SM VADD Fiber Test Services and AMC Fiber Infrastructure Testing Services 23-24
Testing and troubleshooting networks
The installed network can be tested quickly and easily with a fiber-optic power meter. Set the network transmitter to transmit a clock output or other bit stream of known duty cycle. Set the power meter calibration on the proper wavelength and the reading units on watts. To test the received power –the most critical element in the network –disconnect the cable connector at the receiver, attach the power meter and measure the power.
If the receiver power is low, check the transmitter power by disconnecting the source jumper cable at the first available connector and measuring the power at that point. Alternatively, you can disconnect the cable at the transmitter and use a jumper that has been pre-tested and is known to be good to measure the coupled power. If the output is measured through a short network jumper cable (less than 30 feet), no compensation for jumper loss is necessary. For longer jumpers, some compensation for cable loss may be necessary.
If receiver power is low but transmitter power is high, something is wrong with the cables. They must be tested at every connection to isolate the bad cables or connectors. Starting from either the transmitter or receiver end, follow the network cables to every patch panel. Disconnect the connector and measure the power at each point. By making measurements in decibels, you can easily calculate the loss of the cable network to each point by subtracting successive readings.
If you note a larger-than-expected loss in the cable link, test the suspect cable using one or more of the following methods. If a cable has attenuation that is higher than specifications but still transmits light, check the connectors using a microscope to determine if they have been damaged and should be replaced. If the connectors look good, the best solution may be to replace the cable or switch to a spare. If a visual fault locator is available, use it to locate breaks in the fiber and find broken connectors. Under some circumstances, such as high loss in long jumper or trunk cables, an OTDR can be used to diagnose cable faults.
Transceiver loopback testing
The data communications capabilities of the network can be tested with a loopback test. This test uses a calibrated fiber-optic attenuator placed between the transmitter and receiver on a piece of equipment to see if it can transmit data to itself. Many types of network equipment have diagnostics to do loopback testing. This loopback method tests the transmitter and receiver of the unit under standard data transmission conditions over the specified link loss budget.
Some data communications equipment can also institute an electrical network loopback test, where the loopback path is inside the equipment, looping back over the entire data link to the equipment on the far end of the link. If both ends of the link pass a unit loopback test but fail a network loopback test, the problem is in the cables, which then need testing.
Once installation is complete, the cabling plant is tested and network equipment is running smoothly, what is likely to go wrong in a fiber-optic network? Fortunately, not much. One of the biggest selling points for fiber optics has been its reliability. But there are some potential problems that can be addressed by the end user.
Loop Back Testing.
Whether installing new cable, or troubleshooting existing cable, cable testing plays an important role in the process. Common tests for Datacom Cabling include length, wire map, attenuation, NEXT, DC loop resistance, and return loss.
Visual Tracing LS
Continuity checking makes certain the fiber are not broken and to trace a path of a Fibre from one end to another through many connections with the use of a visible light source. This allows us to carry out a first line check to identify a faulty or a functional fibre very quickly.
OTDR Testing
Unlike light sources and power meters which measure the loss of the Fibre optic cable plant directly, the OTDR works indirectly. The source and meter duplicate the transmitter and receiver of the Fibre optic transmission link, so the measurement correlates well with actual system loss.
The OTDR, however, uses backscattered light of the Fibre to imply loss. The OTDR works like RADAR, sending a high power laser light pulse down the Fibre and looking for return signals from backscattered light in the Fibre itself or reflected light from connector or splice interfaces.
At any point in time, the light the OTDR sees is the light scattered from the pulse passing through a region of the Fibre. Only a small amount of light is scattered back toward the OTDR, but with sensitive receivers and signal averaging, it is possible to make measurements over relatively long distances. Since it is possible to calibrate the speed of the pulse as it passes down the Fibre, the OTDR can measure time, calculate the pulse position in the Fibre and correlate what it sees in backscattered light with an actual location in the Fibre. Thus it can create a display of the amount of backscattered light at any point in the Fibre.
Since the pulse is attenuated in the Fibre as it passes along the Fibre and suffers loss in connectors and splices, the amount of power in the test pulse decreases as it passes along the Fibre in the cable plant under test. Thus the portion of the light being backscattered will be reduced accordingly, producing a picture of the actual loss occurring in the Fibre. The OTDR presents this information onto the display allowing the engineer to detect problems in the cable caused during installation. If a Fibre is broken, it will show up as the end of the Fibre much shorter than the cable or a high loss splice at the wrong place. If excessive stress is placed on the cable due to kinking or too tight a bend radius, it will look like a splice at the wrong location.
We have the capabilities, expertise, and approvals necessary to provide you with a cost-effective solution for your Fiber Optic Testing needs, including GR-20-CORE, GR-409-CORE, and GR-13-CORE.
