More stuff to know…
MODULE 10
Testing and Troubleshooting Fiber-Optic Cabling Systems
Testing and Troubleshooting Fiber optic Cabling Systems
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Testing and
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Goal 10
After completion of this module students will be able to identify the
correct steps in accordance with standards in completing Fiber Optic
testing using the power meter and light source and OTDR. Students will
be able to identify correct procedures in testing Fiber Optic segments.
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Objectives :
• Identify that the key considerations that affect a Fiber Optic system are:
Fiber Optic cable all connectors and splices strength of source signal
detector sensitivity
• Identify the importance of using a mandrel when checking short cable runs
• Define how to take a power and loss measurement using the power meter
and light source Identify the passive and active parts of a Fiber Optic
system
• Define tier 1 as required and that it uses an optical power meter and light
source Define tier 2 testing as the use of an OTDR
• Identify the 5 testable link segments in Fiber Optic systems
• Identify the considerations given when testing centralized links, first and
second level backbones and horizontal links
• Identify testing procedures using the one, two and three cabling methods
Read an OTDR display and determine events and distances
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Objectives (continued):
• Identify OTDR controls and features Describe pulse in relation to resolution
• Define the importance of keeping records when testing an optical system State
some of the rules of optical testing
• Define the split-half method of troubleshooting Fiber Optic links
• Identify some of the things to look for when troubleshooting an optical system
• Describe the effects of cabling mismatches in regard to core size, cable elasticity
and core eccentricity Define the importance of correct fiber alignment
• Identify the four items that account for connector insertion loss Define the purpose
of the visual fault locator
• Define the purpose of the Optical Time Domain Reflectometer (OTDR)
• Identify testing and troubleshooting procedures in regards to the active
components of a Fiber Optic system State the purpose of following Electro Static
Discharge (ESD) precautions when dealing with sources and detectors
• Complete an optical troubleshooting scenario
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Testing and Troubleshooting
Optical Systems
A completed Fiber Optic system is a type of working system that relies on many parts
to function. Consideration should be given to all of the system component parts. Any
one part of the system that is not functioning properly can have a large impact on the
total Fiber Optic system operation. The key components of a Fiber Optic system
include:
• the Fiber Optic cable
• all connectors and splices
• the strength and reliability of the source signal the sensitivity and reliability of the
detector
The above are the key considerations that a network connectivity technician should
keep in mind when testing and troubleshooting a system. It is the technician’s job to
get the best amount of signal from where it is, to where it needs to be used.
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Testing Using a Mandrel
A mandrel cuts down the higher order modes of light. It also reduces the light that is traveling inside of the core.
If you test a short run of cable without a mandrel you are looking at light that has not been attenuated. If the cable
was longer than this additional light would be lost to attenuation.
If you zero a meter with a short run of cable, you will be measuring light that will later be lost due to cable
movement.
Figure 10.2.1
The mandrel is used to attenuate light from the source in a controlled manner. Using
the mandrel reduces the fluctuation of the power meter reading caused by changes in
the position of the test cables.
For 62.5µm fiber a 0.8 inch diameter mandrel is recommended; 50µm fiber requires a
1.0 inch diameter mandrel. The mandrel in the instructors workstation is designed for
62.5µm cable.
Figure 10.2.2
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Activity 10.1 Testing
using a Mandrel
Watch the demonstration
video
Configure cables together
and determine the effect of
a mandrel on attenuation.
Show the move and or
conduct a demonstration
or let student’s work with
the equipment and
explore the concepts.
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Active and Passive Parts of a Fiber Optic System
The active system parts of a Fiber Optic system are the
source and the detector. They are considered active because
they actively work as transducers in converting either energy
to light or light to energy.
The passive system parts of a Fiber Optic system are the
cable plant itself including all cable, connectors, splices and
adapters. The passive parts of the system are responsible for all
system attenuation. Since the operating parameters of the
passive parts of a system are known or at least have a standard
measurement, the active parts of the system (sources and
detectors) are the largest factors determining system operational
performance.
