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Air-blown fiber eases installation This new technology is gaining momentum. Should you be using it? Powersoft Corp. relocated from a single office building along Boston’s Route 128 high-tech corridor to a multi-building complex in Bedford, MA. The relocation presented the company with the opportunity to upgrade its cable plant, while at the same time it was faced with several challenges: installing optical fiber among three buildings at an existing campus site in a very short time. These challenges were met by Corporate Connectivity Resources in Wellesley, MA, which designed a fiber-optic campus backbone for Powersoft, and Cablenet Systems in Peabody, MA, which installed the backbone using air-blown fiber. CCR principal Mike Kerwin designed the system. Why did he opt for air-blown fiber? “One reason was a requirement to provide a flexible, high-capacity infrastructure to our clients,” he says. “The changing applications, specifically the demand for document imaging, often require new fiber or more fiber than was in place originally for the traditional concentrator environment.” Powersoft is an aggressive, high growth software company. Its initial public offering this year has fueled dramatic growth, leading to the need of a larger facility. Kerwin’s task was to provide for high-speed, high volume data transfer in a campus environment. “Powersoft has [much] interaction,” he claims, “especially between development and support. We had to provide a system that not only give them the high bandwidth capacity they required, but that would also let them continue to grow at a rapid rate.” The Bedford site consists of three two-story office buildings in landscaped office park. The buildings are situated in a rough triangle with sides of about 600 feet. A maintenance hole located centrally among the buildings supplied a place for a splice enclosure. Tube cable installed first Cablenet began its installation by pulling tube cable through th exiting underground conduit system with traditional pull ropes. The tube cable, similar to innerduct, is flexible and comes on a reel. It comes with factory-installed inner tubes, each of which accommodates a single blown-in fiber bundle. Tube cables come up with up to 19 inner tubes and are available in different designs–armored cable for direct-buried applications, all dielectric outside-plant cable for use in conduit banks and laminated aluminum polyethylene-jacketed cable where a water barrier is needed.The tube cable can be riser-rated for indoor use; it has been listed with Underwriters Laboratories and carries an optical fiber nonconductive riser designation. The trickiest part of the Powersoft installation wa the splice of the tube cable in the maintenance hole. Each inner tube had to be spliced to its designated counterpart to form a ring topology, and then the splices were encased in a waterproof splice closure. The tube cable was tested, first with pressurized air to ensure there were no leaks and then by blowing a 5-millimeter steel ball through th tubes to ensure that there were no obstructions. The fibers could then be blown in from the equipment rooms of the different buildings. Each fiber bundle contains up to 18 fibers, either singlemode or 50/125-micron multimode. The Powersoft multimode fibers were installed in six-tube cables and the entire system was backed up with a redundant fiber. Each building, therefore, is joined by two 18-strand fiber bundles, and four of the inner tubes remain vacant for future use. The fibers were terminated in the company’s equipment rooms in the standard breakout trays. Advantages for installer Cablenet technician Rich Aiello was responsible for pulling the tube cable and blowing the fibers. “At Powersoft,” he says, “the maintenance hole is full of water and all kinds of debris–old sneakers, you name it. We wanted to go through there only once.” The maintenance hole had to be pumped out only one time for installation and splicing of the tube cable. In addition, the underground conduits in the Concord Farms office park were damaged and partially clogged, so it was only possible to pull a tube cable one inch in diameter through the existing system. If the traditional innerduct had been used, only one could have been installed because of size limitations, and one fiber cable instead of six could have been put in. Other advantages of air-blown fiber are that smaller installation crews can be used, and the work can be completed more quickly. With conventional fiber installation, it would have taken a four-person team three days or more to do the Powersoft job, according to Aiello. “With air-blown fiber, it took a two-man crew less than a day–a total of eight man-hours–to install 3000 feet of fiber, he remarks. The fiber installation device is light, portable and uncomplicated. It comes disassembled in a carrying case and can be assembled in about ten minutes with standard hand tools and a few special ones included with the device. Once assembled, the whole system weighs about ten pounds. Compressed air or gas is bled into the inner tube at a measured rate to float the fiber cable, which is paid off a reel about the size of an auto steering wheel using a small motor. The system comes with fiber reels, but the compressed gas must be obtained separately from a local welding supply shop. “The fiber pays out at about 100 feet per minute,” asserts Aiello, “and we’ve easily done 3000 or 4000 feet on a job. You can blow it uphill or downhill. You can do amazing things. In my mind, it’s the best long-term approach.” Rethinking system design CCR’s Kerwin is equally enthusiastic about the technology. From the perspective of the system designer, the technology offers a new way of thinking about optical fiber installation. “It’s going to force people to think differently about fiber,” he says. “It’s not an all-or-nothing thing anymore; it can be viewed as an as-needed utility.” You can, for