Lessons Learned in Testing DWDM Systems Francisco M. Tacoa, TTC Introduction The acceptance of the Internet as a viable means of communication, combined with the accelerated growth of high-speed data services, and the deregulation of the telecommunications market have led to an increased demand for capacity in transport networks. Service providers such as the Bell companies, long distance carriers, and new players such as CLECs (competitive local exchange carriers) and CAPs (competitive access providers) are all facing this problem today. Deployment of DWDM systems brings a number of issues to network managers, including testing issues. This paper discusses key testing considerations when planning, deploying and maintaining DWDM systems in the field. This paper addresses service providers; they are faced with the challenge of integrating and maintaining new DWDM systems in the field today. It reviews the impact that testing DWDM systems has throughout the entire DWDM network life cycle, including: 1) DWDM network planning and design, 2) DWDM network deployment and new service provisioning, and 3) Service maintenance, maturity, and transition Testing of DWDM systems in the field is necessary to ensure proper operation of a new system before loading it with customer s traffic. Testing DWDM systems is also needed when upgrading existing DWDM systems already in operation. In addition, testing is also needed at handoff points between service providers, and also at handoff points between service providers and customers (wavelength services) to guarantee performance and compliance with agreed levels of service. Effective testing through the entire lifecycle of the network, described in Figure 1, can greatly enhance the success of a DWDM system. 1
Figure 1: Telecommunication network lifecycle Lessons learned during network planning and design It is important to design a DWDM network to enable maintenance and efficient fault localization once the network is up and running. This includes ensuring the existing fiber plant can support DWDM traffic, and ensuring optical monitoring points are available throughout the network for signal access. Test existing fiber plant for dispersion If DWDM systems are being planned to carry OC-192 or higher speed channels on older fiber plant, then dispersion can be an issue. Dispersion broadens the pulses in time due to photons traveling through a fiber at different speeds. Tolerance to dispersion decreases linearly with bit rate. Dispersion is of concern for some established service providers who have a relatively old installed fiber plant, and desire to use such fiber plant for OC-192 and higher rates. Dispersion increases the likelihood of bit errors in the signals. The fiber cable s manufacturing process, and especially the installation process can increase polarization of the optical fiber (due to mechanical stresses), particularly if it is performed without extreme caution, or by a workforce with little optics training. A lesson to be learned during DWDM network planning and design is to test for dispersion using dispersion testers. Recommended maximum levels of dispersion exist for different SONET rates. For example, a maximum value of 10 psec worth of dispersion is suggested for OC-192 signals. If it is found that dispersion is a limiting factor in the span, then limiting the distance between regeneration points may keep this problem under control. For local exchange carriers, dispersion may not be the greatest concern due to the relatively short distances between central offices (COs.) Test for non-linear effects OC-192 signals require significantly more power than OC-48 ones. At high power levels, evenly spaced DWDM optical channels may mix non-linearly and form new optical frequency components, as seen in Figure 2. Referred to as four-wave mixing, this phenomenon causes both a loss of energy from desired signals and a generation of unwanted signals, which may coincide with existing channels. To reduce the effects of four-wave mixing, unequal or increased channel spacing may be considered during DWDM system planning and design. Again, this can be a major concern depending on the fiber type and is independent of the transmission rate. This may limit the number of DWDM channels the fiber span can support. A lesson to be learned is to design and order a DWDM system so that four evenly-spaced, contiguous wavelengths can be present in the system during deployment testing. 2
Figure 2: Effects of Four-Wave Mixing on a DWDM Signal Design with optical monitoring access points in mind Another important consideration during DWDM network planning and design is to ensure the DWDM system includes optical monitoring access points. Monitoring access points will supply local test points during transport network maintenance and troubleshooting. Since in most cases DWDM systems will be co-located with SONET muxes that may not feature monitoring access points for high speed signals, deployment of DWDM systems offers an opportunity to introduce access points in the network. These access points are needed to connect a portable test set to the network for nonintrusive analysis. Most vendors of long-haul DWDM systems today include monitoring access points in their DWDM systems at the optical amplifiers. However, passive DWDM systems are increasingly being deployed in the field today. Passive DWDM systems do not use optical amplifiers to boost signals. The lack of optical amplifiers in passive DWDM systems means that these systems often do not include monitoring access points. Therefore, DWDM network design that includes passive DWDM systems should