best Practices for troubleshooting WDm networks with an
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1. If the OSNR is lower than expected evaluate whether it is due to low channel power or high noise or both In the first case analyze the faulty channel with an OSA at different places in the light path to find the location of the excessive loss In the second case test each amplifier as they are the main contributors of noise Figure 3 Example of a signal with low OSNR likely to exhibit high BER 2011 EXFO Inc All rights reserved Application Note 257 CONCLUSION A troubleshooting operation is typically initiated due to an alarm received at the network operation center most often issued by an amplifier or a receiver The guidelines given above usually allow carriers to rapidly pinpoint the defective component which is commonly a splice a connector an amplifier or a transmitter Some of the best practices used by operators to troubleshoot a WDM network with an OSA include gt If there is a suspect for the defective component begin by testing the input and the output of this component gt If there is no suspect for the defective component acquire an OSA trace at the receiver end gt If measurements at the first location were not conclusive carry out an OSA test at other locations by moving toward the transmitter each time gt At each location test channel flatness channel power channel spacing and OSNR the exact parameters measured will depend on the likely cause of failure Applying these best practices a2. wavelength and power at the transmitter validating that they are within specified tolerances 2011 EXFO Inc All rights reserved Application Note 257 CASE 3 Automatic Power Reduction APR Alarm Coming from an Amplifier This type of alarm mostly occurs in long haul networks It indicates that the amplifier reduces its gain typically from about 20 dB down to 10 dB to protect people from exposure to hazardous laser power it is based on the principle that an abrupt decrease in optical energy most likely stems from a fiber break or disconnection This may result in potentially hazardous levels of laser radiation and so the laser rapidly reduces its optical power to a lower safe level Technically speaking APR will be caused by high optical return loss ORL High ORL means that light is reflected back into the amplifier most often from the output port thus disturbing the gain medium Procedure 1 Check recent network documentation to make sure that there was no change around the amplifier new splices causing ORL new connections etc If changes to the network might be the cause of the issue check for dirty connectors and bad splices using an out of band OTDR and a fiber inspection probe 2 If the issue was not identified in step 1 measure OSNR and amplifier gain flatness to see if the amplifier is the cause CASE 4 Reduced Power Ratio Alarm Coming from an Amplifier An amplifier triggers this type of alarm whenever its gain
3. Application Note 257 Best Practices for Troubleshooting WDM Networks with an Optical Spectrum Analyzer by Jean S bastien Tass Product Line Manager EXFO THE CHALLENGE Most telecom operators are facing a growing demand for bandwidth which is fueled by data hungry applications such as video conferencing IPTV video on demand etc while the average revenue per user Is reaching its plateau These trends lead to significant pressure to increase efficiency in field operations avoid repeat truck rolls and develop network test procedures to get It right the first time in order to reduce operating expenses Wavelength division multiplexed WDM networks in which several wavelengths propagate in a single fiber have been rolled out in the vast majority of countries These networks present a high level of complexity due to the large number of active and passive components i e transmitters multiplexers demultiplexers etc that must work properly to ensure top notch quality of service QoS 2 degree ROADM n degree ROADM Figure 1 Example of a WDM network with ROADMs While more basic problems call for a relatively straightforward solution i e no transmission between points A and B requires an OTDR to pinpoint the exact location of the failure more complex problems like a missing channel or high noise level require the use of more advanced troubleshooting tools like an optical spectrum analyzer OSA to rapidly f
4. chnical considerations like the network component that issued the alarm and the likely technical cause of the trouble as well as on operational considerations like the availability of field technicians and equipment In an ideal case the likely defective component is readily known from the alarm origin or from the system information previously mentioned and the first OSA tests will be done at the input and output of this component to confirm the fault If one does not know which component failed the first question to answer is whether the breakdown affects all wavelengths or just one or two If it affects them all then it is a broadband device that has issues an amplifier or the fiber itself If only one or two wavelengths have failed and the defective component is unknown then it is better to start troubleshooting with an OSA from the receiver end because a measurement at this location takes into account everything that happened during transmission It is then possible to work backwards to the transmitter end acquiring an OSA trace at each major component location Assessing Next Gen Networks Issue affectingone ___ Start troubleshooting __ Continue troubleshooting or two wavelenghts from receiver towards transmitter Defective component is unknown Hs Issue affecting Check fiber and all wavelenghts amplifiers Defective component _____ Test input and output is known of defective component Figure 2 Where to troubleshoot In
