OptiX BWS 1600G Troubleshooting                                                                                           ...
OptiX BWS 1600GFigures                                                                                                    ...
OptiX BWS 1600GTroubleshooting                                                                                            ...
OptiX BWS 1600GTroubleshooting                                                                               6 Handling Bi...
OptiX BWS 1600G6 Handling Bit Error                                                                              Troublesh...
OptiX BWS 1600GTroubleshooting                                                                                6 Handling B...
OptiX BWS 1600G6 Handling Bit Error                                                                             Troublesho...
OptiX BWS 1600GTroubleshooting                                                                            6 Handling Bit E...
OptiX BWS 1600G6 Handling Bit Error                                                                                       ...
OptiX BWS 1600GTroubleshooting                                                                                      6 Hand...
OptiX BWS 1600G6 Handling Bit Error                                                                             Troublesho...
OptiX BWS 1600GTroubleshooting                                                                              6 Handling Bit...
OptiX BWS 1600G6 Handling Bit Error                                                                               Troubles...
OptiX BWS 1600GTroubleshooting                                                                              6 Handling Bit...
OptiX BWS 1600G6 Handling Bit Error                                                                               Troubles...
OptiX BWS 1600GTroubleshooting                                                                                            ...
OptiX BWS 1600G6 Handling Bit Error                                                                               Troubles...
OptiX BWS 1600GTroubleshooting                                                                               6 Handling Bi...
OptiX BWS 1600G6 Handling Bit Error                                                                                  Troub...
OptiX BWS 1600GTroubleshooting                                                                                 6 Handling ...
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Handling BER in BWS 1600G

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Handling BER in BWS 1600G

  1. 1. OptiX BWS 1600G Troubleshooting Contents Contents6 Handling Bit Error .....................................................................................................................6-1 6.1 Background Knowledge................................................................................................................................6-2 6.1.1 Relationship Between Optical Power and Bit Error.............................................................................6-2 6.1.2 Dispersion ............................................................................................................................................6-3 6.1.3 Non-Linearity of Optical Fiber ............................................................................................................6-3 6.1.4 FEC Technology ..................................................................................................................................6-3 6.2 Causes ...........................................................................................................................................................6-4 6.3 Methods and Procedures of Fault Locating...................................................................................................6-5 6.3.1 Methods ...............................................................................................................................................6-5 6.3.2 Procedures............................................................................................................................................6-5 6.3.3 Techniques ...........................................................................................................................................6-7 6.4 Classified Fault Handling..............................................................................................................................6-8 6.4.1 Handling of External Faults .................................................................................................................6-8 6.4.2 Handling of Equipment Faults ...........................................................................................................6-10 6.5 Cases of Common Fault Handling .............................................................................................................. 