Fiber is being pushed deeper and deeper into access networks worldwide as service providers continue to address subscriber demands for higher capacity services. Going “fiber deep” could be part of G.fast deployments for telco operators or Distributed Access Architecture (DAA) upgrades for cable companies, but the next step (or end step) is full fiber-to-the-premise (FTTP) or -home (FTTH) and for this providers are utilizing Passive Optical Networks (PON).

As mentioned in my previous post, the key to any service (in this case FTTH) performing as expected comes from having a reliable PON network, and the way to ensure a reliable network is to test and certify it properly during the build or construction phase. It all stems from there.

So how best to go about doing this? Are there any specific challenges that PON networks present (such as complete network end-to-end certification) and how can those challenges be overcome?

As you probably know, PONs are made of multiple components: feeder fibers connected to splitters, connected to drop terminals/fibers, ultimately connecting to a home or business, as shown in the diagram above. In order to ensure PON reliability from turn up through on-going maintenance, it is important to ensure it has been built correctly. Throughout the construction process, there are certification tests carried out (or there should be, otherwise you are obviously a gambling person and putting it all on red… or was it black?). Either way, without testing it’s a 50/50 chance that everything works. The feeder and distribution fibers should be tested in isolation (point-to-point) once they are laid and in a previous blog we’ve looked at why you might want to test the these fibers bi-directionally (Fiber Testing – Get the right perspective). If you are a contractor who’s been asked to do this by the network owner, we’ve also looked at ways to ensure that the test and certification time is minimized (Two ways to cut fiber test time in half). And in case you’re wondering if bi-directional test is really necessary, it is recommended in industry standards IEC 62316 & 60793-1-40 & TIA-FOTP-61.

Once feeder and distribution fibers are in place, the splitters are connected or more likely spliced into place. Then, the network needs to be tested end-to-end to make sure that there is end-to-end continuity, that the splitters are OK and that connectors/splices are all good. Typically this is done with an OTDR. Again, in previous posts we’ve looked at OTDR applications to simplify this (With FTTH/PON… Are you working smart) and as critically, solutions which adopt and automate a multiple test or acquisition approach using varying OTDR pulse widths, which then combine results into a single trace to make it easy to read and to submit for certification purposes.

The varying pulse widths are the key part to testing a PON end-to-end. The end-to-end test is performed only in one direction (uni-directional) from the subscriber end of the network and shooting back towards the local office. You need shorter OTDR pulse widths to test the initial drop/distribution fiber, but a shorter pulse may not have enough energy to pass through the splitter. So, to test and characterize the splitter, you need to test again and use a slightly longer pulse. Then there is the feeder fiber on the far side of the splitter, and in order to reach and test all the way to the end of that you’ll need to use an even longer test pulse.

The challenge with this uni-directional end-to-end test approach is partly the level of results accuracy at the far end, closer to the local office; for example, accuracy of loss measurements made on splices in the feeder fiber. The bigger part of the challenge is the individual test acquisition time and the overall testing time. Longer test pulses used to test the feeder fibers over longer distances and through splitters require longer acquisition times and longer testing times mean less tests per day and more time spent on site certifying the build(s).

So how do you address both the minor accuracy issue and the major time issue?

The answer is to adopt a bi-directional test approach and partially test the PON from each end of the network. Using two OTDRs will cut down on travel time or any other issues associated with relocating a tech and tester from the subscriber-side to the local office-side of the PON. Of course, this does raise the issue of gathering and matching up test results from two OTDRs and post-processing them to create a final end-to-end certification. This adds additional time and effort that usually negates any gains, so any approach like this must also include automation. The level of automation needed doesn’t stop at just the testing from each end of the PON. It must also coordinate the testing process to minimize operator delays with regard to starting tests, and turn it into a single push-button execution. Automation must also take care of the test result management and report generation. This means exchange of results between units, plus combining reports into a single trace/view/result/cert. The end result would be a dual-ended solution that requires only one tech to initiate testing, minimizing test time but also critically eliminates any post-test activity and processing required to retrieve and match test results and generate reports.

Bi-directional: it’s an adjective!

This is exactly what VIAVI offers with its end-to-end SmartLink Mapper (E2E-SLM) application. Locating one OTDR at the local office means you can test the feeder fiber using a shorter pulse width, meaning optimal results accuracy and minimal test time. At the same time, the subscriber end field unit tests the drop/distribution fiber and splitter and doesn’t have to concern itself with the feeder fiber (also reducing test time). Enabling communication between the units means that feeder testing can be triggered from the field unit at the subscriber side and takes care of the test result exchange, allowing report generation directly on the OTDR. This also means that the local office device can be left unattended, freeing up a tech to perform more testing from the subscriber side as it’s not a 1:1 implementation. A single local office unit can support a number subscriber-side field units.

So, by adopting a bi-directional approach to testing the completed PON network, you can significantly improve the quality/fidelity of the test result. This in turn certifies that you have a higher-quality end-to-end PON, while simultaneously saving time during the test and reporting/certifying process.

For more information about PON networks and services, take a look at our PON page or for a guide to what test is required through all phases of the networks lifecycle (build->network activation->service activation->maintenance>monitoring), download/order a copy of our Understanding PON Testing poster. In the meantime, look out for part 3 in this series: Certifying PON with Unbalanced Splitter Architecture.

 

Douglas Clague is currently solutions marketing manager for fiber optic field solutions at VIAVI. Doug has over 20 years of experience in test and measurement with a primary focus on fiber optics and cable technologies, supporting the telecommunications industry. Prior to VIAVI, Doug held positions as manufacturing engineer, solutions engineer and business development manager. Doug has participated on numerous industry panels around fiber and cable technology trends. He attended Brunel University in London and graduated with an honours degree in electrical and electronic engineering.

 

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