Live Fiber Monitoring in CWDM Networks Olivier Plomteux, Senior Product Line Manager, Optical Business Unit To cope with the signifi cant increase for storage and on-demand delivery of digital content, enterprises need to boost their opticalpipe 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 fi lters. CWDM also favors connectivity in a multiple-operator, multiple-network environment, where using only one fi ber on a specifi c trunk (instead of two) can make the difference in establishing connectivity between two sites at an affordable cost and not only in terms fi ber utilization cost, but also in terms of operation and maintenance. However, using fewer fi bers, or increasing traffi c on existing ones, also increases the importance of maintaining their availability at all times. How can I simultaneously deploy traffi c, while testing and monitoring the fi bers 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-refl ectometer (OTDR) is a well-known and effi cient tool for monitoring and troubleshooting optical fi bers in cabled optical networks. OTDR surpasses most other techniques as it enables: Single-ended, one-operator measurement of the fi ber and splices quality Distributed testing along the cable sheath length with proper confi guration, a fi ber fault can be identifi ed and positioned with respect to the OTDR launch position With a fiber fault at a given position 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 fi ber level is essential to benefi t fully from the raw information captured by an OTDR. 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 specifi c fi ber 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 fi bers. This is the main use of EXFO s NQMSfi ber fi ber-testing and monitoring solution. PERFORMING CWDM LIVE-FIBER TESTING After passive CWDM optical fi lters are installed, and circuits have been lighted-up with one or multiple CWDM wavelengths, the fi rst 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 traffi c; this is also called in-service testing. To perform CWDM live-fi ber testing, three methods are possible: Out-of-band, using an additional WDM to MUX the U-band (1650 nm) test wavelength to the line In-band, using one of the CWDM channels as test the wavelength Test through tap or monitor port of CWDM optics when available OUT-OF-BAND 1650 NM OTDR TESTING Out-of-band testing for CWDM requires the use of a 1650 nm 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 fi ber 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 traffi c interruption 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. 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.
Figure 2. Out-of-band OTDR testing at 1650 nm of CWDM links. Figure 3. By-passing a node for testing end-to-end. At the end of the line, the 1650 nm test wavelength is usually rejected by the CWDM fi lter. Care must be taken to ensure that the CWDM fi lter at the end of the path is at least 30 to 35 db isolation at 1650 nm. If not, the same WDM fi lter used for injection and by-pass can be installed at the end of the line and act as a rejection fi lter 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 fi ber core. The difference in fi ber attenuation between 1650 nm from 1550 nm is not larger than the difference between 1310 nm and 1550 nm. In most fi bers seen and available today, the fi ber 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 fi ber 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 fi ber 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.
IN-BAND USE OF A CWDM CHANNEL FOR OTDR TESTING 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 fi lter used for transmission. Figure 4. In-band OTDR testing using one unused CWDM channel. 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. Ports used can be either the transmit or the receive port. Figure 5. In-band OTDR testing using 1470 nm. For establishing end-to-end test, route patching is done at an intermediate site between 1470 nm ports of the CWDM MUX/DEMUX. Only one fiber, either transmit or receive is presented here. For fiber-pair communication, two test ports of the test equipment have to 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.
TESTING THROUGH BROADBAND TAP COUPLER (MONITOR PORT) OF CWDM OPTICS Some CWDM MUX/DEMUX/ADM devices are equipped with taps (1 to 10%) in order to monitor traffi c. These monitor ports are broadband couplers and extract a portion of traffi c usually for testing with protocol analyzers. When testing in the opposite direction than traffi c, the CWDM signal power, even though attenuated by the tap, will be detrimental to perform a proper OTDR measurement unless one selects a fi lter to isolate the OTDR wavelength band from active CWDM channels. The isolation of the fi lter to place between the tap/monitor port and OTDR must be at least 50 to 60 db when testing against traffi c. It must be a band-pass type if the test wavelength is in-band. If the OTDR is out-of-band, a high-pass fi lter for 1650 nm can be used. Figure 6. Broadband tap coupler as a monitor port can be used for OTDR testing but care must be taken to isolate the OTDR test wavelength from active channels. For the case of in-band, one or two cascaded optical add/drop multiplexers (OADMs) can be used as band-pass filters. If the fi ber transmits CWDM traffi c at the same wavelength selected for testing, OTDR will fail measuring. Limitations with this approach are: Major drop (seen in literature up to 23 db or less than 1%) in optical attenuation, cutting large amounts of the available OTDR measurement range; if the monitor port ratio was 10% (10 db), it could be used The test direction is likely always against traffi c at least in the case of fiber pair transmission, which doubles the number of test units to install, one at each end. For single fi ber bidirectional transmission, this limitation does not apply Additional care must be taken to ensure proper isolation of the OTDR from attenuated but unfi ltered CWDM channels 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 course 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 benefi ts 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 diffi culty 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 fi bers available today tend to reduce these non-linearities 1. 1 Raman spontaneous emission is a non-linear effect present in all singlemode optical fibers, which generates broad optical noise in spectral bands relatively far away from the pump signal; in this case, the pump is the carrier channels. The effect typically starts to be non-negligible when the carriers composite level injected in the fiber is above 10 dbm. When DWDM channels are added to CWDM, some Raman noise will be generated with a peak emission +100 nm above the pump. This noise then travels in both directions and toward the OTDR input port through the channel opened for the measurement. This noise at some point can start to impair the ability of the OTDR to perform, such as in dark fiber test cases.
