Testing tips for network managers.
Only a few years ago, the design, installation, and testing of fiber optic systems was a specialized field requiring expensive test equipment and highly trained personnel. Now, fiber optic systems have reached a plug-and-play status making them much easier to design, install, and test.
Long distance telephone and cable television (CATV) applications still dominate the fiber optic market, but other applications such as local area networks (LANs), wide area networks (WANs), data networks, and videoconferencing are increasing. As these proliferate, they are enrolling new users, installers, and test personnel--and driving product innovation in portable test equipment.
FIBER OPTIC SYSTEMS MATURE
Most LAN or campus wiring architectures consist of a number of wiring closets from which individual workstations are connected, usually at distances under 150 feet. Higher-speed links interconnect the wiring closets through backbone or riser cables.
A few years ago, backbone wiring was typically done in coax copper cable and individual workstations were connected using Category 5 (CAT 5) copper, twisted-pair wire. But many installations now implement the backbone with fiber optic cables, according to Hugo Draye, marketing manager for Media Test at Fluke (Everett, Wash.).
The need for greater bandwidth and a reduction in the cost of installation and equipment have helped move fiber into LANs and other short-distance applications. Prices of communications equipment, like transmitters and receivers, are decreasing, as is the cost of installing, testing, and maintaining the network. Here, easier methods to splice and connect fiber have made installation quicker, while more automated test equipment is lowering the cost of test and maintenance.
As fiber moves into LANs, installers have to learn to connect and test both copper and fiber components. As a result, a new category of test equipment has emerged--hybrind testers. These devices feature the capabilities to test both CAT 5 copper cable and fiber optic cable. Vendors such as Wavetek (San Diego, Calif.), Fluke, DataCom Technologies, Inc. (Everett, Wash.), and Scope Communications (Northboro, Mass.) all offer modular hybrid testing instruments.
Another sign of the maturity of fiber optic systems--and the big opportunities in premise and campus wiring--is that new groups of installation personnel are entering the field. For example, electrical wiring contractors are now getting the training and equipment to provide network wiring and test services.
Once the fiber has been physically installed, it must be tested to be sure it meets the needs of the data or communications system. The most basic tests include measuring optical power, verifying link continuity, and determining the link's optical loss.
Installation and maintenance technicians usually perform these tasks, but network managers may sometimes also need to do this. A simple optical power meter, for example, helps determine if the cable plant (fibers, connectors, and patch panels) or the transmitter/receiver electronics are malfunctioning. This knowledge allows the network manager to determine which service group to call.
To run a test to decide which part of the link is misbehaving, the optical power meter is first connected to the system's laser or LED light source to determine if it is operating. If it is working, the transmitter is reconnected to the fiber. The power meter is taken to the other end of the link and connected to the fiber. If there is adequate power to run the network, the problem is probably a bad receiver. If little or no power comes out of the fiber, then something is wrong with the cable plant.
Fortunately, power meters have dropped in price so that they can become part of almost anyone's toolbox. Jim Hayes, president of Fotec (Medford, Mass.), notes that their optical power meters have fallen from the $500-$600 range down into the low $200 range.
If the transmitter or receiver is bad, it can often be replaced with a new plug-in module. But to diagnose the cable plant, you will have to call the repair technician or buy more equipment.
A visible fault locator, for instance, contains a red LED or laser source that is connected to the fiber. LEDs can check fiber link continuity and lasers can "leak" light out, showing bad connectors or splices as well as cracks/breaks in single strand fibers to indicate where to make repairs. These devices cost $500 to $1,000.
MEASURING LINK LOSS
Air gaps in connectors, fiber degradation with time, or too many connections in the link all conspire to increase loss. To see if these losses are too great for the system to operate, you need to perform a loss measurement. This is normally done at the time of installation and is used as a reference during subsequent testing for maintenance or repair.
This measurement places a test transmitter at one end of the fiber and a power meter at the other end. By knowing how much power you inject into the fiber and measuring what is received, you can determine the loss of the link.
Newer equipment has simplified these testing procedures and added storage capabilities for easy comparisons. For example, Exfo's (Vanier, Quebec)product manager Pierre Talbot says, "With one button, you can now do bidirectional, dual-wavelength tests." Backbone systems typically operate with either 850 nm or 1,300 nm LED sources over multi-mode fiber, or 1,310 nm laser sources over singlemode fiber. Exfo also offers a product that packages a light source, power meter, and visible fault locator into one instrument.
Once you have made a loss measurement, how do you know if the loss is too much?
Rich Helstrom, director of marketing at Wavetek, says, "The pass/fail criteria are determined by the loss bud get of the link and is usually the toughest thing for the technician to figure out. This calculation requires knowledge of the approximate fiber length and the manufacturer's loss spec for connectors used on it. Once they have the data, they can then calculate the loss budget."
But recent product advancements are making this task easier, too.
Instruments from Microtest (Phoenix, Ariz.), Scope, and Wavetek can actually calculate the loss budget for the user.
But Fotec's Jim Hayes warns that network managers should be knowledgeable when looking at pass/fail data. "Many technicians use TIA standard 568, appendix H to determine the losses for connectors and fiber attenuation. But that calculation doesn't always reflect real values in today's fiber systems," he says. "By allowing too high a loss budget, the system can pass, but it may mask a bad connector or severe bending losses in the fiber."
An optical time domain reflectometer (OTDR) is often used to locate these types of faults on longer links, but opinions differ as to their ability to detect and locate faults over the shorter distances of premises and campus wiring networks.
OTDRs work by sending pulses of light down the fiber and measuring signals that are reflected back by connectors and splices. By accurately measuring the time of flight of the light pulse to a fault and back again, the OTDR can identify and measure the distance to the fault, so repairs can be made.
Newer OTDR modules can now test multimode fiber over shorter distances. Typical instruments can detect events (faults) at one meter and can isolate and calculate loss/reflection on events as close as four meters apart.
It is important for network managers to understand how advances in fiber optic technology and test equipment can affect network operation. Fiber usage is increasing and test equipment is becoming more sophisticated, more specialized, or cheaper--all to fit different needs. Taking advantage of these trends can help to ensure that networks operate as smoothly and as cost efficiently as possible.
Chinnock is a technology writer specializing in the areas of displays, optoelectronics, and semiconductors. He is based in Norwalk, Conn., and can be reached at email@example.com.
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|Title Annotation:||Technology Information; cable testing power meters|
|Date:||Aug 1, 1998|
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