Autumn 2019 - Optical Connections Magazine


two ways are better than one T echniques of bidirectional testing have been developed across the industry over the past 15 years or so and, aside from predominantly one-way applications, such as media broadcasting, it is now the commonest form of GREATER ACCURACY “When you make bidirectional measurements you improve the results because you have two values instead of one so your accuracy is better. Also, on return loss, which comes from bidrectional testing is necessary because significant differences in transmission quality can have a detrimental effect on the overall performance of the network.” 400GE TESTING Keysight Technologies gave a joint demonstration of Ixia’s AresOne-400 Gigabit Ethernet test system and BIDIRECTIONAL TESTING

different connections in the link, such measurements give you information about outward and return paths. Furthermore, using OTDR, bidirectional testing guarantees you the correct spliceless measurements. In a lot of network cases now, there are different fibre types deployed in different sections from upgrades, extensions and so on. Bidirectional testing tells us where there are different fibre types. If they are spliced together then losses can be different each way; even a loss one way and a gain the other,” she says. Another take on bidrectional testing comes from Andy Moorwood, VP hardware engineering, Network Test Equipment, at Ixia: “Looking at the lowest layer and considering the wide area network, LFPs are inherently unidirectional and may not be co-routed. So, there may be a short path in one direction and a long path in the other direction.” He adds, “Understanding even a Bit Error Rate Test (BERT), the performance of those two paths is important. As you move up to the higher layers, where we get involved with setting up sessions and so forth, notably, layers 4-7 testing, then

network testing. Bidirectional testing is now universally applied to testing for problems such as signal loss and analysis of chromatic dispersion. Ken Gold, director of Test, Monitoring & Analytics Solutions at Exfo, told Optical Connections, “Bidirectional testing is typically a part of 24-7 network operations, especially with virtualised, cloud-connected networks. Networks are becoming commodities so this type of testing is critical to managing customer experience.” idea of sending out a test signal from a portable OTDR system and measuring the reflected signal to identify potential fault locations? Charlene Roux, product line manager at Viavi Solutions, based in Lyon, France, explains, “We want to make sure that when a new construction is handed over to the service provider that it has been properly done, to minimise the impact on measurement errors and to give better accuracy, so characterising the link needs to be performed bi-directionally.” So why has bidirectional testing superseded the historic (one-way)

the Marvell Prestera CX 8580 Ethernet switch at the Open Compute Project Global Summit, in March at the San Jose Convention Center. This demonstration, which showcased the maximum performance capabilities of new 400GE, PAM4-based silicon, was said by the partners to be “the first-of-its-kind full box 400GE test at 12.8 Tbps.” Michael Scholten senior product marketing manager at AFL Global, the diversified communications, networking and testing company, told Optical Connections, “End-to-end bidirectional optical loss test and Optical Time Domain Reflectometry (OTDR) testing is often required by network service providers when deploying and verifying new point-to-point fibre networks, or when deploying point-to-point fibre segments of FTTH PONs. Bidirectional testing achieves the most accurate loss measurements, verifying end-to-end optical loss and return loss limits are not exceeded, checking splice and connector losses and that reflectance limits are met. It ensures the installed network will


| ISSUE 18 | Q3 2019

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