Testing submarine communications networks: EXFO one-on-one

From the testing to the deployment of fiber cables, submarine communication networks come with many challenges. For one, the cost and time associated with submarine networks far exceed that of terrestrial networks—whether it is being deployed or repaired. Furthermore, the number of customers relying on these networks is vast, making a damaged cable critical to repair as soon as possible. Considering these factors, it is essential that these fiber cables be meticulously tested and re-tested before being deployed. TelecomEngine received some insight into the testing of submarine networks from Mai Abou-Shaban, 40G/100G product line manager, Transport and Datacom, at EXFO, including which tests are being performed to the overall process of testing and deploying a submarine network.


TelecomEngine: What are some of the challenges that come with testing submarine networks?


 Mai Abou-Shaban: The key challenging areas in submarine networks are at the physical layer and transport layer: as in the transmission system and the actual services carried on top. Some of the key submarine network requirements include the growing demand for low-latency networks to address packet-based services and more specifically, the time sensitive ones. Traffic-routing diversity is another requirement, as well as minimizing service disruption time when a fault happens—all of these are challenging areas that network operators are facing. From a testing perspective, EXFO provides testing solutions to address each of these challenging areas whether at the physical, transport or services layer; as well as at all stages- from the manufacturers’ development, verification and system integration, and finally, as part of a submarine networks commissioning, turn-up, troubleshooting and maintenance.


Why is it more expensive to repair and upgrade new submarine fiber compared to terrestrial fiber? What does that mean for network operators?


MA: When we talk about submarine networks one thing that we have to note is that the mean-time-to-repair (MTTR) is typically in the magnitude of a few weeks, if not months, compared to terrestrial network where the typical MTTR would be a few hours; hence the higher cost of repair.


This means that telecom operators need to take all required preventative actions. As they say, it’s better to be safe than sorry. Therefore, testing needs to be done at an early stage with the submarine cable manufacturers to capture chromatic dispersion (CD) and polarization-mode dispersion (PMD) of each spool of fiber. Then, these spools get re-tested in the submarine cable plant and repeater manufacturing plant to guarantee that the proper compensation is applied. Once the fiber sections are spliced together, another test is applied to verify the splice quality and to optimize the total CD and PMD of the fiber span. The physical layer testing extends also to cover assembly testing, load testing, lay testing and end-to-end commissioning that includes: optical signal-to-noise ratio (OSNR), bit error-rate testing (BERT) and round-trip delay (RTD) testing. Across all the different stages of manufacturing, deployment and commissioning, testing the accuracy and precision is crucial. For example, BERT is an industry standard test that is commonly conducted to qualify the integrity of the payload as part of the transport link commissioning procedure, and is typically run for 24 to 48 hours on a terrestrial network. However, in the case of the submarine networks, due to the high precision requirements, the same BER testing needs to be extended to seven days.


What are some tests EXFO uses in checking submarine networks?


MA: For physical layer testing, EXFO offers a wide range of optical solutions, including CD and PMD testing solutions, as well as optical spectrum analyzers (OSAs) for OSNR measurement, optical time-domain reflectometer (OTDR), optical attenuators, video fiber scopes and power meters—all to verify the quality of the fiber and actual link.


On the other side, EXFO offers a wide range of transport and datacom testing solutions typically used for link commissioning and service turn-up. These testing solutions support 10G, 40G and even 100G multiservice testing needs including bandwidth perspective, key transmission technologies and packet based services such as; SONET/SDH, OTN (as per ITU-T G.60), Ethernet and Fiber Channel. Again, these solutions offer key testing capabilities including; BER testing, automatic protection switching (APS) service disruption time (SDT) measurement, RTD or latency, RFC 2544 and the new Ethernet service activation standard EtherSAM (ITU-T Y.1564) among many other testing capabilities.


How does EXFO ensure that submarine tests are accurate?


MA: EXFO works with its customers starting from fiber and system manufacturers through networks operators and service providers to ensure that they have the proper testing solutions they need at every stage. Now, these offered testing solutions also vary from development and lab testing solutions to more field, deployment, troubleshooting and maintenance testing solutions.


For example, on one side of the spectrum thorough and advanced testing capabilities with multi-user access and automation capabilities are required by fiber and system manufacturers as part of their product development and verification stages in lab-type environment.  On the other side, there are the portable and simplified testing solutions with capabilities like remote management, battery operation and integrated optical tools, which are needed for field deployment and testing. 


Underwater cables are prone to damage. How can telecom operators monitor their cables constantly to know if the cable was damaged?


MA: Most system manufacturers offer, as part of their submarine transmission system, a network management capability. Sometimes they even support demarcation devices that get installed at different customer sites and allow customers to do end-to-end network and service monitoring. This monitoring system will report all the alarms and errors that happen or potentially might cause a defect. Typically and as part of EXFO’s testing offering, we can simulate and trigger faults- including alarms and errors- using our testing equipment to ensure that the system being tested responds back with the proper fault management information and that it reports back to the network management level.  In live networks, operators monitor their management systems and if they see any indications or potential area of fault, such as signal degradation, they take the required actions to troubleshoot the issue.


What does it mean to test the final acceptance of the fiber and why is it so important?


MA: It’s not only the final acceptance test that is important, but every stage of testing the submarine network is crucial. One thing that we need to keep in mind is that network operators are upgrading the network capacity to 40G and 100G, meaning more traffic is being curried and more customers are being impacted in the case of any fault. We all remember the 2006 earthquake in Taiwan, which left seven out of the available nine submarine cables damaged, as well as the Mediterranean undersea cable that failed four times in 2008. In both cases, the traffic carried on those damaged cables were down, which impacted customers and operators who needed to deal with service level agreements (SLAs). Therefore, testing submarine networks is key for manufacturers and operators who demand stable and resilient networks that are capable of handling any of the demanding services that may be placed on them. These test routines, especially now at 40G and 100G rates, when performed without the proper test equipment, can prove complex and cumbersome to perform.


 

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