Unprecedented demand for fiber links and capacity are driving the deployment of high-fiber-count cables with hundreds or even thousands of fibers. These cables are being deployed by dark fiber providers as well as in traditional telecommunication networks to provide data center interconnections, metro connectivity between central offices, access network in support of FTTH, and edge computing for low latency 5G network applications.
There are real challenges associated with testing these cables. The processes that worked well for a few fibers are much less efficient when scaled to hundreds or thousands. Project managers are faced with a lot more data and reports to handle and verify/audit. Technicians are dealing with the sheer volume of connection/disconnection of the fibers being tested, the manual post-processing for bi-directional OTDR analysis, and “on the fly” spreadsheet tracking of test results. Additionally, reporting of project status and progress becomes inefficient and risks delays in handover of a network or project.
So, how can we create efficiencies that will allow bi-directional test to scale and what do those efficiencies deliver in terms of reductions to certification times?
First let’s take a brief look at an Old Legacy Method (OLM) to compare the approach and certification times with the very latest solutions available.
We will outline a generic Old Legacy Method (OLM) based on test equipment with separate test ports, one for IL and ORL and a second for OTDR (it could be that they are even separate pieces of test equipment altogether). The OLM also does not have the capacity for real time bi-directional OTDR analysis, meaning that for any project where bi-directional OTDR certification is required, there is additional work required to post process OTDR results, which adds time and cost (to be compliant with ITU-T and IEC standards, bi-directional OTDR test/analysis is required to certify a fiber link). Nor does the OLM have the any form of integrated project management, meaning that tracking project progress and pass/fail status is often recorded on separate spreadsheets, which requires manual entry and carries a risk of human error. Plus it interrupts the test flow, leading to poor workflow and lower tech efficiency.
Let’s look at a 12-fiber bundle for a comparison and perform bi-directional IL, ORL and OTDR testing at two wavelengths (1310 & 1550 nm). For this we’ll also look at a dual ended approach with two sets of test equipment and technicians.
So, what does the work or test flow look like? First things first, all 12 fiber connectors or patch panel ports must be inspected and, if necessary, cleaned.
Due to the separate test ports with OLM, there must be a first pass to make the IL and ORL measurements; this has to be synchronized with both instruments connected to either end of the same fiber at the same time. This means the techs have to be in almost continuous communication in order to coordinate the taking of measurements and moving to the next fiber. There are small delays, but delays nonetheless, as the test cord is moved and a test manually triggered. However, the instruments can typically take care of auto saving results, thus saving time and another step having to be manually triggered.
Once the IL/ORL pass has been made you can start on the OTDR tests. In order to save time and be efficient with workflow, some equipment vendors recommend an asynchronous approach where the tech with the main unit starts at fiber one while the tech at the remote end starts at fiber 12. They then simply work their way through the 12 fibers and cross over each other at the “middle” of the bundle around fiber 6 to 7. The only thing you have to watch out for is testing the same fiber at the same time (test collision) which will mess up the result. There are other approaches to test fiber in the same order, parallel 1 to 12, but offset the start of the tests by a few minutes so as to avoid test collisions. But for this comparison, we will stick with the fastest approach OLM can offer, which is the asynchronous one and hope that due to slight variances in the work speed/rate of each tech that there will be no test collisions.
This approach, however, will not fully comply with the TIA and IEC standards for bi-directional OTDR certification, a topic I explored in an earlier blog in this series. When it comes to performing the bi-directional OTDR analysis, you will only be able to evaluate the end connectors of the link with a uni-directional OTDR measurement, and as such the certification is non-standard. To comply and fully certify fiber links as per the standards, you need to use a receive cable for the OTDR test, which means an extra level of coordination is required during the OTDR tests. The main tech testing fiber 1 must ask the remote tech to connect a receive cable to the same fiber, the main tech has to do the same for the remote tech and connect a receive cable to fiber 12. Once the OTDR tests are complete the techs must again coordinate and move the receive cable to the next fiber to enable the testing to continue, so they have to be in almost continuous communication. There is a time associated with moving the receive cable and again there are delays that creep due to the interruption and manual restarting of the test process. Again, auto-save of results will help remove some delays. For this comparison, we’ve estimated OTDR test time of 30 seconds per fiber and the same again for the launch and receive cable management, one minute in total.
While all of this is going on the techs must manually track which fibers they have tested using a separate spreadsheet.
Once all the testing is complete, the results have to be submitted for post processing in order to perform the bi-directional OTDR analysis. This is usually done via USB stick and email or bulk upload to the cloud at the end of a day’s work. Depending on the amount of tidy-up work to correct file names (or other errors) and the volume of tests to process, it can take anything from a few hours to a few days to complete. For this comparison we’ve averaged and estimated an upload and processing time for a day’s work and broken that down to approximately 15 minutes to process a batch of 12 sets of results.
Time spent on each task and test was then grouped and is shown in the time comparison graph below. When using this asynchronous technique and post processing it gave an estimated total test and certification time of just under 40 minutes.
The alternative approach to this was explored in an earlier blog in this series, which looked at what methods could be adopted to reduce the whole test, processing and certification time. Essentially a single test port approach for IL, ORL, and OTDR testing. The integration of a multi-fiber switch and fan-out cable, use of more automation to coordinate the instruments and batch test fibers in a single test sequence, including the automatic use of a receive cable, and removing the need for results upload to the cloud and any bi-directional OTDR post processing.
If we assume the same IL/ORL and OTDR test times but remove the time and delays due to manually moving, coordinating, and testing through 12 fibers (this is now done via an optical switch) and the post processing effort (this is now done on the instrument via the fiber under test), the total test and certification time can be reduced to a little over 22 minutes, yielding a time savings of around 45%.
Further time reductions on subsequent batches of 12 fibers can also be achieved by performing tasks in parallel. Don’t just sit and wait for an automated sequence of 12 fibers to complete; use the time to inspect and clean the next set of 12 fibers so they are ready to go. You can also migrate over each test cord of the fan-out cable as the tests on that fiber are complete so that you are ready to test the next batch of 12 fibers almost immediately. This reduces the full certification time to a little over 14 minutes, yielding a time saving of over 60%.
TestEquity are an approved UK partner for VIAVI
The Impact of Fiber Test Equipment Implementation: Multi-Fiber Certification Time Studies
Content Source: https://blog.viavisolutions.com/2023/03/13/bi-dir-time-studies/
