The choice of outside plant (OSP) fiber optics components begins with last month’s work: developing the route the cable plant will follow. Once the route is set, it is certain where cables will be run, where splices are located and where the cables will be terminated. All that determines which choices must be made on cable type, hardware and sometimes installation methodology.
Most projects start with the choice of a cable. Since OSP applications often use significant lengths of cables, the cables can be made to order, allowing optimization for that particular installation. This usually allows saving costs but requires more knowledge on the part of the user and more time to negotiate with several cable manufacturers.
To begin specifying the cable, one must know how many fibers of which type will be included in each cable. It’s important to know that fiber, especially single-mode fiber used in virtually all OSP installations, is cheap, while installation is expensive. As one industry pundit described it, “Fiber is now cheaper than kite string or monofilament fishing line.” Installation of an OSP cable may cost a hundred times the cost of the cable itself.
Choosing a single-mode fiber is easy. Basic 1,300-nm, single-mode (called G.652 fiber) is adequate for all but the longest links or those using wavelength-division multiplexing. Those may need special fiber optimized at 1,500–1,600 nm (G.653 or G.654). OM3 type laser optimized 50/125 multimode fiber is probably the best choice for any shorter multimode OSP runs, as its lower attenuation and higher bandwidth will make most networks function better.
Including more fibers in a cable will not increase the cable cost proportionally. The basic cost of making a cable is fixed, but adding fibers will not increase the cost much at all. Choosing a standard design will help reduce costs, too, as manufacturers may have the cable in stock or be able to make your cable at the same time as others of similar design. The only real cost for adding more fibers is additional splicing and termination costs, still small with respect to total installed cost. And remember that having additional fibers for future expansion, backup systems or in case of breaks involving individual fibers can save many future headaches.
Common traits of all outside plant cables include strength and water or moisture protection. The necessary strength of the cable will depend on the installation method. All cables installed outdoors must be rated for moisture and water resistance. Until recently, most people chose a gel-filled cable, but dry water-blocked cables now are widely available and preferred by many users. These cables use water-absorbing tape and powder that expands and seals the cable if any water enters. Installers especially prefer the dry cables as the installation does not require the messy, tedious removal of the gel used in many cables, greatly reducing preparation for splicing or termination.
OSP cable construction types are specifically designed for strength, depending on if they are to be direct buried, buried in conduit, placed underwater or run aerially on poles. The proper type must be chosen for the cable runs. Some applications may even use several types of cable. Having good construction plans will help in working with cable manufacturers to find the appropriate cable types and to order sufficient quantities. One must always order more cable than route lengths to allow for service loops, termination preparation and excess for possible future restoration needs.
Like cable types, cable plant hardware types are quite diverse and should be chosen to match the cable types being used. With so many choices in hardware, working with cable manufacturers is the most expeditious way to choose hardware and ensure compatibility. Besides cable compatibility, the hardware must be appropriate for the location, which can be outdoors; hung on poles; buried; underwater; or inside pedestals, vaults or buildings, etc. Sometimes the hardware will need to be compatible with local zoning, e.g., in subdivisions or business parks. The time consumed in choosing this hardware can be lengthy but is very important for the long-term reliability of the cable plant.
Splicing and termination are the final category of components to be chosen. Most OSP single-mode fiber is fusion-spliced for low loss, low reflectance and reliability. Multimode fiber, especially OM3, also is easily fusion-spliced, but only a few splices are necessary. Mechanical splicing will provide adequate performance and reliability. If termination is done directly on multimode OSP cables, breakout kits will be necessary to sleeve fibers for reliability when connectors are directly attached. This takes more installation time than splicing preterminated pigtails on the cables, as is common with single-mode fiber cables, and may not save any costs. Even complete preterminated OSP systems are becoming available, reducing the time necessary for termination and splicing. Talk to the cable manufacturers to determine feasibility of this option.
Choosing the proper components for OSP installations can take time but is important for system operation. Once components are chosen, the materials lists are added to the documentation for purchase, installation and future reference.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.