 dB penalty to the link budget, which needs to be taken into account at the circuit design stage. In cases where multiple sections need to be tested from a central site in one acquisition, then by-pass site(s) using DEMUX/MUX functions have to be planned, which increases link attenuation to another 1 dB.
At the end of the line, the 1650 nm test wavelength is usually rejected by the CWDM filter. Care must be taken to ensure that the CWDM filter at the end of the path is at least 30 to 35 dB isolation at 1650 nm. If not, the same WDM filter used for injection and by-pass can be installed at the end of line and act as a rejection filter for 1650 nm.
Testing out-of-band at 1650 nm has the advantage of quickly revealing unwanted bending of the cable or the individual fiber strand, as this wavelength propagates in the outer diameter of the fiber core. The difference in fiber attenuation between 1650 nm from 1550 nm is not larger than the difference between 1310 nm and 1550 nm. In most fibers seen and available today, the fiber attenuation at 1650 nm is about 0.25 dB/km (typically) compared to 0.2 dB for 1550 nm. For a 60 km link tested at 1650 nm, this is within 3 dB from what would be measured at 1550 nm on a fiber that has no excessive bending. It is therefore recommended to have a construction trace at 1625 nm, if possible at 1650 nm, in order to record the difference of the attenuation measured at both wavelengths for consistency in the measurement taken before and after the fiber goes live. Obviously, the implementation of a 1650 nm testing path throughout the network can, in some cases, be troublesome and adds costs and complexity, which is not always welcome.
The in-band method uses one or multiple CWDM channels for OTDR testing. This is a straight connection of a CWDM-enabled OTDR into the MUX/DEMUX CWDM filter used for transmission.
Despite the fact that one channel becomes ‘dedicated’ to testing, in-band is a test method that can be implemented after the network has been commissioned, which is a great advantage compared to the out-of-band approach. It is also simple to establish a test route. When passive add/drops are used over a ring, the selected test channel will continue and follow the other express channels. When all channels terminate in a site and are regenerated for another section, it simply requires patching between the same channel ports on both sides of the route. Either the transmit or the receive port can be used.
It is interesting to note that service channel (SC), or expansion channel (EC) at 1310 nm or any other wavelength, if available and not in use, can also be selected for in-band testing, similarly to above with less stringent requirements in terms of channel bandwidth for the tolerance on central OTDR wavelength.
Some CWDM MUX/DEMUX/ADM devices are equipped with taps (1 to 10%) in order to monitor traffic. These monitor ports are broadband couplers, and extract a portion of traffic usually for testing with protocol analyzers. When testing in opposite direction than traffic, the CWDM signal power, even though attenuated by the tap, will be detrimental to perform a proper OTDR measurement—unless one selects a filter to isolate the OTDR wavelength band from active CWDM channels.
The isolation of the filter to place between the tap/monitor port and OTDR must be at least 50 to 60 dB when testing against traffic. It must be a band-pass type if the test wavelength is in-band. If the OTDR is out-of-band, a high-pass filter for 1650 nm can be used. For the case of in-band, one or two cascaded optical add/drop multiplexers (OADMs) can be used as band-pass filters. If the fiber transmits CWDM traffic at the same wavelength selected for testing, OTDR will fail measuring.
Limitations with this approach are:
Be sure to read Part 3 of this article as it examines practical examples of in-band remote testing and proactive monitoring.