Technicians need well-honed skills to keep up with DWDM's ever-changing dynamic.
By LARRY JOHNSON
In the past few years, DWDM has emerged as the technology that enables existing networks to expand using optical solutions to provide more bandwidth (information capacity) over existing or new fiber-optic installations. The use of DWDM has dramatically changed the installation, testing, and maintenance procedures of a network system. The role of both the network and outside- plant technician must be to address the requirements of the network and physical plant. This requires technicians to have a fresh outlook on the characteristics and properties of DWDM, including developing their skills in order to successfully install, test, and maintain a DWDM system.
For years, service providers installed fiber-rich cables that were expected to keep up with bandwidth demands for five to ten years. However, explosive growth is forcing organizations to use their remaining dark fibers. By the late 1990s, most carriers were operating at 90% capacity, requiring greater demand of technicians' knowledge and skills. The critical nature and reliability of the network depends on understanding and applying technicians' workmanship while learning new disciplines, technologies, equipment, and techniques.
DWDM uses passive optical coupling techniques to multiplex a number of discrete wavelengths, or optical channels, over a single optical fiber. For DWDM to be effective, the light sources used must operate in the low-loss 1,550-nm infrared window of optical fiber, having narrow enough spectral widths to occupy the same fiber without interfering with adjacent wavelengths. This requires all optical spans to be optical-loss and optical-time-domain-reflectometer tested for both 1,550-nm wavelength and for new installations using nonzero dispersion fibers and the 1,625-nm wavelength as well. In addition, the power meters should have a higher sensitivity to measure the higher power levels required-in most applications, up to +20 dBm.
Laser wavelengths for DWDM applications have been standardized by the International Telecommunications Union (ITU-T G.652 specification). The standard originally specified 100-GHz channel spacing, but has since included 50-GHz as well. Manufacturers are now providing systems with 25-GHz spacings in the entire 1,550-nm band. Today, manufacturers are building systems with 160 wavelengths at 10 Gbits/sec for 1.6-Tbits/sec transmissions over a single fiber. This is equal to 206 million equivalent voice circuits.
In addition, the use of erbium-doped fiber amplifiers have revolutionized the design and economics of high-capacity systems by reducing the need for electrical regenerators. Optical signals can be launched farther, increasing fiber-span distances. A single erbium amplifier can simultaneously amplify all wavelengths carried by the fiber in the 1,520- to 1,565-nm window. The critical nature and operations of DWDM systems require a substantial increase in the support training required of the technicians responsible.
The fibers. Over the years, the fiber industry extensively used standard singlemode fiber optimized for the 1,310-nm window. This fiber was followed in the late 1980s by dispersion-shifted singlemode fiber optimized for the 1,550-nm window. Finally, in the mid-1990s, nonzero dispersion-shifted fiber was introduced, optimized for DWDM technology. Each fiber has differing optical and physical properties technicians must understand in order to correctly test and maintain them.
Cable, closures, panels, and connectors. Fiber counts are now exceeding 1,000 fibers in a cable, primarily with the use of high-density ribbon structures. This impacts how to design, configure, break out, and protect fibers in patch panels and closures. The use of new small-form-factor connectors for both high-density and low-reflectance applications is critical to the operation of the network. The technician must understand new techniques and be supported with a new generation of products (tools and test equipment) to address current as well as future needs, adds, moves, and changes.
Test equipment. The role of test equipment in DWDM applications has evolved to a completely higher level. The knowledge of how new DWDM systems work, the roles of the individual components, and the testing requirements of both the physical plant and the network are all critical.
Tests performed include: insertion loss vs. wavelength, center wavelength measurements, polarization-dependent loss, peak wavelength, return-loss measurements, bandwidth measurements, power measurements, crosstalk measurements, wave-shape measurements, component characterization, wavelength characterization, and chromatic dispersion.
DWDM technology creates the demand for products with much tighter specifications than those required by earlier optical systems. This new generation of optical products will provide opportunities for the future all-optical network (AON) to become a reality. The AON leverages the ability of fiber-optic passive components to split, tap, switch, direct, multiplex, and demultiplex light as needed. The end result is greater flexibility, less complexity, and eventually, lower overall cost of transmission products, enabling protocol-transparent systems.
AONs will allow the owner and/or user of the fiber network to add, drop, multiplex, demultiplex and reconfigure networks. They will also enable increased speeds, self-healing protection capabilities, and service provisioning.
Today's technicians must expand their knowledge and skill levels to keep current in new technologies, products, and techniques used to install, maintain, and troubleshoot advanced optical networks and technologies. The first step is to acquire an understanding of optical technologies from the theoretical to the installed systems. They must understand all four vertical tiers (component, optical subassemblies, systems, service providers) and how each is tested and maintained. This knowledge must be applied to develop skills to assure compliance of standards and proper operation.
Larry Johnson is president of the Light Brigade (Kent, WA), an international fiber- optic training organization. For further information on instructor-led training courses, as well as training on CD-ROM and video, contact the Light Brigade at www.lightbrigade.com or call (425) 251-1240.