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  • PAGE 58 CBM DECEMBER 1998

    The centralized optical fibernetwork design consolidatesall network electronics in asingle main cross-connect (MC) andprovides direct connectionsbetween the MC and workstations.When compared with convention-al, decentralized premises data net-works, this design minimizessignificantly the number oftelecommunications closets (TCs),connectors and splices needed.Consequently, the centralized opti-cal fiber design greatly simplifiesnetwork testing.

    With optical fiber cabling sys-tems, the properties most fre-quently tested are continuity andattenuation. Simply put, continuityshows that a link will pass lightfrom one end to the other, andattenuation measures how muchlight passes from one end of a linkto the other. Usually, only attenua-tion testing is performed becauseit also proves continuity. However,a continuity check can be per-formed very quickly and easilyprior to attenuation testing to veri-fy that fibers are terminated in theright positions and labeled cor-rectly.

    A continuity test can be per-formed on multimode fibers with aflashlight, a walkie-talkie set andtwo people. No tone generator oramplifiers are necessary. Just selectthe color-coded fiber to be tested,darken the area, and shine the flash-light at the end of the fiber. Confir-mation from a partner, in adarkened area at the other end ofthe fiber, indicates link continuity.Attenuation testing is nearly as sim-ple. (Note: To protect your eye-sight, do not look directly into theend of a fiber that may be part of asystem using lasers.)

    The primary parameter field-test-ed in optical fiber cables is attenua-tion. This is the optical power loss,expressed in decibels (dB),between cable termination endpoints. Attenuation is the sum ofloss caused by the physical proper-ties of fiber, splices, connectors,couplers and switches. End-to-endattenuation testing involves using alight source and a power meter tomeasure optical power lossbetween cable termination points.

    In a more sophisticated test, anoptical time domain reflectometer(OTDR) measures and documentsdiscretely all points of loss along acabling link.

    Attenuation testing after installa-tion ensures the network cablingmeets established attenuation spec-ifications and link loss power bud-gets. Documenting the test resultsprovides an invaluable as-builtrecord of the system, useful for anysubsequent troubleshooting oraccountability.

    Acceptance values and test pro-cedures for optical fiber cablingsystems are specified in Annex H,Optical Fiber Link PerformanceTesting, of ANSI/TIA/EIA 568A, theCommercial Building Telecommuni-cations Cabling Standard. Testequipment is accurate and easy touse.

    End-to-end attenuation is per-formed using the insertion lossmethod, comparing the power oflight injected into the fiber at the

    FIBEROPTICS

    by Douglas E. Harshbarger and George H. Sellard

    A Tutorial on Centralized Optical FiberCabling Networks

    Testing and DocumentingAttenuation

    Standards

    Measuring End-to-EndAttenuation

    Part IV

  • DECEMBER 1998 CBM PAGE 59

    near end (P reference) and light exiting atthe far end (P test). A stabilized lightsource, handheld power meter, test cordsand an adapter are required. Testingshould be performed in accordance withTIA/EIA standard procedures: TIA/EIA526-14A, Method B for multimode fiber;TIA/EIA 526-7, Method A.1 for single-mode fiber.

    Attenuation testing begins with clean-ing connectors and adapters and zeroingout the equipment to establish a refer-ence measurement. This is accomplishedby transmitting a reference power levelover a test jumper cable to the powermeter, and then delivering a check powerlevel over the test jumper connected toan adapter and a second test jumper con-

    nected to the power meter. (See Figure1.) This check power level must be with-in 0.5 dB of the reference.

    Next, an optical source/test jumper isconnected to one end of the fiber to betested, and a power meter/test jumper isconnected to the other end. The resultingpower reading is subtracted from the ref-erence measurement to obtain the end-to-end attenuation (Figure 2).

    Analysis with an optical time domainreflectometer (OTDR) verifies acceptablelosses through the cable and connector orsplice joints. The OTDR also will detectany unexpected and potentially trouble-

    some sources of loss, such as a very tightcable bend. In addition, the OTDR mea-sures and documents cable footage,which is a helpful piece of data.