6-11 6.5.1 Bit Errors Caused by Incorrect Fiber Connections ............................................................................ 6-11 6.5.2 Bit Error Caused by Non-linearity of Optical Fiber...........................................................................6-13 6.5.3 Bit Errors Caused by a Faulty OTU...................................................................................................6-15 6.5.4 Bit Error Caused by Unmatched Dispersion Compensation ..............................................................6-16 Issue 01 (2007-01-20) Huawei Technologies Proprietary i
  2. 2. OptiX BWS 1600GFigures Troubleshooting Figures Figure 6-1 Handling flow of bit errors ...............................................................................................................6-6 Figure 6-2 Monitoring B1 byte...........................................................................................................................6-7 Figure 6-3 Networking configuration diagram................................................................................................. 6-11 Figure 6-4 Networking configuration diagram.................................................................................................6-13 Figure 6-5 Networking configuration diagram.................................................................................................6-15ii Huawei Technologies Proprietary Issue 01 (2007-01-20)
  3. 3. OptiX BWS 1600GTroubleshooting Tables Tables Table 6-1 OSNR requirements for various rates.................................................................................................6-2 Table 6-2 Common causes of bit errors ..............................................................................................................6-4Issue 01 (2007-01-20) Huawei Technologies Proprietary iii
  4. 4. OptiX BWS 1600GTroubleshooting 6 Handling Bit Error 6 Handling Bit ErrorAbout This Chapter The following table lists the contents of this chapter. Title Description 6.1 Background Knowledge Describes background knowledge used in handling bit errors. 6.2 Causes Describes causes of bit errors. 6.3 Methods and Procedures of Describes methods and procedures of locating bit error Fault Locating faults. 6.4 Classified Fault Handling Describes methods of handling different bit error faults. 6.5 Cases of Common Fault Describes cases of handling common bit error faults. Handling Bit errors refer to the errors occurring in the code element during the transmission. Bit errors are usually represented by bit. In the SDH frame structure, bytes for monitoring bit errors include B1, B2, M1, B3, G1 and V5. But in DWDM equipment, the OTU only provides non-intrusive monitoring on B1 and B2. This chapter describes the methods of handling bit errors generated in the OptiX BWS 1600G equipment.Issue 01 (2007-01-20) Huawei Technologies Proprietary 6-1
  5. 5. OptiX BWS 1600G6 Handling Bit Error Troubleshooting6.1 Background Knowledge In the DWDM system, many factors can result in bit errors, including optical power abnormity, insufficient dispersion tolerance, too low optical signal to noise ratio (OSNR), non-linearity of optical fiber and performance deterioration of optical components, and so on.6.1.1 Relationship Between Optical Power and Bit Error There are two circumstances under which bit errors are generated due to abnormality of the optical power: The input optical power is below the receiver sensitivity As a result of high attenuation of optical power, the input optical power of the OTU at the receiving end is below the laser’s sensitivity, and thus bit errors occur to the OTU. Two types of laser detector are adopted for OTU at the receiving end: PIN pipe and APD pipe. For the 2.5 Gbit/s system, the receiver sensitivity of a PIN pipe is –18 dBm, or even below –21 dBm if tested in laboratory. If an APD pipe is adopted, the receiver sensitivity is –28 dBm, or below –31 dBm when tested in laboratory. In actual application, since optical fiber is usually very long, if taking the optical path penalty of the system into consideration, a redundancy of 2 dBm should be reserved for minimum receiver sensitivity. By far, the 10 Gbit/s system only adopts PIN pipe with receiver sensitivity being –17 dBm, but when the optical power reaches –14 dBm, an alarm for too low input optical power may be generated. The OSNR at the receiving end decreases The decrease of optical power affects the OSNR at the receiving end. If OSNR redundancy is not large enough, the decrease of optical power may directly lead to the deterioration of OSNR, and thus generating bit error in OTU at the receiving end. OSNR requirements of OTU boards of various rates are listed in Table 6-1. Table 6-1 OSNR requirements for various rates OTU rate OSNR requirements 2.5 Gbit/s (without FEC) ≥20 dB 2.5 Gbit/s (with FEC) ≥16 dB 10 Gbit/s (with FEC) ≥20 dB 10 Gbit/s (with AFEC) ≥17 dB 10 Gbit/s (with AFEC and Super CRZ) ≥15.5 dB In Table 6-1, OSNR requirements are typical values.6-2 Huawei Technologies Proprietary Issue 01 (2007-01-20)
