Anatomy of a ROADM Network (Part 2)—What Should You Test?
Gwen Amice, Senior Application Engineer
Bruno Giguère, Member of Technical Staff, Transport and Datacom Business Unit
Your network engineering team has decided to implement a reconfigurable optical add/drop multiplexer (ROADM) infrastructure. Now what? What issues will jump out at you at the wrong time as you move your project from technology investigation to field trials and finally deployment? You are creating a network topology that collapses multiple rings together. To what extent will this infrastructure affect your troubleshooting capability?
This article will discuss the testing requirements of a ROADM network based on the technology concept that was presented in Part 1 of this article. As in any network, the test requirements cover three different aspects: characterization of fiber, characterization of the light and, finally, characterization of the transmitted service. This is true for the network element but also for the complete end-to-end network.
Testing ROADM Network Elements during Site Turn-Up
Once the ROADM network element (NE) is all hooked up and ready to go, it must be tested from a functionality perspective. The first step is to check its optical characteristics. To do so, the ROADM needs to be configured for attenuation. By using a tunable laser source (TLS) connected to the east ingress port (see Figure 1), a signal is injected into the NE. The east egress port is then connected back, through a variable optical attenuator (VOA), to the west ingress port. Finally, an optical spectrum analyzer (OSA) is connected to the west egress port.
The TLS is then used to sweep through the different wavelengths supported by the NE. Since the number of wavelengths can vary between 32 and 72, depending on the network element vendor, this could become very time-consuming. By testing the first and last wavelength, the test time can be reduced while maintaining great accuracy. Setup will enable turn-up personnel to test the NE’s optical line amplifiers (OLA) and adjust them accordingly.
Figure 1. Testing ROADM network elements: optical line amplifiers
The second step in turning up a ROADM NE is to validate the express routes and add/drop ports. Again, by using a TLS, VOA and OSA, turn-up personnel can test the NE’s routes and ports; by connecting the TLS to the VOA and then to the east ingress port, it will be possible to simulate a low-power wavelength coming into the NE. The OSA must be connected to the east express route or east add/drop port. The TLS needs to sweep through all of the ROADM channels (used, unused and express). The OSA should measure each channel, and the channel loss results should be less than 5 dB with a linearity of 1 dB maximum for all channels. An example of this measurement can be found in Figure 2.
Figure 2. Testing ROADM network elements: add/drop and express routes
Once the ROADM NE is connected to the network and wavelengths are turned up, the performance needs to be characterized. By using the OSA and connecting to different ports of the NE, performance can be verified. An example of OSA traces at different NE monitoring points are represented in Figure 3.
Trace A provides the view from the drop port. The parameters that need to be monitored at this point are insertion loss, flatness level and channel isolation. A high insertion loss indicates a low or poor signal-to-noise ratio. The flatness level needs to be validated, especially if re-amplification is performed afterwards to drop to another part of the network, as the higher-power wavelength will be more amplified than the other wavelength. Finally, channel isolation must be measured for the channels going through the express path.
Trace B represents the view from the express path. Again, channel isolation is very important as it might have an impact on adjacent channels. If the next step is the addition of a wavelength, it is important that the drop wavelength does not appear on the express path.
Trace C provides a view of the add port. The most important measurements for this port are peak power and flatness. A peak power value that is too low on a channel will force the ROADM VOA to attenuate all channels, which would worsen the optical signal-to-noise ratio (OSNR).
The next measurement is the central wavelength. As optical filters are very precise, an offset wavelength will induce additional losses and again deteriorate the OSNR. Then, the second-mode suppression ratio measurement should be performed; this is important because a bad signal will “pollute” adjacent channels.
Figure 3. ROADM performance testing
Finally, Trace D provides the view of the egress port. This is where you will see if the calibration of the ROADM NE was successful. By looking at the flatness of the output, you will know if the VOA and optical power monitor (OPM) were calibrated correctly. The power level will also indicate issues with the NE. The VOA will equalize to the lowest power level, causing channel power anomalies.
Main Cause of Low Channel Power: Dirt
The OPM and VOA will regulate the overall tilt of the ROADM NE in order to get the same power per channel. Therefore, any bad connection or dirty connectors will have a drastic impact on the overall OSNR. Figure 4 demonstrates the difference between a clean, dirty and damaged connector. As ROADMs have an all-optical core, the only parameter that will greatly influence its performance is its cleanliness. Cleaning and connector inspection is vital!
Figure 4. Clean, Clean, Clean and Check!
ROADM OSNR—In-Band or Out-of-Band Measurement?
The answer is in-band. Why? Well, since there is no optical-to-electrical-to-optical conversion in a ROADM NE, the signal is not regenerated and, like the signal, the noise gets switched and amplified! This means that an out-of-band measurement will measure the amplitude of the signal compared to the noise floor of the ROADM NE, not the OSNR. For more information, please read our application note entitled: The ROADM Challenge and the In-Band OSNR Solution.
Testing the Tributaries of the ROADM Network
Once the optical core of the ROADM NE is turned up, the functionality of its transponder cards must be validated. As demonstrated in Figure 5, customer traffic is fed to the tributaries of the ROADM NE and groomed into a 10G OTN signal. Turning up these interfaces means that certain steps need to followed.