    By sending pulses of laser light througha fiber and measuring the level and timedelay of the pulses as they return, theOTDR pinpoints where and how lossoccurs. The resulting signal trace depictsthe optical power as it is attenuated overdistance by the fiber, connectors andsplices. (See Figure 3.) The signal tracedocuments the integrity of the system asbuilt and provides a baseline for futuretroubleshooting. This record could proveto be a critical cost-saver, as digging upinterbuilding cable to correct mistakes isexpensive.

    The centralized design just makeseverything easier and faster, said CharlesB. Orr of Sellard Communications, whohas installed and tested a number of cen-tralized optical fiber networks. With

    direct connections between the singlemain cross-connect and workstations,setup for testing is done only one time.

    The biggest advantage is not having tomove anything around. Orr said. Youonly set up once. With a conventional net-work design,you have to test between thecomputer room and the horizontal cross-connect (HC). Then you have to move tothe HC to test the cable between the HCand the workstations. You have to movepeople, equipment and documentation.

    Also, youre supposed to zero out themeters when you turn them off and onagain, Orr continued. When you test acentralized design, you dont have to turnthe meters off until youre done. Thatsaves time and trouble.

    ConnectorConnector

    -6.10 dBm

    dBm

    Light Direction

    Test Jumper No. 1

    Power Received = P (dBm)ref

    1: Reference Setup

    2: Jumper Check

    InterconnectionSleeve

    Power Received = P check

    OS OM

    -6.70 dBm

    dBm

    Connector Connector

    Test Jumper No. 1

    Test Jumper No. 2

    Connector Connector

    OS OM

    Figure 1 Zeroing out test equipment includes taking a reference measurement with one jumper, and then compar-ing that measurement to another taken with an adapter and a second jumper.

    Test Jumper No. 2Connector

    Test Jumper No. 1

    Connector Connector

    OS

    -9.30 dBm

    dBm

    Connector

    OM

    Connector Connector

    PatchPanel

    PatchPanel

    System

    Figure 2 End-to-end attenuation testing: Light from an optical source (left) is sent through a fiber to an opticalmeter (right) where a power reading is recorded. This reading is compared with the reference measurement toobtain the end-to-end attenuation of the fiber link.Attenuation testing

    begins with cleaningconnectors andadapters and zeroingout the equipment toestablish a referencemeasurement.

    How the Centralized DesignSimplifies Testing

    Testing with an Optical Time

    Domain Reflectometer

  • PAGE 60 CBM DECEMBER 1998

    Completing all testing from a singlelocation not only saves time and effort,but it also improves the accuracy of mea-surements, according to Orr.

    Its recommended that your light

    source remain stationary during testingbecause any jostling can change the refer-ences, Orr said. So staying in one placehelps.

    According to Orr, OTDR testing of cen-

    tralized networks is completed onlyrarely, due to generally short distances inhorizontal cabling.

    If theres trouble and you think theresa broken fiber, you would do the OTDRtest. But that doesnt happen very often.Youve got to get really aggressive withthose cables to break anything, he said.Actually, copper cable is more sensitiveto bend radius than fiber is.

    When OTDR testing is necessary, a cen-tralized design simplifies this proceduretoo, because of the single setup. Andyoure not moving a $30,000 piece ofequipment around, Orr said.

    Testing optical fiber involves workingwith tried-and-true methods, which areuncomplicated and reliable. The central-ized network design makes that proce-dure even simpler.

    Douglas E. Harshbarger, RCDD, ismarket development engineering man-ager, premises systems, for Corning Inc.George Sellard is president of SellardCommunications. Special thanks toPatrick Scanlon of Rochester Institute ofTechnology for his assistance.

    All figures are from Siecor UniversalTransport System Design Guide, ReleaseIII, Siecor Corp., 1995.

    CBM

    Distance (feet or meters)0 300 600 900 1200

    Relat

    ive Lo

    ss (d

    B)OTDR

    Test FiberBox

    Patch Panel Fusion SpliceMechanical

    SplicePatch Panel(Fiber End)

    A B C D E

    Figure 3 In this sample OTDR signal trace, slopes indicate gradual attenuation over a section of fiber, while verticaldrops represent point losses at connectors, splices and faults. Spikes indicate reflective events such as connectors.The final spike marks the end of the fiber.