  6. 6. OptiX BWS 1600GTroubleshooting 6 Handling Bit Error6.1.2 Dispersion The dispersion of single-mode fiber includes chromatic dispersion and polarization mode dispersion (PMD). An insufficient dispersion tolerance of the system will lead to occurrence of bit errors at the receiving end. Chromatic dispersion can be compensated through the dispersion compensation fiber (DCF). Dispersion of optical fiber is usually measured with dispersion coefficient with the unit of ps/nm⋅km. The dispersion coefficient of G.652 fiber is 17ps/nm⋅km, and that of G.655 fiber is 4.5 ps/nm⋅km usually. The dispersion tolerance of 2.5 Gbit/s optical transmitting module is large, needing no compensation. But the dispersion tolerance of 10 Gbit/s optical transmitting module is small (hundreds of ps/nm), therefore, the signal needs dispersion compensation after being transmitted for a certain distance. On G.652 fiber, the signal needs dispersion compensation after being transmitted for a distance of 30 km, while on G.655 fiber, the signal needs dispersion compensation after being transmitted for a distance of 100 km. PMD is a random value, which cannot be compensated through DCF. But if the rate of a single-channel signal is below 10 Gbit/s, the effect on the system is not great.6.1.3 Non-Linearity of Optical Fiber The non-linear effects of optical fiber are related to the input optical power to a large degree. If the input optical power is very high and optical fiber transmission distance is long, non-linearity of fibers may seriously affect the performance of the system, resulting in performance deterioration at the receiving end and occurrence of bit errors. Therefore, in the 40-wavelength DWDM system, the input optical power should be limited within 20 dBm.6.1.4 FEC Technology The OTU of the OptiX BWS 1600G adopts Reed-Solomon forward error correction (FEC) technology, which can correct up to eight bytes in any location for every 255 bytes, boasting powerful error correction capability. The out-band FEC function can improve the OSNR budget of the DWDM transmission system and increase the transmission distance. In addition, the FEC function can reduce bit error rate in line transmission, and alleviate the bad effects on the signal transmission quality caused by the aging components or deterioration of fiber performance, thereby improving the communication quality of the DWDM transmission network.Issue 01 (2007-01-20) Huawei Technologies Proprietary 6-3
  7. 7. OptiX BWS 1600G6 Handling Bit Error Troubleshooting6.2 Causes The occurrence of the following alarms or performance events means that bit errors have occurred to the equipment. B1_EXC, B2_EXC, B1_SD, and B2_SD alarms BEFFEC_EXC and BEFFEC_SD alarms RSBBE, RSES, RSSES, RSUAS and RSCSES performance events MSBBE, MSES, MSSES, MSUAS, and MSCSES performance events Table 6-2 shows the common causes of bit errors. Table 6-2 Common causes of bit errors Fault type Causes Handling External causes External interference See this chapter. Equipment grounding problem See this chapter. Optical power abnormity (over-high or over low) See Chapter 5. and OSNR deterioration Dispersion tolerance problem See this chapter. Non-linearity of optical fiber See this chapter. Environment problem (extra-high equipment See this chapter. temperature) Equipment faults OTU board failure or performance deterioration. See this chapter. Optical amplifier board failure or performance See this chapter. deterioration.6-4 Huawei Technologies Proprietary Issue 01 (2007-01-20)
  8. 8. OptiX BWS 1600GTroubleshooting 6 Handling Bit Error6.3 Methods and Procedures of Fault Locating6.3.1 Methods Alarm and performance analysis Section-by-section loopback Replacement Meter test For details, refer to Chapter 2 "Basic Thoughts and Methods for Fault Locating" in this manual.6.3.2 Procedures Procedures for handling bit errors are shown in Figure 6-1.Issue 01 (2007-01-20) Huawei Technologies Proprietary 6-5
