This section includes:

a. Testing procedures for verifying horizontal optical fiber cabling after initial installation, and

b. Troubleshooting tips for systems that have been up-and-running. Backbone cabling verification is not covered in this document.

 

4T.1. SYSTEM VERIFICATION TESTING AFTER INITIAL INSTALLATION

Benefits of verification testing. Testing affirms that the loss does not exceed acceptable limits, and ensures that the cable system meets the user's attenuation specifications. It also provides documentation that will be essential to a troubleshooter in gauging whether or not the readings he is getting are normal.

4T.1.1. TEST EQUIPMENT

For basic testing you will need the following equipment: a power meter, an optical source, two test jumpers (of the same connector type and fiber core size), and an interconnection sleeve. You will also need one of the following instruments to determine the exact location of any faults you find: an OTDR (optical time domain reflectometer), fault finder, or visual tracer.

4T.1.2. REQUIRED TESTS

Testing the optical properties of fiber optic cable involves measuring two characteristics: attenuation and bandwidth.

Attenuation is the measure of signal loss during its travel through the cable, from transmitter to receiver. A small amount of loss is unavoidable, acceptable, and not noticeable by the data. But because the number of spliced/connections can have an effect (more interruptions = more chance for loss), and workmanship/handling is another contributing factor on performance, it is important to test after installation to ensure the cabling system is performing to specification. TIA 568A compliance requires an end-to-end attenuation test with results within the published specifications.

Bandwidth is a measure of the information-carrying capacity of the cable. The quality and length of the fiber determine bandwidth. Installer handling has no affect on this. It is important that a cable system's bandwidth provide the information-carrying capacity required by the end-user. Bandwidth can be verified by simply affirming the documented specifications of the installed cable type. An actual field test is only necessary if this is not sufficient to determine bandwidth, or if the installer's practice is to run a field test anyway, or if the end-user requires it.

4T.1.3. TESTING PROCEDURE

You will want to read the test equipment manufacturer's instructions for testing, but here is the general procedure that complies with EIA/TIA-526-14, Method B: Optical Power Loss Measurements of Installed Multimode Fiber Cable Plant.

Before testing, make sure all connectors, jumpers and sleeves have been properly cleaned.

Step 1. First you need to take a reference reading. Connect a test jumper from the meter to the optical source, set the meter and source to the same wavelength, turn them on, and record the power reading in decibels (dB). This is your REFERENCE READING.

Wavelength settings are generally 850 /1300 nm for multimode, and 1310/1550 nm for single mode. In premises environments all these wavelengths except 1550 nm are commonly used.

(Note: To comply with EIA/TIA-526-14, the light source or OTDR must operate within the range of 850 +/- 30 nm for multimode, and 1300 +/- 20 nm for single mode. Also, the power meter must be calibrated and traceable to the National Institute of Standards Technology.)

Step 2. Next, connect a second jumper (of the same size fiber as the test jumper) to the first test jumper, joining them with an interconnection sleeve. Turn the meter and source on, and record the power level shown on the meter. This second reading is the CHECK READING.

Compare the 'Check Reading' with your initial 'Reference Reading', to make sure that the second jumper did not increase attenuation by morethan .75 dB. To do this, subtract your initial Reference Reading from the Check Reading. There should be no more than .75 dB of loss between the first and second reading. (Note that .75 dB is the TIA 568A threshold, but if desired you may also use the manufacturer-specified Guaranteed Maximum Mated Pair Loss for the specific connector you are using.) 

If the reading is satisfactory, proceed to Step 3, the end-to-end attenuation test. If not, clean all connectors except the source connection point, and repeat the Check Reading procedure.

Step 3. Now you are ready to perform the end-to-end attenuation test. Leave both jumpers attached to the optical source and power meter, but disconnect them at the interconnection sleeve. Take the meter and its jumper to one end of the cable being tested, and take the source to the other end, Record the reading; this is your official ATTENUATION TEST READING. Subtract the reference reading (recorded earlier) from the test reading you have just taken, to determine the end-to-end attenuation. Document this reading.

Attenuation should be measured and documented in both directions and at both applicable wavelengths (mentioned in Step 1). Note that a manufacturer may recommend that an OTDR be used to measure attenuation of fibers that will be left unterminated. A visual tracer can be used to confirm continuity.

4T.1.4. BANDWIDTH VERIFICATION

It is not necessary to perform a field test to verify bandwidth if documents or cable labeling allow you to see that the proper bandwidth fiber has been specified. Perform an actual field test only if documentation of the fiber bandwidth is not available.

4T.1.5. OTHER THINGS TO DO BEFORE SYSTEM START-UP

Use the power meter to check the power levels of the transmitter and receiver, after they have been installed and before the system is used. This lets an owner or troubleshooter quickly determine if the electronics are working properly, and provides a valuable maintenance record for subsequent troubleshooting.

Document your findings.

4.2. TIPS FOR TROUBLESHOOTING AN INSTALLED SYSTEM

An optical fiber cabling system that has been correctly installed and tested will require minimal maintenance while providing many years of reliable service. However if a problem does occur in a system, here are some tips to help you troubleshoot it easily.

If all systems have failed, check for a power failure. If power is fine, simply use a methodical approach to isolate the problem. Start by checking the transmitters and receivers.

First, using a power meter, measure the received power at the receiver. If any light is coming in, then you know that the transmitter and cable are fine so the problem would be the receiver. If however there is no light coming into the receiver, check power at the transmitter.

Only after eliminating the transmitter and receiver as the problem would an OTDR, fault finder or tracer be used to locate a break in the fiber. Note that in LANs, most problems tend to be concentrated in the areas where there is the most access to the fiber -- i.e., patch panels. Using Leviton Telcom fiber optic cabinets with locking doors will help prevent such problems in the first place.