Figure 10.4.1
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Activity 10.2 Fiber Optic Systems
1. Explain why a mandrel is used in testing.
To cut down on the higher order modes and light traveling in the cladding
2. The passive components of a Fiber Optic system are:
Cable
connectors splices
3. The active parts of a Fiber Optic system are:
Sources and Detectors
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Testing Methods
Fiber Optic systems are tested by levels or tiers. There are two levels
of testing Fiber Optic systems. As a technician, you should test a
system to make sure it will work as part of your guarantee to the
customer. If attenuation values are too high it is your job to find and
correct the problem.
Tier 1 testing is required. When testing at tier 1, a technician uses a power
meter and light source. The power meter and light source determine the loss
of an entire segment.
Tier 2 testing uses and OTDR (Optical Time Domain
Reflectometer). An OTDR is an optical radar that can map the
attenuation of an entire segment. Attenuations are seen for each
cable connector and splice in a link.
Figure 10.6.2
Figure 10.6.1
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Fiber Optic testing standards define three ways of conducting tier 1 testing. Each way is correct. The differences in the
methods are based on the number of reference cables and connectors. Each method has different attenuation values.
Each method is designed for different situations. Two of the methods have additional dB that needs to be added to your
calculations.
The methods all concern how the power meter is “zeroed” then
taking into account connector loss and connecting the test
cables.
Method A uses two patch cords.
Method B uses one patch cord.
Method C uses three patch cords.
Look at the methods above. Note that method A uses two
patch cords. Method A is the most common. Often you check
attenuation from an adapter in the equipment room and the
adapter on a face plate. You already use two cables.
Figure 10.7.1
Testing and Troubleshooting Fiber optic Cabling Systems
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Testing Method A – The Two Cable Method
A quality connector should not exceed 0.5dB. You may estimate the loss of the connector in your calculations.
The two cables are connected to the light source and power meter using an adapter. The power meter is then
“zeroed”. Using this method a technician really has no idea the attenuation of the referenced cable or
connectors. Often this method is used when dealing with different types of connectors. Once zeroed do not
remove the cable from the power meter or light source.
Figure 10.8.2
Figure 10.8.3
Figure 10.8.1
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Testing Method B – The One Cable Method
Connect the reference cable between the power meter and light source. Zero the meter and establish the reference
value. Disconnect the cable from the light source. Attach the reference cable to one side. Connect the light source
to the cable and system under test. Once tested subtract 0.5dB from the reading to get the link attenuation. When
using method B, the 0.5dB represents one mated pair and test cable number 2.
The one cable method is known as a “single loss”. The loss is
only on one side of the link. This method checks an installed
patch cord on the other end of the segment.
Figure 10.9.1
Figure 10.9.2
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Testing Method C – The Three Cable Method
Method C uses a “golden reference cable”. This cable is called golden because you are assuming a quality cable with
good connectors. This cable is the reference that you are testing against.
Connect the three cables and zero the meter. Do not remove the cable from the source or power meter
once the reference is established. Replace the “golden” cable with the segment under test. When using
method C no other losses need to be included into a calculation.
Figure 10.10.2
Figure 10.10.1
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Testing
The C-Tech Fiber Optic color code card provides for the maximum attenuations of cables, splices and connectors.
These are maximum values and usually more realistic numbers are used to get a better idea of system
performance. Rounding numbers also gives a quick idea of system performance.
0.5dB for a mated pair of connectors 0.2dB for each splice
3.0dB for each km of 850nm fiber (this is approximately 0.1dB for each 100 feet) 1.0dB for each km of
1300nm fiber (this is about 0.1dB for each 300 feet)
Splice
connector
Cable
connector
Figure 10.11.1
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Tier Two Testing
The Optical Time Domain Reflectometer (OTDR) can look at an entire Fiber Optic system and map it for attenuation. The
OTDR is like a radar set for a fiber system. It sends a light pulse and it looks at reflections. The time it takes the
reflections to return determines how far they are from the tester.
Remember from earlier modules that anytime light strikes a surface light is reflected back towards the source.