  9. 9. OptiX BWS 1600G6 Handling Bit Error Troubleshooting Figure 6-1 Handling flow of bit errors Start T2000 1 Any external Yes Remove the interference? interference source No 2 Remove the Any grounding Yes grounding problem? fault No 3 Is B1 at receive end Yes Remove the SDH and transmit end the fault same? No 4 Is B2 at receive end Yes Remove the SDH and transmit end the fault same? No 5 Is optical power Yes See handling optical abnormal? power abnormity No 6 Enable the FEC Is FEC function of OTU Yes function of OTU board disabled? board No 7 Is optical non-linearity Yes Reduce the input severe? optical power No 8 Yes Replace the fan Is fan abnormal? No 9 Yes Is board performance Replace the board degraded? No 10 Yes Replace fibre, clean Is fibre connection problem? connector or reroute No No Is bit error Return to step N removed? Yes Contact Huawei for End help6-6 Huawei Technologies Proprietary Issue 01 (2007-01-20)
  10. 10. OptiX BWS 1600GTroubleshooting 6 Handling Bit Error6.3.3 Techniques In removing faults such as bit error, flexible use of some analysis techniques can be helpful for locating the fault quickly.Determining the Channels Involved with Bit Errors Bit error occurs either in all channels or in specific channels. Bit Errors in All Channels The fault exists on the line (between MPI-S and MPI-R), you should check the main path of the system, including optical amplifier, line fiber and related fiber connection. Bit Errors in Specific Channels If bit errors only occur in some of the channels, maybe something is wrong with those channels themselves or the system is working in the critical state, for example, OTU is faulty, or the signal accessed to the client side is abnormal, or connection of fiber in a single station is abnormal. In this way, the fault can be located quickly.Skillful Use of B1 OTU provides non-intrusive monitoring on B1 byte in SDH signal frame, as shown in Figure 6-2. Figure 6-2 Monitoring B1 byte A B DWDM side DWDM side O O O O Monitor B1 byte T T T T Monitor B1 byte U U U U Client side The OTU at the transmitting end of station A detects the number of B1 errors and occurrence time, and then it transmits bit errors transparently to station B. The OTU at the receiving end of station B also detects the number of B1 errors and occurrence time. The difference of B1 errors at station A and station B is the number of bit errors generated between stations A and B, that is, the bit errors generated in the transmission process of the DWDM equipment. In this way, you can determine whether the bit errors are generated at the DWDM side or at the client side. In addition, you can know the number of bit errors generated at the client side and that at the DWDM side. The OTU with FEC provides four performance events of error correction data to reflect the number of corrected bit errors and the number of uncorrected bit errors on DWDM line. FEC_COR_0BIT_CNT: count of corrected 0bitsIssue 01 (2007-01-20) Huawei Technologies Proprietary 6-7
  11. 11. OptiX BWS 1600G6 Handling Bit Error Troubleshooting FEC_COR_1BIT_CNT: count of corrected 1bits FEC_COR_BYTE_CNT: count of corrected bytes FEC_UNCOR_BLOCK_CNT: count of uncorrectable frames If the number of bit errors during 24 hours is within the error correction range, the system works normally without bit errors, but FEC error correction performance events are reported by the T2000. The number of corrected errors is consistent with the number of bit errors generated in the DWDM system. If the number of 24-hour bit errors is beyond the error correction range, the B1 error performance events or alarms are still reported by the receiving end.Replacement If bit errors occur in only one direction of the system, the replacement method is recommended. By observing whether bit errors or error correction performance varies with replacement, you can easily locate the fault. The following can be replaced: Optical Fiber Replace the optical fiber in one direction with the fiber in the other direction. Exchange the fibers connected to the "IN" port with that to the "OUT" port of FIU at both ends. OTU The OTUs at the receiving end (such as LWF) can be exchanged without differentiation of wavelength. The faulty board can be replaced by an idle or standby OTU. The OTU at the transmitting end (such as LWF) corresponds to different wavelengths. On the site, if there is no standby board with the corresponding wavelength; replace the faulty board with the OTU of back-to-back OTM station in the other direction. Optical amplifier board Replace with the optical amplifier board in the other direction. Note that the types of optical amplifier boards should be the same.6.4 Classified Fault Handling6.4.1 Handling of External FaultsChecking External Interference The following external interference may cause bit errors: Electromagnetic interference brought about by external electronic equipment Electromagnetic interference from the power supply of equipment Electromagnetic interference generated by thunderstorm electricity and high-voltage line To prevent external electromagnetic interference, preventive measures are important, such as: The power consumption equipment in the equipment room should be well grounded. The interference degree of radio-frequency device should comply with the requirement. The transmission equipment is suggested to use separate power supply.6-8 Huawei Technologies Proprietary Issue 01 (2007-01-20)
