My TN Connection

I read an interesting article in the October 2007 issue of Cabling Installation and Maintenance magazine. The article, entitled Using an OTDR: How to keep it simple, does a good job of explaining how you can easily use a tool that is often thought of as a very intimidating piece of equipment.

What is an OTDR
An OTDR (optical time-domain reflectometer) is a tool used by cable contractors to test and certify their fiber installations. It is also used by Telcos and Service Providers to troubleshoot fiber issues in their large public networks. While this tool can be expensive, it is often justified based on their high volume of work involving fiber optic cabling. Yet, a network administrator of a smaller enterprise level network could benefit from the use a tool like an OTDR, even if they only need it once or twice a year. These infrequent users can still affordably gain access to this equipment through rental companies who often rent OTDR’s on a weekly basis.

Calculating Link Loss
OTDR’s can perform a loss-length test similar to an OLTS (optical loss test set) device, which provides the amount of signal loss over a particular fiber link. Factors that contribute to signal loss include cable attenuation, connectors, and splices. This is valuable information when selecting fiber based networking devices. Media converters, for example, are used to make connections between new fiber being installed in a network and the existing twisted pair based devices. Most media converter manufactures will publish link budget statistics for each of their products. A link budget is the amount of light available to transmit a signal. It’s necessary for the link budget of a particular media converter to be greater then the amount of signal loss measured by the OTDR for the given fiber link. When subtracting a signal loss from a link budget results in a positive number, the media converter has enough power to drive the signal over that particular fiber link. This is especially important in long-haul fiber transmissions.

OTDR’s provide other information in addition to the loss-length test, such as:

• Single-ended testing of fibers
• Measuring the distance to the end of the fiber
• Checking the quality of each piece of the fiber link
• Documenting characteristics of the fiber link
• Troubleshooting faulty fiber links

If you are a fiber veteran or just starting to work with fiber cabling, the article in this month’s Cabling Installation and Maintenance is a great resource to increase your fiber testing knowledge.

Additional sources of information
Fluke Networks is a manufacture of cable test equipment and they offer several on-demand webcasts including “Testing and Troubleshooting a Premises Fiber Network with OTDR Technology”. If you are interested in OTDR testing this webcast is worth attending.

Also, Transition Networks offers a Fiber Optic Basics white paper that discusses the basics of fiber optic technology. If you would like to get an easy introduction to calculating link budgets, without the expense of buying or renting an OTDR, this white paper provides the guidelines of how to manually calculate link budgets without the use of test equipment.

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