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Benefits of Fiber Optic and Passive Optical LAN Test

Mark Mullins

In recent years, passive optical LANs have gained significant popularity as an alternative to horizontal copper structured cabling in a variety of enterprise spaces.

The technology brings fiber out of the riser backbone and data center, and with that comes the need for fiber technicians to test these systems out in the horizontal space.

Let’s take a closer look at these passive optical deployments.

How Do They Work?

Passive optical LANs are a point-to-multipoint fiber architecture that use passive optical splitters to divide the signal from one singlemode fiber into multiple fiber signals. The signals are transmitted simultaneously in both directions over separate wavelengths using wavelength division multiplexing (WDM) technology—1310nm for upstream data and 1490nm for downstream data.

Available in a variety of split ratios such as 1:8, 1:16, and 1:32, optical splitters basically serve the same purpose as a network switch, but they are not electrically powered—that’s why the technology is referred to as “passive.”

The singlemode fiber that arrives at the splitter originates at an optical line terminal (OLT) typically located in a data center or main equipment room.

From the splitter, multiple fibers connect to optical network terminals (ONTs) that convert the optical signal into multiple balanced signals for transmission over twisted-pair copper cabling to end devices.

What are the Benefits?

Because passive optical LANs use singlemode fiber, they are not limited by the 100-meter channel distance of copper but instead can reach distances of 20 kilometers. This is ideal for large facilities, or really any facility where 100 meters is not feasible.

In addition to eliminating the distance limitation, the primary cost-saving benefits of passive optical LANs include the ability to eliminate telecommunications rooms and the associated power and cooling infrastructure. The smaller, lighter singlemode fiber cables used in these systems also reduces pathway and space requirements.

Other benefits touted by proponents of passive optical LANs, and of fiber systems in general, include improved security and eliminating the crosstalk and EMI/RFI concerns associated with copper cabling.

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How are they Tested?

Just like any fiber optic system, a passive optical LAN requires insertion loss testing. And just like any fiber system, the overall channel loss is based on the end-to-end path between application specific equipment—the OLT and ONT in the case of the passive optical LAN. That means that everything in between—cable, connectors, splitters, and splices—attributes to loss. And just like any fiber optic system, connector cleanliness remains vital. That means the connectors should be inspected for contamination.

For passive optical LANs, the acceptable insertion loss is a minimum of 13dB and a maximum of 28dB at a 20km distance. The singlemode fiber used in a passive optical LAN should also be tested at both the 1310nm and 1490nm wavelengths. And test reference cords must include the angled polish contact (APC) style connector to match those used in passive optical LANs.

Best practices for passive optical LAN testing will be included in the upcoming international standard IEC 61280-4-3, which in keeping with existing TIA and ISO/IEC standards, specifies a light source/power meter for Tier 1 testing and an OTDR for Tier 2 testing in the upstream direction.


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