A pending TIA/EIA document refines and defines test methods needed for fiber installed in building and premise networks.
January 1, 2004
The industry has long been working on clarifying standards for optical fiber testing. Much work has been done in recent months on explaining proper test procedures, which will be reflected in the publication of the TIA/EIA TSB140 document.
TSB 140 describes field-testing of length, optical loss and polarity in optical fiber cabling using an optical loss test set (OLTS) optical time domain reflectometer (OTDR) and a visual light source such as a visual fault locator (VFL).
Its purpose is to clarify optical fiber testing to ensure overall network integrity and performance by enhancing and defining the existing ANSI test standards for fiber installed in buildings and premises networks. (i.e. ANSI/TIA/EIA-526-7 for singlemode and ANSI/TIA/EIA-526-14A for multimode fiber).
TSB 140 will not replace ANSI/TIA/EIA-526-7 or ANSI/TIA/EIA-526-14A standards, which were written almost 20 years ago.
Rather, it will serve to enhance these testing procedures by refining and defining the test methods needed for fiber installed in buildings and premises networks.
One area of particular interest within the TSB 140 is explaining the methodology for testing of small form factor (SFF) connectors. At this point, TSB140 is undergoing final review and will address the SFF issue in updated annexes. It is expected to be ratified later this year.
About SFF Connectors: Introduced a number of years ago, Small Form Factor (SFF) connectors have a footprint similar in size to copper-based RJ-45 connectors, and are about half the size of conventional fiber connectors.
This reduced size helps users increase port density, reduce the cost of hubs and switches, lower patch-panel and enclosure costs, reduce jumper costs and improve design flexibility in datacom, networking and telecom applications.
The main driver behind the growth in SFF connectors is the increasing demand for smaller components in network systems. Since SFF connectors allow users to have considerably more interfaces on a single card, they increase the flexibility and versatility in network systems, and make them less expensive to maintain.
The industry has been working towards a standard SFF fiber connector for pluggable modules. There are a number of vendors currently fighting it out in the standards space, and at this point, there is no clear indication as to which will dominate.
Leading candidates include MT-RJ by Tyco, VF-45 by 3M, LC by Lucent and Opti-Jack from Panduit, each of which offers specific design features. A number of these optical fiber connectors emulate the RJ-45 8-pin modular connector, which is the most widely used in copper connections for private networks.
The MT-RJ was designed as a multi-source solution for smaller, less expensive fiber interconnect systems. It is a very small duplex connector with a moulded body and two fibers. While it can be used for epoxy-polish termination in the field, the current designs use ‘cleave and leave’ technology, with a pre-polished fiber stub.
The VF-45 is a fiber-to-the-desk product suitable for converters and switches, and is mainly used for multi-mode applications. It features a simple design that can be implemented at relatively low cost.
LC, which is mainly used in single-mode applications, is based on proven single fiber ferrule technology. There are standard epoxy and pre-polished quick mountable versions available for different installation needs.
Opti-Jack has been on the market for some time. It features a duplex design with a plug/jack configuration.
This connector has two 2.5 mm out-diameter ferrules within an 8-pin modular jack type housing. Its wider fiber spacing makes it ideal for transceiver applications and can be used in both single-mode and multi-mode applications. Field termination is easily accomplished with common optical fiber tools.
Other new offerings on the market include the LX.5 from ADC Telecommunications — a single-fiber connector that can be duplexed, as well as the SCDC/SCQC connector, which offers both dual-contact and quarto contact versions and can be interchangeable with the MT-RJ.
SFF Testing: SFF presents a unique challenge within the testing environment. Using single-fiber testers on dual-fiber connectors is awkward and prone to errors. Dual-fiber testers are preferred, especially when performing Small Form Factor testing. However, there are some special considerations to take into account that will be outlined below.
The testing of premises fiber optic cabling links requires precise methods for referencing to obtain accurate and valid results. The referencing procedure uses one patch cord per fiber link to be tested.
Since only one patch cord (per link) is part of the reference, the test results will include loss from the fiber cable under test, plus the connections at both ends.
One particular testing method provides an accurate measure of the loss in the fiber link. However, there are some shortcomings inherent in this method when testing SFF connectors that require some adaptations:
When going from the reference setup to the test setup, it is necessary to disconnect one end of the patch cord from the tester when using a single jumper method.
It is important to never disturb the connection at the output or source end, otherwise the reference is lost and will require re-referencing. Proceeding without re-referencing will seriously compromise the test result.
Therefore, it is essential to never disconnect the patch cord from the source (Output) end. However, to test SFF connectors that have transmit and receive fibers in the same connector, you must disconnect from the source (Output) end, which violates proper referencing and test procedures using a single jumper method.
When disconnecting the patch cord from the detector, dirt and other elements can easily cause damage to the detector.
Using this method requires that users have the same type of connector on the tester as they are testing in the fiber link. It is not uncommon to see multiple types of connectors in one installation.
While these problems can arise, adaptations can be made to the testing method to accommodate them.
The addition of a short “test jumper” with a connector will provide accurate test results that contain the loss for the fiber cable plus the connections on both ends.
This offers a number of advantages, including loss results that conform to specified standards, as well as the ability to use hybrid patch cables to connect test equipment to the links.
Ultimately, this process allows users to test different types of connectors, as well as eliminates the need to disconnect patch cords from the test equipment. This reduces the possibility of errors caused by reinsertion of patch cords or the risk of contamination or damage of test equipment fiber interfaces.
Without the proper equipment and methodology, testing fiber optic links to specified standards can be a complex exercise. The methods explained in TSB 140 can simplify the process with fiber test adapters that allow for accurate SFF link testing to ensure proper reference settings, regardless of the connector being used. This improved accuracy and efficiency will become even more critical as new standards come into play this year.
Brad Masterson is Canadian Product Manager for Fluke Networks and a member of the CNS Editorial Advisory Board. A Certified Engineering Technologist, he has been involved in the field of networking and network testing since 1995.