Designing and Building Centralized Fiber Networks

Last month, we mentioned centralized fiber local area networks (LANs) as a place where fiber optics are more cost effective than copper wiring. What exactly is a centralized fiber network and how is one designed and built? The traditional LAN cabling layout, as described in the original EIA/TIA 568 standard, follows decades-old telephone system designs. It divides the LAN into “horizontal” cabling no longer than 90 meters, connecting the desktop to a hub in a wiring closet. The “horizontal” cabling connects to a “backbone,” which, by virtue of bandwidth and distance requirements, is generally fiber optics already. The backbone connects all the hubs to a central communications facility, usually called a main telecommunications closet, where the complete LAN is managed and connections to the outside world are terminated. Every closet needs a hub to interconnect the desktop to the backbone, which requires space, power, UPS, air conditioning, and installation. Most closets have punchdowns, patch panels, and other passive hardware too. With fiber optic cabling, you aren’t limited to 90 meters. You don’t need horizontal and backbone links, or even a telecommunications closet! You just need one link, a pair of fibers straight from the desktop to the main telecommunications closet. You don’t need a hub, power, and backup power for the hub; racks and space for all that; or installation and maintenance labor. At most, with fiber you have an intermediate passive patch panel to connect desktop cables to the backbone cable and some extra (and inexpensive) fibers in the backbone cables. The EIA/TIA 568 committee has issued TSB-72, which recognizes a “centralized fiber optic network” solution for an all-fiber network. The newest version of the 568 standard is being written in three parts, and the fiber optic section, 568-B3, has already been published. You can ignore the “horizontal” and “backbone” nomenclature still used. In fact, fiber doesn’t need to be segmented like that. But you can assure your customers that an all-fiber network meets the “568” standard. Designing or installing a centralized fiber architecture is simple. First, you must know how many desktops you will be serving. Each will need two fibers for connection to the network, one transmitting and one receiving. Some networks use simple two-fiber cables to each desktop, while some will run an extra pair for future expansion. All are terminated in a work area outlet, often shared with a Category 3 copper cable for the telephone. (That need will vanish soon, when we run “voice over Internet Protocol (VoIP)” on our computer networks.) The backbone cable consolidates the desktops in a zone. At the zone connection, you terminate all the desktop cables and connect them to a backbone cable. In a centralized fiber network, one difference with a horizontal copper/hub/backbone fiber architecture is the number of fibers in the backbone cables. You will be connecting each desktop directly to the communications room, so you will need two fibers in each backbone cable for each desktop, plus spares. In a network with hubs, the backbone cable uses two fibers to connect the hubs to the communications room, so backbone cables have fewer fibers. The backbone fiber optic cable will therefore have more fibers in a centralized fiber network. However, large fiber count cables are not bulky, heavy, or even expensive. You will need space for terminations, but the new small form factor connectors, such as the LC, MT-RJ, FiberJack, and Volition, are tailor-made for these installations, where they take up a fraction of the space of conventional connectors. Some users have even used splices instead of connectors, with even less space necessary. In the communications room, racks of patch panels may be necessary to terminate all the backbone cables if you have many users. Then patchcords will run off the switches and hubs, which are kept in one location. The most obvious advantage of a centralized fiber network is that all the network electronics are concentrated in the central communications room. Having all the electronics in one location makes it much easier to manage, troubleshoot, and make continual moves, adds, and changes (MACs). It also means that in large networks you do not need electronics in each building, which can be a security issue. Note also there is no need for electronics in the telecommunications closet. Fiber needs only passive connections, just like the telephone system. Does this mean the telecommunications closet is obsolete? We’ll address that issue next month. HAYES is founder of Fotec, the fiber optic test equipment company and the Cable U training in cabling installation. He can be reached at

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