instance, put in the tube cable and then wait for the cost of opto-electronics to decrease before installing the fiber. You do not have to run 144-fiber cables everywhere and then find you are not using your capacity. You can blow in two fibers and come back later to blow in more, if necessary. You can purchase only the capacity you need. Kerwin points out that air-blown fiber is good for rehabilitating existing installations, as was the case at Powersoft, because it can be installed easily in existing conduits, without excavation, and the work can be done quickly and without disturbing other work crews. It is appropriate for colleges and schools too. “They want to recable only once every 100 years,” Kerwin says, “so it’s best to put the best possible system in place today.” It is also useful where many changes are anticipated, and work must go on in a building despite rewiring. In a hospital, for example, fiber bundles can be blown into vacant inner tubes from the equipment room without disturbing patients on the upper floors. The air-blown fiber system available in North America was developed by British Telecom in London in the early 1980s. Used first commercially in Europe and then Japan, British Telecom has master-licensed the process to Sumitomo Electric Corp. in Research Triangle Park, NC. Sumitomo completed its first commercial installation in the United States in 1991 and still does some installations through an independent subsidiary, but today the company prefers to sublicense to design and installation companies such as CCR/Cablenet. Sumitomo’s manufacturer for cabling system products, Kurt Templeman says sublicenses exist in different regions of the country, but these agreements are not exclusive, and the entire United States is not fully covered. About 100 air-blown fiber installations have been completed nationwide. Templeman points out there are many differences between the designing for conventional fiber cable versus air-blown fiber. With conventional cable, for example, you have to plan for splice points, but air-blown fiber depends on continuous cable runs, so there is no need for splicing. “The key,” he adds, “is that I can create any path I want once I’ve installed the network of tube cables. As the network changes, I can add the fiber I need, when I need it, where I need it. I don’t have to do all the planning upfront or try to guess what all my needs are going to be for the next five years.” This technology could be especially valuable in the case of a service interruption caused by a cable dig-up. Imagine, for example, that your cable has been cut by a backhoe. If your cable run is 500 meters you could blow the broken fiber out in about 10 minutes and then blow in unbroken replacement fiber in about the same time. To repair the cut tube cable, you cut out the damaged section, insert a new section and couple the two together quickly with standard push-fit connectors. Some questions raised The method may raise practical questions for some, however. Is there extensive tensile stress on the fiber when it is blown in? Is the fiber sturdy enough to be blown in, blown out and then reused at another location? Is there a cost premium for this new technology? According to Templeman, tensile stress is minimal because there is no pig or parachute attached to the end of the fiber. A tip is put on the end of the fiber, but it is simply to keep the fiber from getting caught on the joints of the push-fit connectors where the tube cables connect together. The fiber bundle is carried the entire length of the tube by the viscous flow of air. The only rationale for the motor at the blowing head is to pull the fiber bundle off the reel and insert it into the tube. “As to reusing fiber,” states Templeman, “we’ve done a significant amount of testing, and what we’ve found is that as long as all installation procedures are followed, there’s no problem in the fiber bundle handling the applications for which it was designed. It doesn’t deteriorate over time.” In laboratory tests, for example, fiber bundles have been blown in and out more than 50 times without damage–an extreme case for a real-world environment. Sumitomo has also performed aging, temperature and humidity testing on fiber bundles, and water immersion testing on the tube cables. The cost equation is more difficult to balance. The materials are more expensive. The fiber bundles for instance, must be carefully manufactured so they feed through the blowing head without jamming and slide along the inner tubes without lubrication. On the other hand, labor cost may be lower. CCR’s Kerwin estimates that the labor cost of blowing in fiber at a later time is only about 15% of what it would cost to dig out and replace innerduct. This points to the best installation scenario for air-blown fiber. It occurs, according to Templeman, where the applications are fluid and unpredictable. “I would not recommend the product,” he says, “where the applications are not changing drastically over the next few years. You need requirements for additional fiber and for configuration changes.” The technology is probably also unsuited to very large fiber runs, such as trunk lines between telephone company central offices, and to very small ones, where only a few fibers are required and where there is little chance of growth or change. In the right application, though, air-blown fiber is no more expensive than a conventional fiber installation, claims Kerwin, and it offers great advantages in other areas. He offers his clients both applications. “We meet with the customer and discuss the advantages of both traditional and air-blown fiber methods,” he says. “We use this discussion as a platform to find out where the client is anticipating his technological growth. Then we look for the right fit. It’s usually a cost-effective long-term plan to go with air-blown fiber, but it depends on the client’s present and future needs.”

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