  12. 12. OptiX BWS 1600GTroubleshooting 6 Handling Bit Error Power supply should be configured with a large capacitor preventing surge and line frequency interference. Do not build an equipment room in an area where lightning is frequent or near high-voltage power cables and be prepared for lightning protection.Checking Equipment Groundings Poor grounding of equipments and cables in the equipment room is another cause for bit errors. Therefore, when analyzing causes of bit errors, check the following items: Grounding of the cabinet of the transmission equipment Grounding of side panels on the cabinet of the transmission equipment Grounding of subracks Grounding of signal cables Grounding of DDF and ODF Grounding of the NM equipment, and various power consumption equipments Whether the interconnected equipments are jointly groundedChecking Ambient Temperature The ambient temperature of the equipment room must comply with the requirements. Either too high or too low temperature may cause bit errors.Checking Optical Power In the DWDM system, transmission distance is usually very long, which requires the equipment to provide a large number of fiber jumper, adjustable attenuators and ring flanges. Bad contact of fiber connector, bending or over tight binding of fibers, bad ambient environment and minute operation may increase the optical power attenuation of fibers. Therefore, when bit error occurs, check the optical power of each point, if the bit errors are caused by cable performance deterioration, high connector attenuation, or incorrect fiber connection, repair cables, adjust optical attenuator or reconnect inter-board fibers correctly. If bit errors are generated due to optical power abnormity, see the Chapter 5 "Handling Optical Power Abnormity" in the manual.Removing Non-Linearity of Optical Fiber Too low optical power may result in bit errors, so it is with over-high optical power, which may lead to non-linearity of signals. To eliminate bit errors resulting from non-linearity, you can query the optical transmitting power on the T2000 and keep them within the specified range.Checking Dispersion Compensation For the single-channel signal at the rate of 10 Gbit/s, the dispersion compensation should be reasonable. In system design, apart from the dispersion budget, note the following points: The types of fibers of the whole system The type of the dispersion compensation module and compensation distance Whether the distribution of dispersion compensation modules is reasonableIssue 01 (2007-01-20) Huawei Technologies Proprietary 6-9
  13. 13. OptiX BWS 1600G6 Handling Bit Error Troubleshooting The effect of dispersion on the system adopting SuperWDM technology is great. Both over compensation and under compensation will affect the OSNR at the receiving end, and thus generating bit errors.6.4.2 Handling of Equipment FaultsBoard Performance Deterioration OTU performance deterioration Signals at the client side experience O/E/O conversion on the OTU, during which, faults in any stage may lead to performance deterioration and further result in generation of bit errors. Furthermore, the instability and drift of wavelength of the laser at the transmitting end, or insufficient isolation for neighboring wavelengths after being multiplexed may also result in the generation of bit errors. If bit errors are generated as a result of the deterioration of the OTU’s performance, the faults can be removed by replacing the faulty board. Optical amplifier performance deterioration The pump laser of erbium-doped optical amplifier may bring about very strong amplified spontaneous emission noise (ASE), which may decrease the OSNR, and thus resulting in bit errors. If bit errors are generated as a result of the deterioration of optical amplifier performance, the fault can be eliminated through board replacement.Fan Abnormity If the fan becomes abnormal, equipment temperature may increase, resulting in the emergence of bit errors. Abnormity of the fan may be caused by blocked air outlet, for example, the air filter is blocked. In this case, clean the air filter immediately. Another possible cause is the fan is faulty, in this case, replace the fan immediately.6-10 Huawei Technologies Proprietary Issue 01 (2007-01-20)