We test safety, reliability and performance of fiber optic components (FOC), including connectors, fiber cables, fiber distribution frames, splice closures, pedestals and indoor/outdoor fiber cabinets.
- Getting faster service delivery
- Benefiting from global expertise with local presence
- Reducing your risk while protecting your brand
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Best Prictices.
1.Ignorance of dust’s impact. Trained and certified installers understand both the importance of clean fibers and connectors, and the consequences of dust. In this example, tens of thousands of “cleave-and-crimp” connectors were to be installed in a large facility under construction. The construction activities resulted in a dusty environment. The connectors required creating a cleaved fiber end and inserting that end into the connector. The dusty environment resulted in dust settling onto the cleaved fiber ends end/or connector ends. High loss resulted.
- Importance of microscopic inspection. Avoidance of dust contamination and microscopic inspection are the well-known rules for trained and certified installers. Published studies of installation problems indicate dirty fiber ends have represented as much as 85% of installation problems. The connectors in Example 1 were installed without microscopic inspection of the ferrule end to assess contamination; installers and their supervisor did not recognize the need and benefit of inspection. Without this knowledge, high-loss connectors were replaced instead of cleaned. It is possible that some of the high loss and cost in Example 1 were due to dust on the ferrule end. Such dust would be simple and inexpensive to remove.
- Failure to follow manufacturer’s instructions. Trained and certified installers are disciplined to follow the manufacturer’s instructions. the connector instruction manual explicitly stated: “Do not clean the cleaved fiber end.Contamination from cleaning after cleaving can and does often result. The inexperienced installers consistently cleaned the cleaved end. T
- Violation of Local Compliance rules. As cable systems are installed within buildings and such systems require compliance with the Local Authorities , trained and certified installers know how to comply with these requirements.
You must install equipment and cabling in a neat and workmanlike. One of the implications of this is the routing of your cables cannot interfere with access to equipment.
Avoiding interference has two benefits—convenience in accessing ports and avoidance of reduced liability. If a technician needs to move one cable to access equipment or a port, there is a risk of damaging cables or interrupting signals.
5.Choosing the lowest-cost installer. Here are two examples of what can happen by choosing the low-cost installer. Trained and certified installers understand two specific aspects of splicing: how to splice fiber without gas bubbles, and how to interpret OTDR traces of such splices.A fiber contractor, a friend of mine, lost a splicing bid to the low bidder. Several weeks later, the client called my friend and stated, “None of the fibers work.” The client asked my friend to redo the splicing. My friend’s technicians reported that OTDR testing indicated every one of the splices had low loss and high reflectance. The reflectance was due to a gas bubble in every splice. The excessive reflectance from gas bubbles corrupted the optical signal on every fiber.
Evidently, the low bidder knew how to measure loss properly with an OTDR but did not know two basic facts: a properly made fusion splice has no reflectance, and it has no gas bubble. Consequence, Installation cost more than doubled.
Trained and certified installers know that following the rules for cable installation eliminates fiber breakage. Following these rules results in control of both the load and the bend radius.
- Inadequately trained installer. While OTDR testing a replacement cable for a client, this author noticed that many of the single mode connectors had losses closer to the maximum than the typical values. From experience installing and supervising more than 50,000 connectors, I knew there were installation errors. Microscopic inspection of the connectors revealed that polishing scratches had not been removed completely. Consequence: The power margin, power level reduction allowable before link failure, was reduced. As a result, the reliability of the link was reduced.
- Inadequate training in connector inspection. Trained and certified installers know how to interpret microscopic appearance of connector ends, as microscopic inspection is the last step prior to connector insertion.
- Cable buried incorrectly. Trained and certified installers know to bury cables at a depth that prevents damage. cable installers buried the fiber cable at a depth of about 3 inches. Two days later, the landscaper put his shovel into and through the cable Consequence: A one-hour task had to be done twice, at twice the cost.
the normal burial depth for fiber cable is approximately 30 inches or below the frost line, whichever is deeper. The normal depth of disturbance by the plow as 3 to 4 inches.Consequence: Significant cable repair cost, which I estimate at high five figures.
- Inadequate cable installation training. Trained and certified installers know to attach cable tied without damage. Such knowledge is required for fiber certification. Visual inspection of the cable revealed cable ties installed with a cable tie tool. The cable tie tool created bend radius violations under each of the ties. Removal and replacement with hand-tightened ties restored link operation.
Cable ties are not recommended for use on indoor fiber-optic cables, instead hook-and-loop ties are, as it is essentially impossible to violate a bend radius under a hook-and-loop tie.
- Failure to monitor oven temperature. Trained and certified installers know that temperature control is required for proper curing of connectors installed with epoxy. After polishing connectors installed with epoxy. specific knowledge is essential for successful fiber installation. The trained installer may have some of this knowledge.