5. exhibit wrong gain value due to aging or if the input power is out of range and they can also show gain tilt gain not uniform in a given spectral range Transmitters might not work according to specifications due to wavelength drift wrong output power level etc Bulk heads and splices another common source of failure might show losses above the specified tolerance which is why an inspection probe is a valuable instrument to include in every field technician s arsenal OADMs and ROADMs reconfigurable OADMs usually exhibit good stability though they can at times cause issues Multiplexers and demultiplexers are also stable because they are passive components If they work properly at turn up they will not deteriorate over time especially since they are generally used in well controlled environments like central offices Passive components if installed properly seldom cause issues while active ones can degrade over time Defective Component Type of Failure Amplifier Wrong gain due to aging Average Amplifier Wrong gain due to input Often power out of range Amplifier Gain tilt Average Bulk heads and splices Excessive loss Very often Transmitter Wavelength drift Sometimes Transmitter Power drift Sometimes ROADM Wrong wavelength assignment Sometimes configuration issue MUX and DEMUX Wrong MUX DEMUxX installed Very rare Table 3 Common causes of failures PRACTICAL CONSIDERATION MONITORING PORTS One of the challenges
6. ind and fix an issue However most telecom operators do not have troubleshooting procedures so field technicians are left wondering what and where to test and what to do with the results As a consequence troubleshooting is often done by a senior field technician with the help of one of the few field team managers that has an in depth understanding of WDM networks and OSAs In this paper we will discuss some of the best practices used by operators to troubleshoot a WDM network with an OSA in order to resolve any breakdown as quickly and efficiently as possible TYPICAL SIGN CALLING FOR WOM NETWORK TROUBLESHOOTING A typical troubleshooting intervention with an OSA stems from an alarm received at the network operation center NOC The table below highlights the typical alarm and the component issuing them Amplifier Automatic power reduction Automatic power shutdown Receiver High bit error rate Failure of one or several wavelengths Transponder Loss of signal Table 1 Typical alarms On top of alarms some systems provide extra information like the received power at each channel the bit error rate BER per channel the input and output power at each amplifier etc Although not specifically part of the alarm this kind of information can be extremely valuable to determine what to test and where WHERE TO TROUBLESHOOT Let s assume that the issue has occured for the first time The location of the first OSA test depends on te
7. is lower than expected most often when it is aging or degrading Procedure 1 Go to the amplifier location and measure the gain it should be taken into account here that the amplifier gain depends on the input power 2 Compare the gain with the expected specification to verify that the amplifier still performs adequately CASE 5 Automatic Power Shutdown Alarm Coming from an Amplifier This alarm is triggered by an amplifier when the input power falls below the threshold Procedure 1 Measure the channel power at the amplifier location with an OSA 2 If the measured power is above the threshold the amplifier settings may need adjustment or the amplifier might be defective If the measured power is below the threshold go to step 3 3 From the amplifier input use an OTDR to locate any splices or connections causing excessive loss 4 If the problem is not identified in step 3 work your way backwards toward the transmitter by measuring channel power at each component location to find the component causing excessive loss and insufficient gain CASE 6 High Bit Error Rate Alarm Coming from the Receiver o C_003 C_004 An alarm issued by the receiver indicating high BER can 4 TAR result from almost any component between the transmitter 3 TTS Fi T dBm and the receiver high BER is often related to low OSNR Procedure 1 Measure OSNR at the receiver with an OSA 1529 9 1530 1530 1 1530 2 1530 3 1530 4 1530 5 2
8. llows carriers to resolve most network issues in a matter of minutes or hours thus increasing their operational efficiency decreasing their number of truck rolls and delivering best in class service to their customers EXFO s FTB 5240S BP family of optical spectrum analyzers is therefore a key ingredient for operators to achieve effective troubleshooting EXFO Corporate Headquarters gt 400 Godin Avenue Quebec City Quebec G1M 2K2 CANADA Tel 1 418 683 0211 Fax 1 418 683 2170 info EXFO com Tass Toll free 800 663 3936 USA and Canada www EXFO com EXFO America 3400 Waterview Parkway Suite 100 Richardson TX 75080 USA Tel 1 972 761 9271 Fax 1 972 761 9067 EXFO Asia 100 Beach Road 22 01 03 Shaw Tower SINGAPORE 189702 Tel 65 6333 8241 Fax 65 6333 8242 EXFO China 36 North 3 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 23 8024 6810 Fax 44 23 8024 6801 EXFO Finland 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 CE Ca gt Assessing APNOTE257 1AN 2011 EXFO Inc All rights reserved WwW Eo E z Printed in Canada 11 11 A E t
9. of troubleshooting WDM networks is that in most cases there is live traffic on the fibers that cannot easily be turned off or re routed Monitoring ports that enable a field technician to make an OSA measurement without stopping the traffic are critical In general amplifiers have monitoring ports but multiplexers and demultiplexers do not especially the older ones One can circumvent the lack of a monitoring port by using a previous port in particular if there are only passive elements between the two monitoring ports traffic can also be re routed though this is not ideal Fortunately the trend is now to include monitoring ports in most components Also the ratio between the power at the monitoring port and within the fiber varies from one port to the other and it is not always known sometimes the tap loss is written on the device This variability becomes important to consider if a field technician is interested in absolute measurements but if he wants to make relative measurements e g measure channel flatness which is the most common case the variability becomes unimportant since the relative signal powers are independent of the monitoring port properties 2011 EXFO Inc All rights reserved Application Note 257 DETAILED TROUBLESHOOTING PROCEDURES FOR DIFFERENT TYPES OF ALARMS AND FAILURES Now that we have discussed the general principles let us take a closer look at specific alarms and failures and propose a trouble