also take into account the need to specify rack-mountable splitters. For example, rackmountable 90:10 splitters, which usually include multiple splitters so that more than one signal can be accessed, can be effectively used to create permanent monitoring access points in DWDM systems that do not already include monitoring points. Watch out for older fiber connectors and fiber splices As a final consideration during DWDM network planning and design, it is important to review the type of fiber optic connectors to be used with a new DWDM system. Older fiber connectors, such as Biconic and D4 types, may cause excessive laser reflections, which can potentially degrade the performance of a new DWDM system. Older fiber connectors and splices is a primary concern for some established service providers who have a relatively old installed fiber plant, and desire to use such fiber plant with DWDM systems. Because of the laser reflections they can potentially generate, replacement of older fiber connector types with SC or a similar connector type is highly recommended. SC or similar fiber connectors show much less reflections and loss characteristics than older fiber connectors. SC connectors also resolve the potential issue of over-tightening fiber connectors, which could occur should FC connectors be used. The type of splices used in the fiber plant is also an important consideration. Fusion splices are highly recommended, since they show low reflection characteristic. Mechanical splices in fiber cables are not recommended for DWDM systems since mechanical splices tend to cause excessive reflections. Proactive replacement of any mechanical splices with fusion splices is highly recommended as part of a DWDM system planning and design. Case study 1: Established regional service provider planning on using DWDM as a fast and economical way to enhance inter-central office capacity in high-traffic corridors. 3
This provider planned on using aging fiber plant with new DWDM system. Due to the relatively low speed of the signals to be carried by the new DWDM system (OC-3 and OC-12 SONET-framed signals), and because of the relatively short distances between COs (less that 30 miles in all cases), after testing dispersion was found not to be an issue. However, this provider was using Biconic fiber connectors. After testing for ORL, it was found that excessive reflections (worst than 20 db) would exceed the DWDM system manufacturer-recommended value. This service provider decided to replace all fiber connectors with SC connectors to improve reflections, which significantly reduced reflections to the DWDM terminals. This proactive practice resulted in streamlined DWDM turnups, and helped this carrier avoid costly kickback situations during later DWDM deployments. In addition, this service provided demanded optical monitoring points from its DWDM equipment supplier as one of the system requirements. Lessons learned during network deployment and new service provisioning It is critical to test the operation of a DWDM system during deployment under realworld environment and actual operating conditions to check overall performance and verify the health of the system before loading it with customer traffic. This will ensure actual network load and interactions with legacy network systems (including SONET/SDH ADMs and existing fiber cables) do not adversely impact the newly deployed DWDM system. Testing new DWDM systems as they are deployed adds value to service providers by helping to quickly identify and isolate problems in the network. This will reduce costly delays in DWDM network turnup. Also, proper testing of new DWDM systems during network deployment and service provisioning will eliminate time and manpower-consuming kickback situations. A number of key tests are recommended during deployment to ensure optimum performance of the DWDM system. Qualify the fiber plant Fiber plant qualification is a key step to ensure a successful DWDM system deployment. Fiber plant qualification is of concern for some established service providers who have a relatively old installed fiber plant, and desire to use such fiber plant with DWDM systems. Fiber plant qualification tests are typically intrusive and require the system to be out-of-service. A number of service providers are increasingly hiring thirdparty contractors to perform these pre-qualification test. Health of the fiber plant is usually the responsibility of the fiber plant owner (typically either service provider or fiber owner), and verification of the fiber system s health includes testing associated fiber splices and connectors, patch panel interfaces, any attenuators, and fiber reels. Fiber plant qualification is typically done in new fiber installations, when existing fiber is to be used with a transport system upgrade (such as when DWDM equipment is installed in an existing SONET/SDH network), or when fiber is leased or sold. As an initial step in the deployment of a DWDM system, it is important to ensure all fiber optic connectors in the cards and in the racks are clean. It is also important to ensure good connections in the DWDM system. 4
