Meeting 10GBase-S loss budgets with MTP cassettes

Third-party testing demonstrates flexibility in 10G network designs.

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Third-party testing demonstrates flexibility in 10G network designs.

BY GARY BERNSTEIN, RCDD AND DENNIS MAINES, LEVITON NETWORK SOLUTIONS

10 Gigabit Ethernet (10GbE) introduces a challenging 2.6-dB insertion loss limit on OM3 fiber cabling. This tight limit has led some industry observers to suggest it is not possible to meet the standard using regular, as opposed to low-loss, MTP cassettes. Some have also suggested it is not possible to run fiber cabling the full 300-meter distance specified in the 10GBase-S standard when using MTP cassettes, whether those cassettes are standard or low-loss.

This article will describe independent testing contracted by Leviton and conducted by Intertek Testing Services, to perform benchmark testing of OM3 fiber system performance. The testing proved that the 10GBase-S standards can in fact be met with the latest generation of OM3 fiber systems using standard MTP cassettes, up to the full 300-meter length limit. In addition, the results show the ability of the tested system to support migration to 40- and 100-GbE applications.

Multimode standards and challenges

For many years 62.5/125-μm (OM1) and conventional 50/125-μm (OM2) multimode fiber have dominated shorter-distance communications applications, such as within a building or on a single-site campus. The fibers easily support applications ranging from 10-Mbit/sec Ethernet to Gigabit Ethernet. Their relatively large core sizes simplify connections and are ideal for use with low-cost light-emitting diode (LED) transmitters.

With the introduction of, and demand for, 10-GbE, typical in data centers, the physical limitations and properties of multimode optical fiber are being severely tested. Multimode fiber provides many allowable paths for light to travel down the fiber, and due to intermodal dispersion not all pulses arrive at the destination simultaneously. This characteristic, as well as other issues, ruled out the use of OM1 and OM2 fiber to support 10GbE.

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The Institute of Electrical and Electronics Engineers (IEEE; www.ieee.org) created the 10GBase-S ("S" standing for short wavelength) multimode fiber specification with the goal of providing 10GbE operating distances of 300 meters. Laser-optimized OM3 fiber was specified in TIA-492AAAC to support both the IEEE standard and economical vertical-cavity surface-emitting lasers (VCSELs) that operate at the 850-nanometer and 1300-nm wavelengths. Fiber that meets the standard delivers an effective modal bandwidth of 2000 MHz•km at a distance of 300 meters.

The TIA-568-C.0 standard allows for a maximum 2.6-dB insertion loss and 300-meter length with OM3 fiber cabling systems. While manufacturers frequently submit their copper cables for third-party testing to verify performance that meets or exceeds applicable standards, they typically have not had fiber cabling systems, including OM3, tested. This lack of independent verification can leave designers and operators of OM3 fiber networks uncertain what the true parameters are in designing premises applications.

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The tests conducted by Intertek were set up as shown here, in accordance with TIA-568-C.0 Generic Telecommunications Cabling for Customer Premises, Annex E: Optical Fiber Field Test Guidelines.

For example, the maximum insertion loss for MTP-LC cassettes or modules as stated in the TIA specification is 1.5 dB. An unusually complicated–but not extreme–configuration might consist of four cassettes with a total of eight mated pairs. If each of these cassettes was at the headroom limit, then the insertion loss of 6.0 dB would exceed the allowable limit. Some vendors have addressed this possibility by offering low-loss cassettes. These cassettes typically come at a cost premium of approximately 50% over standard cassettes.

Channels approaching the 300-meter maximum allowable distance for 10GbE have the same potential issue. Insertion loss for OM3 cable is typically 3.0 dB per kilometer. The TIA standard allows 0.75-dB insertion loss per connector mated pair. For example, a 300-meter channel configuration with six mated pairs could have a maximum insertion loss of 3.9 dB, well above the 2.6-dB limit. Undoubtedly, this explains why some vendors have recommended that users stay well below the 300-meter standard-specified length limit.

Putting fiber to the test

How much headroom does a 50/125-μm laser-optimized fiber system actually provide relative to the specification, and what is the impact to the network designer? To address these questions, we at Leviton contracted with Intertek Testing Services to test our OM3 fiber system including MTP trunk cables, MTP-LC cassettes and harnesses, MTP adapter brackets and LC-LC OM3 patch cords. The testing included three different OM3 channel configurations, as follows.

300 meters, 6 mated pairs
53 meters, 8 mated pairs
62 meters, 9 mated pairs

All channels were set up and bidirectionally tested with readings measured against maximum allowable insertion loss as specified in TIA-568-C.0, using Annex E guidelines. Specifically, Intertek performed the forward-direction link measurement by placing a reference-quality jumper on a Rifocs-brand optical meter and measuring the link insertion loss by connecting the test jumper of the source to one end of the cabling link, and the test jumper of the meter to the cabling link's other end. Both ends of the link were disconnected and the ends were interchanged and reconnected to perform the reverse-direction measurements.

For Configuration 1, the 300-meter channel with 6 mated pairs, the average maximum channel insertion loss was 2.04 dB, providing a 0.56-dB margin under the 2.6-dB limit.

For Configuration 2, the 53-meter channel with 8 mated pairs, the average maximum channel insertion loss was 1.83 dB, providing a 0.77-dB margin under the 2.6-dB limit.

For Configuration 3, the 62-meter channel with 9 mated pairs, the average maximum channel insertion loss was 1.87 dB, providing a 0.73-dB margin under the 2.6-dB limit.

All three configurations were found to comply with all applicable criteria of the TIA-568-C.0 standard. The connectors tested not only meet the maximum published performance requirements, but also the insertion loss specifications spelled out in the table.

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Configuration 1, set up as shown here, was a 300-meter channel with 6 mated pairs.
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Configuration 2, set up as shown here, was a 53-meter channel with 8 mated pairs.
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Configuration 3, set up as shown here, was a 62-meter channel with 9 mated pairs.

It is clear that 10GbE specifications can be met without the additional cost of low-loss cassettes. These tests, providing independent verification of OM3 fiber channel performance, show substantial margin under TIA insertion-loss limits for 10GbE using standard cassettes. With the assurance of these results, network designers have the freedom to use the complete 10GBase-S standard including distances up to 300 meters and the maximum number of specified connectors.

In addition, the test results show that the latest-generation OM3 fiber systems provide a solid foundation for future installations of 40GbE and 100GbE. The IEEE approved the 802.3ba standard in June 2010; it specifies a maximum insertion loss of 1.9 dB for 40/100GbE using OM3 cabling. The systems tested here are already right on the border of meeting that specification, indicating that organizations will be able to upgrade with either no or very limited system changes.


Gary Bernstein, RCDD is director of product management, fiber and data center and Dennis Maines is senior applications engineer, fiber and data center, with Leviton Network Solutions (www.leviton.com). This article is based on their white paper "Overcoming OM3 performance challenges."

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