Testing new equipment and proper techniques guarantee fiber performance. Installers should follow the TIA/EIA buildings test method, which closely correlates with current LAN requirements.
BY BOB JENSEN
Fiber certainly has the bandwidth to support the higher-speed protocols increasingly demanded by companies today. But the performance-dependent protocols such as Gigabit Ethernet leave such small margins for error in the loss budget, that accurate test results are a necessity. To make sure LANs provide optimum performance, it's critical that installers use the right test methods and have the right equipment.
One of the long-standing benefits of deploying fiber in LANs is that it has always be en relatively easy to test, depending largely on attenuation measurements. However, in high-performance LANs, how attenuation is calculated is very important. According to recent re search by 3M, approximately 77% of LAN installers currently use a test method that's not appropriate for premises installations, one that can underestimate the attenuation loss in a system.
ANSI/TIA/EIA-526-14A specifies two test methods for multimode cabling: "Method A", access-provider testing, and "Method B", buildings testing. Access-provider testing is typically used by telecommunications companies and long-haul carriers. Since connector loss in these applications is not an issue, this technique measures the loss for the cable-but only accounts for one connector. Unfortunately, when used to measure attenuation in premises networks, this omission can skew the results of attenuation calculations. When referencing test instruments to this method, not taking into account the loss from the other connector can result in a 2% error level. Consequently, depending on the reference method chosen, the error level could range from 3% to 25%.
For results that correlate more accurately with current LAN requirements, installers should use the buildings test method, which takes into account the loss from both connectors. Using this method, one of the test cords is referenced so that the cable can be tested once an installer is dispatched to each end. The referenced cable remains with the source, and another test cord is placed in the test configuration, clean and free of defect. Effectively, the test results provide the loss for two connections and the cable, while the test-cord loss is negligible.
When the results from these two test methods are factored into a loss budget for an entire network, the difference can be considerable. Taxing the maximum attenuation for cable and connections, as specified in ANSI/TIA/ EIA-568A, let's ex amine an example of a three-story building with a cable extending from a centralized equipment room to a workstation that involves four connections. Measurements taken using the access-provider method incorporate three connections into the loss readings plus the cable, resulting in a total measurement of 2.56 dB. The buildings test method includes all four connections plus the cable, resulting in a total loss of 3.31 dB. This discrepancy could be detrimental to the services installed, especially if the channel is used for Gigabit Ethernet, where the specified cabling-loss limit is 3.2 dB.
While the buildings test method provides the most accurate measure of the loss in premises fiber links, this method does have some drawbacks related to ease-of-use and testing flexibility. Test results could be compromised if installers disturb the connection at the source, which could occur when moving the testing setup.
This disturbance could happen if the test cord is disconnected from the source instead of the connector, or if dirt or dust contaminates the detector.
In LANs that use small-form-factor (SFF) connectors with transmit and receive fiber in the same unit, a modified buildings test procedure can be used that overcomes some of the shortcomings listed above.
In this scenario, the test set functions as both a source and meter. The duplex cable and connectors are connected, and installers take a reference using two jumpers, referencing both test cords and one connection. Once again, a defect-free patch cord must be placed in the test configuration. That yields the same outcome as the buildings test method, because the results include the loss for two connections and the cable, with the additional test cord adding negligible loss.
This adaptation of the buildings test method does not require the same type of connector on the tester as that which is tested in the network; the two jumpers used for referencing allow for the use of hybrid patch cords. Consequently, this test method provides the flexibility to test fiber links with any type of connector, regardless of the connector on the test set. While this test procedure is a modification of the TIA-568A test recommendation, it is the approach endorsed by SFF connector manufacturers for the testing of SFF connector links.
The premises market requires updated test instruments to address new applications and the needs of users. These instruments should be fast, durable, handheld units that are intuitive to the technician. Most of the optical time-domain reflectometers (OTDRs) available for use in premises networks are large, a drawback that will hopefully be remedied in next-generation equipment. OTDRs and source and power meters should also have a familiar feel and software to reduce training time. Documentation is important, because the software needs to incorporate cable-management utilities and be downloadable to cable-management software packages. Similarly, a power meter should automatically provide a zero reference on every test.
The capability to measure specific parameters with both a light-emitting diode and a vertical-cavity surface-emitting laser to verify Gigabit Ethernet performance is an other requirement. Since OTDRs are used in trouble shooting, a visible light source em bedded in the unit will also work to the installer's ad vantage. While OTDRs must continue to measure fiber length as well as the location and severity of events, the results need to be shown graphically-with an auto mated adjusting of the trace on a screen and in a table format.
Testing requirements for fiber-optic cabling continue to change. As a result, new procedures and test equipment are needed for premises cabling. The test equipment should accommodate both the access-provider method and the buildings approach. The latter approach ("Method B") specified in ANSI/TIA/EIA-568A, is the preferred testing method for buildings, or premises networks. However, an adaptive approach using an additional test jumper is recommended for testing SFF connectors and for testing cabling setups involving different connector types. With greater awareness of this test methodology, accurate measurements of link performance for Gigabit Ethernet or multigigabit-over-fiber can be ensured.
Bob Jensen is technical-services team leader for volition network solutions at 3M Telecom Systems Div. (Austin, TX). He wrote this column on behalf of the TIA Fiber Optics LAN Section (FOLS). FOLS member companies include 3M, AMP, Allied Telesyn, Berk-Tek, Belden Wire & Cable, Comm Scope, Corning, LANCAST, Lucent Technologies, MicroLinear, Ortronics, Panduit, Siecor, Siemon Co., SpecTran, Sumitomo Electric Lightwave, and Transition Networks. For more information from FOLS, please visit www.fols.org.