The OTDR launches a light pulse at T1. The light pulse strikes an event. An event is anything in the optical link
that returns a reflection. The event is timed and is one half the time/distance. To determine the distance to an
event divide the travel time by 2 for send and return time.
Figure 10.12.2
Testing and Troubleshooting Fiber optic Cabling Systems
Figure 10.12.1
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Mapping a Link
The OTDR launches a pulse of light into the fiber. Any discontinuity
(connector, splice, excessive bend, etc.) in the link will cause a
reflection. By observing the trace on the OTDR, you can measure
attenuation over distance and attenuation due to connectors, splices or
damage.
An OTDR test set is used to determine:
Figure 10.13.1
the attenuation of a link
the attenuation of each event the distance to an event
An OTDR is a more comprehensive test of an optical link. It provides
attenuation for each event. For example, when testing with just a power meter
and light source a technician has no idea of the attenuation of each cable or
connector. A segment may pass with a lot of loss at a dirty connector. An
OTDR can show each event and in the case described, a technician could
locate and clean the dirty connector.
An OTDR can produce a permanent record of the system that can be used to
certify the system to a customer for use. The information can be saved and
compared later for additional testing or troubleshooting.
Testing and Troubleshooting Fiber optic Cabling Systems
Figure 10.13.2
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Reading an OTDR Display
The OTDR displays optical power versus distance. The OTDR display represents everything that a signal must
pass through in a fiber run.
Notice how the reflected power decreases over time. This is because a Fiber Optic cable has attenuation and the
farther a signal travels in a Fiber Optic cable the more it is attenuated. All cables will have this decrease in signal
reflectivity over time as represented by the plot.
Connectors in the fiber system are represented as the spikes, or areas of reflection. A splice is also represented on
the plot to indicate an area of less reflection than a typical connector. If the cable were broken the OTDR test set
would indicate a sharp
drop off in the plot similar to the "End of cable run" in the diagram.
If a plot ended before that and the technician knew that the run was longer, it would indicate a break in the fiber
cable, a severe bend or other reason for signal loss at that point.
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Reading a Display
As discussed earlier the OTDR display indicates attenuation over distance. Power is on the Y axis and time or
distance is on the X axis.
Reflective events like connectors, breaks and splices can be seen. The display shows attenuation of the event as
well as the event’s distance from the OTDR.
Reflective events can be expected, because a map of the links has been provided. Events can also be
unexpected. An example of an unexpected event would be a break or too tight a bend radius.
Note the events in the picture and compare the event distances and attenuations.
Figure 10.15.1
Testing and Troubleshooting Fiber optic Cabling Systems
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Activity 10.3
The OTDR
Watch the
demonstration
video and take
notes.
Show the
movie and
explain the
concepts.
Testing and Troubleshooting Fiber optic Cabling Systems
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OTDR Controls
The OTDR has two main controls, one is amplitude of the signal and
the other is the pulse width
adjusting these controls a technician can The narrower the transmitted pulse of
ability of an OTDR to show more detail. should be adjusted as to show the cable
to overload the display. Select a distance long as the one you are testing.
automatic and manual Features
OTDRs vary in functions widely but most manual and automatic settings. Some
information required by the OTDR would manually. That information would include
core diameter, or numerical index. A also select wavelengths, ranges or pulse
resolutions. Some OTDRs report passes or fails testing.
Testing and Troubleshooting Fiber optic Cabling Systems
Figure 10.17.1
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Resolution
Narrow pulse widths allow for better resolution. Remember from earlier reading, in an OTDR distance and time are the
same thing. Since a pulse can be longer in time it will also be longer in distance. The diagram shows that resolution is the
ability to see individual events that are close together. A narrow pulse width is able to distinguish three separate events while
a large pulse width sees only one.
Narrow pulse widths do have a limitation. OTDRs function by sending a strong light through a fiber. To make the light
stronger it is necessary to keep it on longer. Pulse widths should be adjusted as narrow as possible while still allowing
sufficient light on an event.