Traffic coming into a ROADM network must be aggregated efficiently in order to maximize each wavelength. Different services can be groomed into one wavelength using technologies such as generic framing procedure (GFP), virtual concatenation (VCAT), link-capacity adjustment scheme (LCAS) and optical transport network (OTN). These technologies provide the adaptation, containers and automation necessary to enable multiple services to run on the same network.
Figure 5. Anatomy of a ROADM network element
During network element turn-up, you will need to verify that the network element works according to defined specifications.
Interface Specification Test
The interface specification test is used to determine that the appropriate OTU rates are supported and that synchronization recovery can be properly achieved by the device under test (DUT). This test is essentially done to verify the proper interoperability of equipment from single and multiple vendors. As demonstrated in Figure 6, a test signal is sent to the tributary of the ROADM NE and physically loop backed from the optical egress to the ingress port and then resent to the test equipment. By measuring the optical power, frequency and frequency offset, we can ensure that the interfaces are working within specifications.
Figure 6. Interface specification test
Device Under Test Response Test
The device under test response test allows the user to use a stimulus (error or alarm) and detect if the DUT responds with the proper consequential actions for the injected errors or alarms. By verifying the response of the upstream and downstream DUTs, we can make sure that they work within specifications. Figure 7 provides an example of a loss of signal (LOS) injected at the tributary of the network element. In this particular network configuration, the LOS will provoke an optical channel transport unit alarm indication signal (OTU-AIS) on the network side and an optical channel transport unit backward defect indication (OTU-BDI).
Figure 7. DUT response test
Mapping/Demapping of Client Signals
The mapping/demapping of client signals test enables the test equipment to determine if the DUT correctly recovers (through stuffing) the optical channel payload unit (OPU) payload under the synchronous and asynchronous mapping specifications, in terms of the required frequency offset tolerance. In this test, an input client signal of 10 Gbit/s with varying offset is transmitted into the OPU by the DUT, as shown in Figure 8. This test is used to verify that the client signal has been properly mapped into the OPU without errors or alarms.
Figure 8. Mapping/demapping client signal test
Appropriate FEC Behavior
In order to determine the appropriate FEC behavior of a DUT, the test equipment generates correctable or uncorrectable FEC errors (distributed over the FEC portion of the frame). This ensures that errors are detected and reported at the G.709 NE.
It also ensures that correctable errors do not affect traffic. In the same type of setup as demonstrated in Figure 6, a number of errors are sent through the tributary. Once the signal is received at the egress of the tributary, it is analyzed for errors. If the number of errors exceeds the correction capability of the DUT, they will be reported as uncorrectable errors.
Many Fibers, Many Tests
An example of a ROADM network is depicted in Figure 9. The core of the network in the center of the green oval shows several multidegree ROADM network elements connected to each other through fiber links. In this example, any signal could be switched to any other ROADM network element. As network operators don’t know which link will be used by each wavelength, they must test all fiber links for chromatic dispersion (CD) and polarization mode dispersion (PMD).
Figure 9. Example of a ROADM network
Figures 10 and 11 illustrate a good example of why each link needs to be tested. In Figure 10, there are two different wavelengths connected to a multidegree ROADM NE. As each wavelength comes from different portions of the network, the resulting CD will be different at every wavelength.
Figure 10. ROADM impact on optical network
Figure 11. Impact of ROADMs on an optical network CD
Figure 11 provides a view of one of many possibilities at the receiver. Because of this network behavior, chromatic dispersion compensation must be perfect and adapted to each wavelength.
Final Turn-Up of Customer Circuit
Once the installation of network element is complete, it is time to turn up revenue-generating customer circuits. The first step is to verify that the different tributary cards have the proper configuration. Most turn-up issues are related to incorrect card configuration.
The second step is to ensure that there is end-to-end circuit continuity. By starting a BER test and by enabling and disabling the remote loopback, the installation specialist will ensure that end-to-end continuity exists. An example of network connectivity is illustrated in Figure 12. Note that instead of a remote location tributary loopback, another test instrument can be used. This remote test instrument will provide a way to segment the test into two directions instead of being round-trip. Should an error occur during turn-up, having two instruments will provide information on which direction is causing the problem, thus cutting down on troubleshooting time.
Figure 12. Final customer circuit turn-up test
The next steps provide additional information on the circuit being turned up. By testing the automatic protection switching (APS) capabilities of the network, the service provider will ensure that backup facilities exist and that service-level agreements can be met. Another additional test that can be conducted is round-trip latency. This measurement is quite important when delivering delay-sensitive services like cellular/mobile backhaul.
The final step for turn-up is long-term integrity testing, which is done through a 24-to 72-hour bit-error-rate test (BERT). By meeting the integrity specified in the SLA, service providers ensure that their revenue stream is not affected by their new installed services.
Conclusion
As WDM systems scale from city to continent, ROADM network elements provide the scalability, resiliency and simplicity required to collapse multiple regions into one network. The convergence of packet and optical circuit technologies has created the largest optical market area in multiservice provisioning platforms. Reductions in OPEX and CAPEX resulting from the use of optical systems have been enormous and will continue to evolve.
Although ROADM networks add flexibility to overall network architecture, network design engineers need to keep in mind that under this flexibility lies new complexity as different wavelengths can be routed to any path across the ROADM network. A fiber that was once used to carry 2.5 Gbit/s traffic might start delivering 10 Gbit/s or 40 Gbit/s signals.
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