10. operators also measure OSNR as a key parameter to assess the quality of the transmission there is a direct relationship between OSNR and BER in systems without forward error correction FEC as long as other impairments are not present dispersion non linear effects etc A minimum OSNR of 15 dB is often required at the receiver but a value of at least 20 dB is often preferred The IEC standard 61280 2 9 recommends that to properly measure OSNR the OSA needs to be able to measure 10 dB more than what is required therefore an optical rejection ratio of 30 dB is required for 20 dB OSNR testing Parameter to Test Acceptable Value Channel central wavelength Within the system s acceptable range for channel central wavelength Channel power Within the system s acceptable range for channel power Channel or power flatness Depends on the manufacturer Typically 0 5 dB to 2 dB between weakest and most powerful channel Channel spacing According to ITU grid of system Typically 25 50 or 100 GHz Optical signal to noise ratio gt 15 to 20 dB at the receiver According to system specifications for other locations Table 2 Parameters to test with an OSA and acceptable values Application Note 257 WHAT ARE THE COMMON CAUSES OF FAILURE There are a number of components that can fail such as amplifiers transmitters bulk heads and splices and optical add drop multiplexers OADM Amplifiers will often be in cause because they can
11. shooting procedure for each of them Please note that functionalities like gain tilt control in amplifiers or equalization in ROADMs can affect the signal and therefore have an impact on the procedures described below CASE 1 CASE 2 Power Related Problem Affecting All Wavelengths Excluding High BER Such an issue can arise from a fiber cut excessive loss e g a dirty connector somewhere in the light path or a bad amplifier defective or poorly configured Procedure 1 Carefully analyze the alarm from the equipment management system EMS as they transmit from one amplifier to the next an amplifier signal that is out of band If the EMS signal gets through then the likely cause is a defective or poorly configured amplifier Test the amplifier gain If the EMS signal does not get through then the probable cause is a fiber break or excessive loss 2 Go to a place downstream from the location of the suspected failure If the fault location cannot be guessed go to a place where an OTDR can be used 3 Use an out of band OTDR to locate the position of the excessive loss or the fiber break by testing in both directions with particular attention to the upstream direction 4 If the OTDR does not reveal any break or excessive loss the issue might be an excessive loss in another segment of this fiber a segment is defined as the fiber between two amplifiers Move to another segment and repeat the procedure from step 3 Issue Affec
12. the case that the issue Is recurring and cannot easily be fixed then a monitoring solution should be considered WHAT PARAMETERS SHOULD BE TESTED AT EACH LOCATION The list of parameters tested depends on the component that is being tested and the likely cause For instance if the breakdown occurred in a CWDM coarse WDM cellular backhaul without any amplifiers then a measurement of power and wavelength will be enough In most other cases many operators tell us that they usually measure channel central wavelength power flatness optical signal to noise ratio OSNR and sometimes channel spacing Channel central wavelength is defined as the wavelength center of mass i e the average wavelength of the signal power Channel flatness the difference between the weakest and the most powerful channels is an important value to test because non uniform channel power will undergo different gains in amplifiers which will further increase the power difference between channels Some operators rely on the tolerance specified by the component manufacturer for channel flatness while others set a specific requirement internally Acceptable ranges used in the industry for channel flatness vary from 0 5 dB to 2 0 dB Channel power relates to the signal power of each wavelength while channel spacing measures the wavelength separation between two channels Depending on the DWDM dense WDM configuration this value can be 25 33 50 or 100 GHz Many
13. ting One or Two Wavelengths and the Failure Location is Unknown This type of troubleshooting requires the use of a wavelength sensitive measurement device like an optical spectrum analyzer as not all wavelengths exhibit a issue Procedure 1 Go to the location of the suspected failure it might be known from the system alarm or the system information If the fault location is not known go at the receiver side 2 With an OSA measure the power OSNR and central wavelength of the faulty channel Ideally test at the DEMUX if a monitoring port is available rarely the case or at the amplifier at this location 3 a If the channel central wavelength is not as expected then this is a transmitter drift problem Go to the transmitter for further troubleshooting b If the power is lower than expected there could be excessive component loss or insufficient amplifier gain in the light path Analyze the faulty channel with an OSA at different places in the light path to identify the location of the issue c If the OSNR is lower than expected evaluate whether it is due to low channel power or high noise In the first case see step 3b In the second case test the gain and noise figure of each amplifier as they are the main contributors of noise 4 If the problem is identified in step 3 check all the locations between MUX and DEMUX in no particular order specifically focusing on the amplifiers 5 If the cause is not yet determined measure
14. u Next Gen Networks
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