Testing and Troubleshooting Fiber optic Cabling Systems
Figure 10.18.1
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0
Saving Trace and Data Information
Trace overlays may also be stored electronically for future reference.
Event tables may also be saved. It is important to have a copy of this information,
not only for your records but for the customer as well.
Introduction to Network Cabling Fiber Optic-Based Systems
Figure 10.19.1
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Testing and
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The Rules of Testing
• Use proper equipment including testers, reference cables and adapters.
• Clean all connections prior to testing.
• Only test systems when they are off-line.
• Follow safety procedures.
• Test in the correct system wavelengths.
• Test a system both ways (bi-directional) unless otherwise specified.
• Record measurements to one significant digit in the decimal place. For example:
10.4dB or 7.2dB, not 10dB or 7dB.
• Insure that the reference/zero values remain effective. Re-zero the meter or change
reference cables as necessary due to changes in temperature, changes in operating
parameters or if you suspect reference values have changed.
• Reference cables must be of the same fiber core size numeral aperture and
connector type as the system under test.
• The power meter and light source must be set to the same wavelength.
• Reference cables must be from 1 to 5 meters long.
• Once reference values are established, do not remove cables from power meter or
light source.
• Light sources shall be 850nm or 1300nm
Introduction to Network Cabling Fiber Optic-Based Systems
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Testing and
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Activity 10.4 Fiber Optic Testing
1. Match the testing method with the number of cables used.
method A
1 cable
method B
2 cables
method C
3 cables
2.The testing method that adds no connector loss is:
1 cable method
2 cable method
3 cable method
3. The first test you should perform on your installation is the:
horizontal link
backbone link
centralized link
none of the above
4.State two rules of testing
Grade to Standard_________
_________________________
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Testing Fiber Optic Systems
•When testing a Fiber Optic system a cabling specialist will check the
passive portions of a system by link segments. In a Fiber Optic system
there are five testable link segments. Compare the link segments with the
diagram.
•centralized link (A to E)
•main cross-connect to the horizontal cross-connect (A to B)
•main cross-connect to the intermediate cross-connect (B to C)
•intermediate cross-connect to horizontal cross-connect (C to D)
•horizontal cross-connect to the outlet (D to E)
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Testing Fiber Optic Systems Con.
Figure 10.22.1
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Testing the Centralized Link
When testing the centralized link a technician checks the Fiber Optic system
from the outlet back through the telecommunications room to either the
Entrance Facility or an Equipment Room. A test of the centralized link is
considered an end-to-end test meaning it checks the complete system at
once.
The standards also state that if there is a consolidation point in the horizontal,
you may increase the attenuation by an additional 0.75dB. The centralized link
test should be the first test performed to verify total system performance. If the
centralized link test fails, then a technician would go on to troubleshoot the
problem by performing either a check of the backbone link or a check of the
horizontal link.
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Testing and
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Testing the Centralized Link Cont.
New Term
Centralized link - The total
Fiber Optic system: from
outlet to Entrance Facility or
Equipment Room.
end-To-end test - A test of
the centralized link-from
entrance point to the outlet.
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Testing and
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Testing the Backbone Links
First level backbone (a to B or a to C) and Second level backbone (C to D)
When testing the first or second level backbone cabling, a technician will test
the link segment at both applicable wavelengths in at least one direction.
Because the layout and size of backbone links differ, the attenuation of the link
should not exceed standard parameters - cable attenuation, connector insertion
loss plus splices (if any)
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Troubleshooting
The Split-half method is one of the methods that technicians use to
quickly troubleshoot and locate a malfunction in a system.
method - A
method of
troubleshooti
ng whereby
you divide the
possible
trouble area
into halves
repeatedly
until you
locate the
problem.
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Introduction to Network Cabling Fiber Optic-Based Systems
Introduction to Networking Fiber Optic-Based Systems
(Version 3.3)
© 1998-2012 by C-Tech Associates, Inc.
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Introduction to Network Cabling Fiber Optic-Based Systems
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Fiber Optic System
Components
QUESTIONS?
Module Test Time!
Introduction to Network Cabling Fiber Optic-Based Systems