  14. 14. OptiX BWS 1600GTroubleshooting 6 Handling Bit Error6.5 Cases of Common Fault Handling6.5.1 Bit Errors Caused by Incorrect Fiber ConnectionsSystem Overview In a project of 40x2.5 Gbit/s (channel) system, station A and station B, fully configured with LWC1 boards, serve as OTM stations, offering 6 channels of services. The overall distance between them is 150 km, and the attenuation on the line is 37 dB, as shown in Figure 6-3. Figure 6-3 Networking configuration diagram Station A Station B 150km SDH OTM OTM SDHFault Symptom In deployment commissioning, the test of 24-hour BER between station A and station B is passed, but it is found on the T2000 that a large number of FEC-corrected errors occur to the performance data of all LWC1s at station B that receives signals from station A. But the number of corrected errors at station A that receives the signals from station B is 0.Troubleshooting Analysis: The LWC1 adopts FEC. The occurrence of a large number of corrected errors indicates that bit errors occur to DWDM line in transmission. But the 24-hour BER test between station A and station B is passed, it indicates the number of bit errors is within the FEC correction range, that is, those errors have been corrected. Handling procedure: Querying alarm and performance events Step 1 A large number of FEC-corrected errors occur to the LWC1s at station B that receives the signals from station A, and the number of corrected errors at station A that receives the signals from station B is 0. ----End Conclusion: Correction occurs in one direction. Querying input optical power Step 1 Query the type of the receiving lasers at the DWDM side of LWC1 of station B and find they are all APD pipes.Issue 01 (2007-01-20) Huawei Technologies Proprietary 6-11
  15. 15. OptiX BWS 1600G6 Handling Bit Error Troubleshooting Step 2 Query the input optical power at the DWDM side of LWC1 of station B and obtain the value of –15 dBm, higher than the sensitivity. Since error correction is found in all 6 channels, it shows that the fault does not exist in the OTU. Step 3 Query the optical power of each point on the T2000, and find that the optical power transmitted from station A to station B is similar to that transmitted from station B to station A, but a large number of corrected errors are found only at station B that receives the signals from station A. ----End Replacing the board Step 1 Since corrected errors occur in one direction, notify the personnel at station B to exchange the optical amplifier boards in two directions. But FEC correction is still found. ----End Conclusion: The fault on the OAU is excluded. Adjusting optical power Step 1 Error correction may be related to non-linearity of fibers, so increase and reduce the optical power input into optical amplifier board, the number of corrected errors varies, but error correction still exist, and the smaller the input optical power is, the greater the number of corrected errors will be. ----End Conclusion: Non-linearity of fiber is excluded. Replacing fibers Step 1 Exchange the optical fiber connected with the "IN" port and "OUT" port of the FIU at station A and station B respectively. View the NM and find that error correction disappears. Restore the original connection, and error correction does not appear either. The fault is cleared. ----End Conclusion: The fault is that the optical fibers at the line side of FIU are connected incorrectly.Conclusion and Suggestion Normally, if the optical power and the OSNR meet the requirements, the system should not generate bit errors or correction performance data (small FEC corrected errors will be exist if the OSNR below 20 dB at the receiving end). Occurrence of correction performance data indicates that system performance becomes deteriorated. Find out the causes to avoid hidden troubles. If bit errors only occur uni-directionally, replace the optical fiber, OTU, and OAU in turn to locate the fault.6-12 Huawei Technologies Proprietary Issue 01 (2007-01-20)
  16. 16. OptiX BWS 1600GTroubleshooting 6 Handling Bit Error6.5.2 Bit Error Caused by Non-linearity of Optical FiberSystem Overview In a chain network, as shown in Figure 6-4, both station A and station B are OTM stations, between which there are four OLA stations. Eight channels (CH1-CH8) of 2.5 Gbit/s services have been configured between station A and station B. Figure 6-4 Networking configuration diagram Station A Station B Station C Station D Station E Station F OTM 17dB OLA 22dB OLA 21dB OLA 20dB OLA 14dB OTMFault Symptom During deployment commissioning, the input and output optical interfaces at the client side of the LWC1 of station F are looped back through the optical fiber and a fixed 10 dB optical attenuator is installed between them. At station A, test bit errors with a meter and some channels cannot pass the 24-hour BER test.Troubleshooting Analysis: Initialize the current performance data