In addition to dispersion qualification described before, an important fiber plant qualification test to be done when deploying DWDM systems is to check optical return loss (ORL) in the fiber span to carry the DWDM traffic at the wavelength region to be used (1550 nm). As transmission speeds increase beyond the OC-12 rate, the effect of optical reflections in carried traffic increases significantly. Unless special electronics or optical isolation is built into the laser transmitter, errors will result if excessive amounts of light are reflected back to the laser sources in a DWDM system. If excessive ORL is measured, then an optical time domain reflectometer (OTDR) test set can be used to locate the sources of excessive reflectances in the fiber plant. Another recommended fiber qualification test to be done when deploying DWDM systems is to check optical insertion loss (OIL) in the fiber span to carry the DWDM traffic at the wavelength region to be used (1550 nm). Loss in a fiber plant is due to intrinsic fiber loss, losses associated with connectors and splices, and bending losses due to cabling and installation. Get electronic fingerprint of DWDM system Once the fiber plant has been properly qualified, the next recommended step to be performed is to qualify the DWDM system. This can be effectively done by verifying and electronically recording three key parameters in the DWDM span: channel center wavelength, channel peak power, and channel signal-to-noise ratio (SNR.) Both established service providers and new ones can greatly benefit from this practice, since the recorded information can be invaluable during future system troubleshooting. As indicated in Figure 3, overlying electronic fingerprints of a DWDM system taken at different times can indicate variations useful to detect problems in a DWDM system being maintained. This needs to be done to verify proper operation of the DWDM span, and for system benchmarking purposes. Figure 3: Overlaying DWDM traces In order to carry out these three tests, as mentioned before, it is necessary to populate the DWDM system with at least four evenly spaced, contiguous channels. This is necessary in order to study proper operation of the system, and to account for all possible known problems in this type of system, including dispersion and four-wave mixing effects. These tests should be done before loading the system with customer s traffic. Therefore, they should be done on an out-of-service basis. 5
Verification of channel center wavelength is a critical measurement. This parameter is perhaps the most susceptible to variations through the life cycle of a DWDM system. This reading measures the location of the DWDM channels in the frequency spectrum. Adjacent channels in a DWDM system may be spaced as little as 0.4 nm today, and possibly 0.2 nm and less in the future. Therefore, there is a need for very high accuracy of wavelength measurement. This measurement is important since misplaced or shifted channels may overlap adjacent channels, resulting in crosstalk, and eventually errors. Channels may shift as a result of drift in the laser sources. This can be the result of aging and temperature effects in the DWDM system. Because of this, a center wavelength test requires multi-wavelength meter-like capabilities in a test set. It is recommended to do a long-term test (at least 24 hours) soak test when DWDM systems are turned up to verify wavelength stability. Figure 4 graphically illustrates this measurement. The purpose of channel peak power testing is to ensure even optical power distribution after amplification over the entire DWDM bandwidth. Optical power in a single DWDM channel can range from as little as -30 dbm to as much as 0 dbm. Also, optical power per channel varies with location in the system, being largest after amplification. Noise floor and gain characterization are also important and a portable test set is invaluable on this. Figure 4 graphically shows this measurement. Measurement of channel signal-to-noise ratio (SNR) is another key measurement for each of the DWDM channels. A high SNR is necessary for error-free operation of the DWDM channels, however it is not a sufficient condition since other problems such as four-wave mixing and dispersion can result in degraded signals. Improperly tuned EDFAs can be identified by a low SNR reading. Figure 4 graphically illustrates this measurement. Center wavelength Peak power Signal to noise ratio Figure 4: Peak power, SNR, and center wavelength measurements Check for initial wavelength, power and SNR fluctuations Measurement of laser wavelength drift (change in center wavelength), peak power fluctuations and changes in signal to noise ratio over a period of several hours is recommended to measure the DWDM system stability. The laser s center wavelength can drift with temperature (and age over the long term), causing crosstalk and resulting in bit errors. Power can also fluctuate as a result of a number of reasons -- environmental, hardware, noise etc. Consequently, variation of these critical DWDM parameters is important to know. These measurements can be made non-intrusively from an optical 6
monitoring access point. Having a time representation of these variations in a chart mode can be invaluable to determine the root cause of these problems. Bit error rate test (BERT) the pipes The tests mentioned above are necessary to verify the proper operation of a DWDM system during deployment. However, they are not sufficient to completely qualify the DWDM system before traffic is commissioned. This is the case because DWDM equipment hardware still needs to be verified for proper operation, and possible presence and effects of four-wave mixing and dispersion, if any, need to be examined. Because of these reasons, a bit error rate test (BERT) needs to be conducted on the whole DWDM system. Since most DWDM systems today are used to carry SONET/SDH channels, the capability to generate a framed SONET/SDH signal with a test pattern is also required. This is why it is very effective and highly desirable to have both DWDM test capabilities and BERT capabilities in the