Tier 1 Testing using an OTDR on Installed Fiber Cabling?
The Telecommunications Industry Association (TIA) is committed, by formal consensus, to develop standards for the benefit of industry and is represented by hundreds of companies. While participating companies cooperate to develop standards, each have their commercial business interests to protect. Sometimes, these companies over-reach to meet their business goals. Fortunately, the TIA voting and balloting process works well as a means to check and balance sometimes rogue proposals.
As early as October of 2013, the notion of allowing an OTDR to be used instead of a light source power meter (LSPM) was introduced in TIA. Before that time, the LSPM was accepted for “Tier 1” testing (e.g. certification testing) while the OTDR was accepted for “Tier 2” testing (e.g., optional characterization of fiber optic installations). At the October 2014 TIA meeting, Tier 1 testing using an OTDR, to measure overall loss, was brought up again within the context of revisions to a testing standard, TIA-568.3-C.
By an overwhelming consensus vote, Tier 1 testing using a single-mode or multimode OTDR was NOT approved for TIA-568.3-C. The reasons the OTDR was rejected for Tier 1 testing were as follows:
- FAILED because data was shown by one OTDR supplier showing “correlation” with an LSPM – it is insufficient to change a standard based on one supplier’s data.
- FAILED because testing used a lab setup under best case conditions (i.e., external polarization controllers were used).
- FAILED because OTDR measurements were made in one direction. Bi-direction testing, is the preferred OTDR test method – unidirectional testing was used and assumes the launch and tail cords, needed to measure permanent link total loss, have the same optical characteristics. For practical reasons, this is not true.
- FAILED because testing did not account for the most common field problem – dirty connectors causing non-linear OTDR response. Dirty connectors cause strong reflections and tailing in OTDRs making measurements difficult.
- FAILED because it was assumed the multimode OTDR was encircled flux compliant as was the multimode LSPM – not all commercially available OTDR are encircled flux compliant.
- FAILED because the OTDR makes measurements based on relative/reflected power. The LSPM makes direct measurements and is more accurate. This is the reason why strong reflections at connectors cause OTDRs problems.
- FAILED because uncertainty calculations for OTDRs, because of complexity, have not been attempted by metrology experts in standards bodies.
With the due diligence and review of proposals by TIA members, the industry continues to be well served by new and evolving standards. These standards provide guidance for the installer, owners, contractors and others required to maintain fiber optical cabling. Despite proposals to position products in standards, attempts are frequently viewed as self-serving. This case study is one example.