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 fi ber 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 fi ber can be tested from both sides at the same time. It can also help diagnose and locate specifi c fi ber degradation that could be missed or reported with lower precision, if tested from one side only. Figure 7.CWDM simple network with OTDR monitoring on each live fiber unidirectionally or optionally 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 refl ectometer (OTDR) can measure in total from one end, while detecting and locating small degradations. A single fi ber carrying CWDM in both directions closes the two long east/west paths this fi ber can be tested with one additional unit since the hub test unit cannot measure well enough beyond the attenuation created by the fi bers connected to it. Again, even for this fi ber, surveillance and remote testing can be performed at 1470 nm or bidirectionally with 1470/1490 nm channels. Assuming that each CWDM fi lter 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 and 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 defi nition, is 40 db for this module. Figure 8. 10 µs pulse width;15 seconds OTDR acquisition on a 20 db attenuation live CWDM fiber. The above trace was taken with 10 dbm CWDM TX injection power in live-fi ber 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 bandpass filters. Below, we compare the same link tested dark and live, as per above conditions (in red the dark fi ber case). Figure 9 illustrates an implementation example of cable and fi ber monitoring from a central location to two remote premises connected optically from end to end.
Figure 9. Comparison between a dark (red) and live (dark) 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 OTDR at opposite side is shooting at same time. 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 fi bers in the main loop in co- and counter-propagation and at the passive nodes, the OTDR signal from one of the fi bers is re-directed to a maintenance dark fi ber of the access line. Figure 10. Implementation example in CWDM all-optical network, connecting remote sites such as base station or SAN/datacenter to central location with 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 ways? 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 fi bers with up to 20 to 22 db fi ber attenuation at the most. For these levels of fi ber attenuation, ability to detect and locate a large and/ or refl ective degradation can be precise and easily done with a single-end, 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 diffi culty to locate 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 refl ections exist, the refl ecting events measured on the OTDR may hide events that are further down the fi ber 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 fi ber has to be monitored on 100% availability, then testing from both sides becomes a must. 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 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 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 case of a refl ective event, such as illustrated below, when tested with multiple pulse widths, it leads to a noticeable difference in positioning accuracy: Figure 12. 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 NQMSfi ber is a centralized test and monitoring system solution to provide 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 fi ber-monitoring application. This application opens the door to in-band testing without any traffi c impairment nor additional loss budget needed 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. Figure 11. 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. EXFO Corporate Headquarters > 400 Godin Avenue, Quebec City (Quebec) G1M 2K2 CANADA Tel.: +1 418 683-0211 Fax: +1 418 683-2170 info@exfo.com Toll-free: +1 800 663-3936 (USA and Canada) www.exfo.com EXFO America 3701 Plano Parkway, Suite 160 Plano, TX 75075 USA Tel.: +1 800 663-3936 Fax: +1 972 836-0164 EXFO Asia 151 Chin Swee Road, #03-29 Manhattan House SINGAPORE 169876 Tel.: +65 6333 8241 Fax: +65 6333 8242 EXFO China 36 North, 3 rd Ring Road East, Dongcheng District Beijing 100013 P. R. CHINA Tel.: + 86 10 5825 7755 Fax: +86 10 5825 7722 Room 1207, Tower C, Global Trade Center EXFO Europe Omega Enterprise Park, Electron Way Chandlers Ford, Hampshire S053 4SE ENGLAND Tel.: +44 2380 246810 Fax: +44 2380 246801 EXFO NetHawk Elektroniikkatie 2 FI-90590 Oulu, FINLAND Tel.: +358 (0)403 010 300 Fax: +358 (0)8 564 5203 EXFO Service Assurance 270 Billerica Road Chelmsford, MA 01824 USA Tel.: +1 978 367-5600 Fax: +1 978 367-5700 APNOTE236.1AN 2008 Printed in Canada 10/10