same portable test set. This is an intrusive test that needs to be done on an out-of-service basis. If the result of the BER test is acceptable, then the DWDM system should be tested for alarms detection both at the local and at the network operations center (NOC) levels. Once this test is successfully completed, the DWDM system can be considered ready for new service provisioning. If this is not the case, then troubleshooting becomes necessary to isolate the source of the problem so it can be fixed. Check the services riding the DWDM span Once the whole DWDM system has been successfully qualified, usually the next step is to put customer s traffic on the system, and to check the integrity of these services. Customers pay service providers for these services; therefore it is imperative that network operators check these services to ensure they are running problem-free. Checking these services is critical if providing drops directly to customers. It is also recommended inside the service provider s network. The type of tests to be performed depends upon the rendered service. The most common services to be tested in high speed networks today are SONET/SDH services, ATM services, DS3 services, and increasingly IP traffic, as shown in Figure 5. Since there is customer s traffic in the system, this test must be done non-intrusively using the optical monitoring access points. Verification of OC-12 and OC-3 SONET/SDH pipes can be done on an in-service basis via either a SONET/SDH or a DWDM monitoring access point. The specific measurements to be done should include verification of bit interleaved parity (BIP) bytes including B1s, B2s and B3s, and frame errors. BIPs indicate presence of errors in the SONET/SDH signal. Likewise, frame errors indicate problems in the SONET/SDH span. It is also important to check for SONET/SDH alarms, including FEBEs (REIs), FERF (RDIs), and AIS alarms to ensure proper operation of the SONET/SDH elements. DS3 Concat (ATM, IP, Data) SONET/SDH Configuration layer 7
Physical layer Figure 5: Service-level check If DS3 pipes are sold to customers as a service, then verification of these signals is important. The specific tests to be done on DS3 signals include frame errors, CRC errors (if applicable) and parity errors. Again, these can be verified from DS3 or higher speed monitoring access points. Finally, if ATM services are sold to customers, then testing key parameters of these signals is key to ensure proper operation of the system. The checks to be done on the ATM signals should include verification of HEC errors and (for DS1 and DS3 ATM signals) PLCP errors, if applicable. Case study 2: Greenfield competitive local exchange carrier deploying OC-48 DWDM system with brand-new fiber plant as a way to future-proof capacity in their network. Once manufacturer deployed this system, five wavelengths were set on the system. Then turnup crew proceeded to take electronic fingerprint of DWDM system by monitoring and recording DWDM traffic at EDFA cards with DWDM channel monitor test set. These readings showed a wavelength had significantly less power than the other four wavelengths running in the system. After checking the setup of the DWDM system, technicians decided to re-clean fiber connectors associated with the problem wavelength. It was then found that one of the fiber connectors was dirty, causing loss of optical power on the problem wavelength. Once cleaned, all power measurements checked ok. The ability to pinpoint marginal differences in channel peak power in the system from day one saved this carrier the expense and time needed to troubleshoot at a later time when problems arise due to the marginal power content of this wavelength in their system. Lessons learned during service maintenance, maturity and transition There are testing considerations to keep in mind once service is running on a DWDM system. These considerations can be classified into two inter-related groups: preventive maintenance, and problem troubleshooting. This paper focuses on preventive maintenance. Follow a proactive maintenance plan Because of the high volume of revenue dependent upon DWDM systems, and also because of the more common practice of leasing fiber cables today, an ongoing, proactive maintenance plan is highly recommended to ensure continued DWDM network integrity. The main objective of preventive maintenance is to identify problems before they become service-affecting. It is no longer sufficient to rely on the SONET/SDH network s protection and fault isolation capabilities to ensure the network s reliability and uninterrupted service. An effective way to achieve DWDM network reliability is via conducting performance monitoring on the DWDM system. 8
Once service is up and running, a proactive approach to maintenance is recommended. Some DWDM system manufacturers recommend in-service monitoring on 6-month intervals, since aging of the DWDM system can lead to laser wavelength drift and fluctuation of power levels. In-service monitoring of the critical DWDM parameters using the monitoring jacks at the optical amplifiers, or rack-mounted splitters is an effective method of identifying problems before they affect customer s traffic. One recommended test is to obtain a graphical representation of the optical spectrum on 6- month intervals, and be able to save this information electronically (electronic fingerprint) for future analysis and comparison. This analysis can be achieved, for example, by overlaying optical spectrum traces of a system on a GUI. This can reveal any wavelength drift or channel peak power fluctuation in the DWDM system as it ages. TTC