and test bit errors with a meter again. Observe for a period of time, then view the T2000 and find that no bit error occurs to the signals sent from station A to station F. However, after 16 hours, bit errors occur in some channels that station A receives from station F. The channels reporting bit errors and the number of bit errors are not stable. Since bit errors appear uni-directionally, the fault can be located through replacement. Handling procedure: Querying performance events Step 1 After 16 hours, bit errors occur in some channels that station A receives from station F. The channels reporting bit errors and the number of bit errors are not stable. ----End No bit error occurs to the signals sent from station A to station F. Conclusion: Bit errors occur uni-directionally. Checking optical power Step 1 Check the optical power of the amplifier at each station in NM, and find it is identical with the value in engineering commissioning. Step 2 Check the optical power of the LWC1 at station F in the T2000, and find it is identical with the value in engineering commissioning.Issue 01 (2007-01-20) Huawei Technologies Proprietary 6-13
  17. 17. OptiX BWS 1600G6 Handling Bit Error Troubleshooting ----End Replacing the board Step 1 Notify the personnel at station F to replace the LWC1 corresponding to the channel with bit errors. Bit errors still exist, and bit errors in some channels increase, while in others reduce. It shows that bit error is not related to the LWC1. Step 2 Since bit errors occur uni-directionally, notify the personnel of each station to exchange westbound and eastbound optical amplifiers. But bit errors still exist. Step 3 Restore the original board configuration. ----End Adjusting optical power Step 1 On the premise that OSNR is guaranteed, increase the input optical power of the optical fiber with bit errors (at station F transmitting signals to station A). Observe the variation of the number of bit errors on the NM, and find the higher the optical power is, the greater the number of bit errors will be. Reduce the input optical power of the fiber, and find that the lower the optical power is, the smaller the bit error number will be. ----End Conclusion: It is determined that bit errors are caused by the non-linearity of optical fiber. Reduce the input optical power and bit error problem will be solved. Adding an attenuator Step 1 Add a fixed 3 dB attenuator after the optical amplifier board at the transmitting end without decreasing the optical OSNR so as to reduce the input optical power, thus alleviating the effect of fiber non-linearity. ----End Test again, and bit errors disappear. Step 1 Observe for five days and find that the system works normally. ----EndConclusion and Suggestion The probability of bit errors caused by fiber non-linearity is little, and bit errors appear at random for it is mainly related to the performance of line fiber. Since the OSNR at the receiving end may be good when bit error is caused by fiber non-linearity, the fault is likely to be hidden. Whether bit errors are caused by fiber non-linearity or not can be determined by increasing or reducing the input optical power. If the input optical power is increased, bit errors increase accordingly. If the input optical power is reduced, bit errors reduce accordingly.6-14 Huawei Technologies Proprietary Issue 01 (2007-01-20)
  18. 18. OptiX BWS 1600GTroubleshooting 6 Handling Bit Error6.5.3 Bit Errors Caused by a Faulty OTUSystem Overview Figure 6-5 illustrates a project of a 40-wavelength system with a full service configuration. All OTUs in the system are LWFs. Figure 6-5 Networking configuration diagram Station A Station B Station C Station D Station E Station F Station G OTM OLA OLA OLA OLA OLA OTMFault Symptom In deployment commissioning, disable the FEC function of the LWF. Attach a meter at station A and perform loopback at station G for BER testing. One bit error is generated every one hour in the CH29 at station A that receives the signals from station G, but the LWF corresponding to CH29 at station G has no bit error. There is no bit error in the other channels.Troubleshooting Analysis: Since only one channel has bit errors, there is nothing wrong with the board and fiber line between the signal multiplexing and signal demultiplexing. Then the factors causing bit errors include: The OSNR of CH29 at the MPI-R point of station A is below the standard value. The input optical power of the LWF at the receiving end of station A is abnormal. The LWF at the receiving end of station A is faulty. The LWF at the transmitting end of station G is faulty. Handling procedure: Step 1 Test the OSNR of CH29 at the MPI-R point with an optical spectrum analyzer. It is normal. Step 2 Check the input optical power at the DWDM side of the LWF of station A. It is in the normal receiving range. Step 3 At station A, check the fibers and the connections between D40 and the LWF. At station G, check the fibers and the connections between the LWF and M40. The optical fibers and the connections are normal. Step 4 At station A, use a normal LWF to replace the LWF corresponding to the 29th channel, but bit errors occur in less than one hour when testing with a meter. The LWF of the station G is suspected to be abnormal. Step 5 Replace the faulty LWF board at station G with the standby board of station A. There is no bit error within 24 hours.Issue 01 (2007-01-20) Huawei Technologies Proprietary 6-15
