otdrs and rftss meet growing fiber test requirements

March 1, 1998

otdrs and rftss meet growing fiber test requirements

The latest otdrs and remote fiber test sets are overcoming price and performance concerns to achieve increased market acceptance.

Stephen Montgomery ElectroniCast Corp.

Telecommunications is changing very rapidly from analog, voice-based, nationwide networking to digital, multimedia (voice, data, and video), international transmission. New fiber-optic technologies, such as Synchronous Optical Network, dense wavelength-division multiplexing (dwdm), optical amplifiers, photonic switch and switch matrices, and Asynchronous Transfer Mode, help meet the increasing demand for new end-user services and "killer applications." However, although optical fiber can transport many more channels of voice and data than can copper, this capacity can mean an increase in the number of customers who are at risk should a problem occur. Therefore, the need for advanced test equipment and systems is real and immediate.

Meeting multimedia demand

One of the driving factors of this multimedia demand is the ability of optical fiber to support high transmission rates. Telecommunications service providers already have large installed bases of fiber-optic cables and will continue to deploy fiber to meet the increasing demands of their customers for new services. This trend increases the importance of a cost-effective preventive maintenance (monitoring) and field-maintenance program for fiber-optic communications networks.

Optical time-domain reflectometers (otdrs) are the most sophisticated fiber-optic instrumentation currently available and provide the most data relating to the optical signal link under test. According to ElectroniCast Corp.`s Fiber Optic Test & Instrument Equipment Market Forecast Service, the current method of using mini-otdrs in the field needs to be complemented with remote fiber test systems (rftss). These otdr-based systems are placed in strategic network sites or optical-fiber amplifier locations and enable the testing of optical signal networks via a network computer. otdr-based rftss locate faults in fiber-optic cable networks, periodically test the optical signal link, and verify cable installation and optical-fiber splice loss. The rfts helps improve overall communication network reliability and maintainability and reduces operating and maintenance costs. The systems minimize the time required to locate problems, improve restoration service personnel`s response time, and save money.

User concerns

The telecommunications industry has several concerns regarding rftss, however, including their ability to adequately measure the performance of networks that employ new technologies such as dwdm systems, optical-fiber amplifiers, optical crossconnects, and optical signal transport systems. Many in the industry believe that research and development of new modules or new software will be necessary to overcome this perceived shortcoming. Currently, some users say, even "turnkey" rftss require changes in their software to meet the unique needs of each network, particularly where these new technologies are in use. Also, the cost of rftss compared to the cost of field use of otdrs or optical fault locators has slowed the implementation of rftss in some networks. The combination of these factors has led some service providers to rely on self-designed status monitoring equipment and software or other solutions, rather than rftss.

Test system manufacturers have begun to address these user concerns. For example, Wandel & Goltermann Communications Test Solutions, Eningen, Germany, recently released a product description of its Optical Fiber Monitoring System (ofm). According to Steffen Vogel, the company`s product manager of fiber-optic test equipment, the ofm is a step toward a full-featured rfts. The ofm is a software package extending the capabilities of the company`s Domino fiber oft-30/40 product-line by performing periodic otdr measurements of fibers and comparing the traces against a reference trace. Depending on the result of the comparison, the trace actually measured is marked "passed" or "failed."

otdr and rfts operation

Currently, optical fibers are linked to the rftss through fiber-optic switches. In this scenario, along with the modular otdr and the optical test modules, are optomechanical 1 ¥ N switches that allow one port, or otdr, to be linked to N fiber ports. Since the price-per-port costs are high, the acceptability of rftss is restricted for several applications or service provider types.

According to Ken Garrett, director of strategic accounts at jds fitel Inc., Nepean, ON, Canada, the most common procedure employed to check the condition of an optical line is shown in Fig. 1. In this scenario, fibers are selected within a particular cable and tested using an otdr. This procedure involves using a 1 ¥ N switch to select an individual fiber and sending an optical signal through it to measure its performance characteristics.

Using an otdr as in the example represents a disruptive test that does not offer the ability to view fibers that are carrying transmissions (see Fig. 1). The need to evaluate fibers while they are carrying data was one of the first implementations of using a wavelength-division multiplexer (wdm) for test access (see Fig. 2).

"Specific wavelengths were chosen as telemetry or monitoring wavelengths, where optical test equipment could be inserted without disrupting the transmission wavelength. These test wavelengths are located on the extreme ranges of the transmission spectrum [1525, 1620, or 1650 nm] and do not interfere with transmission wavelengths. This allows carriers to check for network integrity as well as fully utilize all fibers within the cable simultaneously," according to Garrett.

Turnkey systems

Gerry Pesavento, director of sales and marketing at DiCon Fiberoptics Inc., Berkeley, CA, says that several otdr companies provide rfts turnkey systems, including GN Nettest, Anritsu, Ando, PK Technology, Hewlett-Packard, Wavetek, and exfo. Monitoring systems have been deployed in Brazil, China, Taiwan, Spain, Japan, France, Germany, and other countries. In fact, Pesavento estimates that the largest market is outside of the United States.

As cited above, the cost of using 1 ¥ N switches has limited the use of rftss. However, 1 ¥ N switch costs have come down dramatically in the past few years--to approximately $150 per channel for volume users--making the fiber monitoring systems less expensive. For example, DiCon has developed Model VX500, a very small 1 ¥ N switch.

Meanwhile, some former state-owned telephone companies in Europe (ptts) are using active fiber monitoring, which involves using filter wdms to insert the otdr test signal onto the same fiber as the data traffic. These systems cost about $300 more per fiber, but they are more accurate because they measure fibers with traffic, rather than without traffic, as described in the example above.

"Some ptts are buying otdr cards and 1 ¥ N switches and using local integration firms to develop custom monitoring software, rather than purchasing turnkey systems. The software is the most challenging part of the system," Pesavento says.

There are two primary applications for fiber-optic switches used with otdrs: rftss, and fiber and cable factory testing (production and qualification). An example of using otdrs and switches for fiber and cable testing is shown in Fig. 3.

There are currently about 20 manufacturers of otdrs supplying the North American marketplace. Successful competitors in this market will be those who are most adept at forecasting the changes and convergence in the customer base and in product performance requirements, and who adjust their marketing and product development to match these windows of opportunity. u

Stephen Montgomery is president of ElectroniCast Corp., a market and technology consultancy based in San Mateo, CA.

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