has found that the most common failures in DWDM systems to date often involve slight variations in the laser sources of these systems. Variations in wavelength can cause a channel to fall outside the passband of the optical filters in the system, creating a signal degradation condition, or even a total channel loss. Concerning channel peak power measurements, channel power fluctuations is an early sign of laser failure. In addition, many DWDM system components are very sensitive to temperature variations, and failures in the associated temperature-controlling sub-systems may lead to DWDM system malfunction. These are some of the potential failures that can occur during the life of the DWDM system. Any deviation of the key DWDM signal parameters (channel peak power, center wavelength and SNR) beyond the manufacturer s or service provider s specifications should trigger troubleshooting of the system. In addition to periodically monitoring the DWDM signals and system, the SONET/SDH layer in the transport system should be monitored periodically on an inservice basis by measuring the B1, B2, and B3 bytes as well as by checking presence of SONET/SDH alarms. This can be done at the same time periodic testing of the DWDM signals is done, by demuxing the DWDM signal down to the SONET/SDH layer. Since most DWDM systems have very limited visibility into the SONET/SDH layer of the signals, one way to achieve this monitoring is by using a test set that can see the DWDM layer and drop out individual wavelengths. Then, this test set can be used to filter out wavelengths, one at a time, for subsequent SONET/SDH overhead and alarms analysis with this same test set. This feature is invaluable during network troubleshooting that involves problem sectionalization thru DWDM spans. Case study 3: Long distance carrier deploying new OC-12 DWDM system with existing fiber plant as a way to enhance interoffice (IOF) capacity in their local network. As part of their high-speed network s preventative maintenance plan, technicians connected a DWDM channel monitor test set to the local monitoring access point for a regular checkup. They also did a quick check of the carried OC-12 signals by selecting and dropping wavelengths, one at a time, to a higher-speed SONET analyzer for further surveillance. While conducting this procedure, one technician noticed alarms in one of the dropped OC-12 SONET signals. After further analysis it was found that one of the OC-12 muxes had a faulty line card, and it was sending Line RDI alarms in the network. This prompted replacement of the card, clearing the false condition. The ability to select 9
and drop a DWDM wavelength out to a SONET analyzer allowed this carrier s technicians to preemptively detect and correct a problem that could have potentially resulted in a failure on an unprotected OC-12 signal in their network. Future trends in DWDM testing As service providers migrate toward all-optical networks, there will be a need to manage and monitor signals at the optical layer. To accomplish this, a technique known as digital wrappers is being developed. A digital wrapper is essentially a frame that encapsulates each wavelength and adds overhead information on a per-wavelength basis. This overhead would include information such as origin of a wavelength, destination of a wavelength, strength of a signal, whether the wavelength has passed certain points in the network, error detection and correction, automatic protection switching, and type of carried traffic. Digital wrappers would be added at the entry points of the wavelengths in an all-optical network, and removed at the exiting points of the network. In addition to transponder-based systems, DWDM systems without transponders (remodulators) are also currently being deployed in conjunction with new SONET/SDH systems. In this case, the laser in the SONET/SDH add-drop mux (ADM) is already tuned to a wavelength specified in the ITU-T grid (1550.12 nm for example) obviating the need for a transponder. Testing these DWDM systems is currently performed at the SONET/SDH ADM at a lower tributary rate, such as OC-12 in an OC-48 DWDM system. There is a need to BERT these systems at the DWDM rate; for example OC-48 rate. However, to BERT at the DWDM line rate, selectable wavelength lasers are required to modulate the line signal at the wavelengths specified in the ITU-T grid. Future test sets will require selectable wavelength lasers to validate these systems more rigorously. Another development that will have an impact on testing is the future deployment of wavelengths in what is known as the S-Band optical region, which is located in the (1470-1500) nm region. With thousands of dollars of revenue per hour depending on a single fiber, verification of DWDM systems in the field creates enormous testing challenges. Testing all these channels simultaneously, expeditiously, and automatically will be a requirement to guarantee system performance. This will require multi-channel test sets with splitters and optical amplifiers that incorporate wavelength selectable lasers. Summary Operating and maintaining a transport network with DWDM systems in place is a challenge. The large amount of traffic carried by a single strand of optical fiber in these systems, combined with ownership issues dictates proactive and rigorous testing, as well as quick, effective troubleshooting. An effective testing strategy and the right test tools can be of tremendous assistance as DWDM equipment and systems continue to migrate to both service provider networks and into technicians lives. 10
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