  19. 19. OptiX BWS 1600G6 Handling Bit Error Troubleshooting Step 6 The LWF of the station G is determined to be faulty. ----EndConclusion and Suggestion When bit errors occur to the system, first determine whether they occur to multiple channels simultaneously. If so, check the optical path shared by all channels; if only some channels have bit errors, check the separate optical channel. In the OptiX BWS 1600G, important service boards like the OTU and the OAU must be prepared with a standby board, and each type of board must has more than one standby boards. As for other boards like the SCC, SC2/SC1 and so on, the standby board can be prepared as required.6.5.4 Bit Error Caused by Unmatched Dispersion CompensationSystem Overview In a project of 40×10 Gbit/s DWDM system, station A and station B are OTMs, and the distance between them is 80 km. The engineering documentation provided in deployment requires that an ITU-T G.655 fiber be used bi-directionally between A and B.Fault Symptom During engineering acceptance, it is found that all LWFs at station A that receives signals from station B report a large number of corrected errors. In 15-minute performance data, thousands of RSBBE are reported. The number of corrected LWFs at station B that receives signals from station A is small and there is no bit error.Troubleshooting Analysis: The LWF has FEC function to correct the bit errors generated in the transmission over the DWDM equipment. If the number of bit errors is beyond the capability of FEC, besides the corrected performance data, the number of uncorrectable bit errors and alarms will be reported. It indicates that the system works normally. If bit errors occur uni-directionally, it indicates that the fault is related to unidirectional optical fibers and boards. Handling procedure: Step 1 Check the output and input optical power of each OAU and OTU at station A that receives signals from station B. Both boards are normal. Step 2 Error correction and bit errors occur to all boards. The OTU fault is excluded. Step 3 Replace the OAUs at the receiving end and transmitting end, but the fault still exist. The optical amplifier board fault is excluded. Step 4 Exchange the optical fibers connected with the "IN" and "OUT" port of the FIU at both station A and station B, and find that error correction and bit errors at station A that receives signals from station B disappear, but all OTUs at station B that receives signals from station A6-16 Huawei Technologies Proprietary Issue 01 (2007-01-20)
  20. 20. OptiX BWS 1600GTroubleshooting 6 Handling Bit Error report a large number of corrected errors and bit errors. It indicates that bit errors are related to optical fiber. Step 5 Check the information about optical fibers, and find that the optical fibers between station A and station B along the direction B to A are comprised of three sections. The optical fiber in the middle is more than 20km long, with the type of G.652. Step 6 Replace the middle optical fiber with a ITU-T G.655 fiber, and then bit errors disappear. The fault is cleared. ----End The dispersion coefficient of the signal in the 1550 nm window is 4.5ps/nm·km on a ITU-T G.655 fiber. The dispersion tolerance of the LWF is 800 ps/nm. But for a ITU-T G.652 fiber, the signal usually needs dispersion compensation when transmitted beyond 30 km. In this case, error correction and bit errors are caused due to inadequate dispersion compensation.Conclusion and Suggestion The 10 Gbit/s signal has small dispersion tolerance, so the transmission distance without dispersion compensation is short. During system design, the dispersion budget of the system must be considered. An optical fiber with a small dispersion coefficient (for example G..655) can be adopted or dispersion compensation can be implemented. In engineering deployment and daily maintenance, you should be familiar with the type of the optical fibers adopted in the system and the distribution of dispersion compensation modules.Issue 01 (2007-01-20) Huawei Technologies Proprietary 6-17

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