The choice of premises fiber optic components is affected by -several factors, including communications equipment, physical routing of the cable plant and building codes and regulations. If the design is a corporate local area network (LAN), the design will probably include a fiber optic backbone, connecting computer rooms to wiring closets. The wiring closets house switches that convert the fiber backbone to unshielded twisted pair (UTP) copper for cable-connected desktops and either copper- or fiber-to-wireless access points. Some desktops, especially in engineering or design departments, may require fiber to the desktop for its greater bandwidth. Extra cables or fibers may be needed for security systems (alarms, access systems or closed-circuit television cameras) and building management systems.
Design of the fiber optic cable plant requires coordinating with those involved in the network in any way, including IT personnel, company management, architects and engineers, etc., to ensure all cabling requirements are considered and to allow the sharing of resources.
As in outside plant design, consider the fiber choice first. Most premises networks use multimode fiber. However, many users now install hybrid cables with single-mode fibers for future expansion. The 62.5/125 micron fiber (OM1 fiber) that has been used for almost two decades has mostly been superseded by the new 50/125 laser-optimized fiber (OM3), as it offers substantial distance advantages.
Virtually all equipment will operate over 50/125 OM3 fiber just as well as it did on 62.5/125 OM1 fiber, but it’s always a good idea to check with the equipment manufacturers to be sure. Remember in the design documentation to include directions to mark all cables and patch panels with aqua-colored tags, indicative of OM3 fiber.
Cable in premises applications is generally either distribution or breakout cable. Distribution cables have more fibers in a smaller diameter cable, but require termination inside patch panels or wall-mounted boxes. Breakout cables are bulky, but they allow direct connection without hardware, making them convenient for industrial use. Fiber count can be an issue, as backbone cables now have many fibers for current use, future expansion and spares, making distribution cables the more popular choice. The cable jacket must be fire-retardant per the National Electrical Code, generally OFNR-rated (riser) would work unless the cable is in air-handling areas above ceilings where OFNP (plenum) is needed. Cable jacket color for OM3 cables can be ordered in aqua for identification as both fiber optics and OM3 50/125 fibers.
If the cable is going to be run between buildings, indoor/outdoor designs are available that have dry water blocking and a double jacket. The outer jacket is moisture-resistant for outdoor use but can be easily stripped, leaving the fire-rated inner jacket for indoor runs.
Fiber optic connector choices also are changing. STs and even SCs are succumbing to the success of the smaller LC connector. Since most fast (gigabit and above) equipment uses LC connectors, using them in the cable plant means only one connector needs to be supported. The LC offers another advantage for those users who are upgrading to OM3 fiber. The LC connector is incompatible with SC and ST connectors, so using it on 50/125 fiber cable plants prevents mixing 50 and 62.5 fibers with high fiber mismatch losses.
Premises cables need to be run separately from copper cables to prevent crushing. Sometimes they are hung carefully below copper cable trays or pulled in innerduct. Using innerduct can save installation time, since the duct (which can be purchased with pull tapes already inside) can be installed quickly without fear of damage, and then the fiber optic cable pulled quickly and easily. Some applications may require installing fiber optic cables inside conduit, which requires one to minimize bends in the conduit or provide junction boxes for intermediate pulls to limit pulling force. It also may be necessary to use fiber optic cable lubricants to reduce friction.
Hardware necessary for the installation will need to be chosen based on where the cables are terminated. Premises runs generally are point-to-point and are not spliced. Wherever possible, allow room for large radii in the patch panels or wall-mounted boxes to minimize stress on the fibers. Choose hardware that is easy to enter for moves, adds and changes but lockable to prevent intrusion.
In premises applications, it is worth considering a preterminated system, which uses backbone cables terminated in multifiber connectors and preterminated patch panel modules. If the facility layout is properly designed, the cable manufacturer can help you create a plug-and-play system that needs no on-site termination. The cost may be competitive to a field--terminated system.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.