Single-Unit vs Dual-Unit Test for Bi-Directional OTDR Certification


One of the challenges with bi-directional (bi-dir) OTDR testing is the logistics and the amount of affordable, available, or required test equipment.

A single-unit approach is usually considered to try and save on cost—one piece of test equipment versus two and one tech versus two—but it has its drawbacks. For one, it does not verify continuity. This is a small but important thing, as verifying continuity ensures that you are testing the same fiber in both directions and that the link is terminating at the correct patch panel (or other) location. But that’s not the biggest issue. With a single-unit approach, a tech (and instrument) must test fibers from one end of a link, then relocate to the other end of a link and test from the other direction. What’s missing here is the implementation and connection of a receive fiber. Basically, however you do it, you’re going to need a second tech.

A fiber link that has been bi-dir OTDR tested but without the use of a receive fiber means you can perform bi-dir analysis (loss averaging) for the link elements (i.e., splices) but only uni-directional (uni-dir) for the end connectors. This means you have not fully bi-directionally certified the link and as such it’s a non-standard certification.

So how to tackle this with just one OTDR?

You could go for loopback approach, which enables the tech with the OTDR to manage connection of a receive cable, but it means an extra tech is needed at the far end to manage connection of a loopback device. But the bigger issue with loopback testing is you can only test half the distance that your OTDR is capable of as you are testing two fibers at the same time.

You must also ensure that the method used for bi-dir OTDR testing includes the use of both launch and receive fibers so that you can measure and characterise the connector ends of a link. In that way you are fully compliant and testing to the industry standards.

Why would any dark fiber customer, service provider, or network owner want to take ownership of fiber that hasn’t been certified correctly/fully to a known standard? The answer is they wouldn’t (or at least they shouldn’t).

Some single unit approaches are just plain wrong, and by that I mean they don’t adhere fully to the standards for bi-dir OTDR test. These are sometimes referred to as asynchronous, and can omit receive fibers, which can leave you unable to perform the averaging of measurements for those end connectors and leave you with an incomplete set of bi-dir results and therefore incomplete or non-standard certification.

The VIAVI approach with two instruments takes care of all of this exactly how the standards specify and does it with no extra manual effort or interruption to test flow and workflow.

In a future blog in this series I’ll look at bi-directional test processes using two instruments and how to achieve full bi-directional fiber certification (IL, ORL and OTDR) in the most efficient way.

TestEquity are an approved UK partner for VIAVI
Content Source: