Choosing the right fiber LAN architecture

For years, network planners have recognized the significant performance benefits of optical fiber: low attenuation, high bandwidth and smaller size, to name just a few. But until recently, many of those who sought these benefits in their installations felt they had to force-fit optical fiber into a copper-based design.

Paul Kolesar,

Lucent Technologies

For years, network planners have recognized the significant performance benefits of optical fiber: low attenuation, high bandwidth and smaller size, to name just a few. But until recently, many of those who sought these benefits in their installations felt they had to force-fit optical fiber into a copper-based design.

That is no longer the case. Industry standards have evolved to accommodate fiber, and today several architectures are available to support fiber use, not only in building backbones and telecommunications closets but for horizontal subsystems as well. An understanding of the differences between these architectures will help network planners design the fiber local area network (LAN) that best meets their needs.

Two such architectures affect the design of the equipment room, riser backbone and closet subsystems. The first is the hierarchical star architecture, a two-level star topology with crossconnect facilities in the equipment room and the telecommunications closet on each floor. The hierarchical star gives planners maximum flexibility in locating fiber-based LAN electronics, which may be distributed in the closets or centralized in the main equipment room.

The second architecture is called centralized optical-fiber cabling. This is a single-level star topology with a single crossconnect in the equipment room. From there, all links run to the work area, either in a home-run design (in which the cable is pulled through the closets in a continuous sheath) or by using an interconnection or splice in the closet. This streamlined design simplifies the installation of centralized equipment while reducing the cost of cabling, electronics, administration and maintenance. It is supported by the Telecommunications Industry Association (TIA--Arlington, VA) in its Telecommunication Systems Bulletin 72.

Zone-cabling approaches

Moving on to horizontal and work-area subsystems, network planners have a variety of new zone-cabling options, in addition to traditional individual runs and outlets, that work with both hierarchical star or centralized cabling architectures.

The first zone-cabling approach uses break-out cable from the closet to the outlets. At each zone, the outer sheath is removed and individual fibers are dropped and terminated behind the work-area outlets. In cases where cable-pulling labor costs are high, this approach can reduce installation costs. These savings may be offset in this case, however, by higher cable costs for break-out cable. This approach is compliant with the TIA`s telecommunications wiring standard, tia/eia-568a.

Another approach uses high-count optical-fiber building cable from the closet to a consolidation point (a common drop point to a group of work areas) in each zone. At this point, the cable is spliced or interconnected to low-count building cable, which is dropped and terminated behind individual outlets at each work area. This approach can reduce cable-pulling labor costs. However, these savings are offset by the additional hardware and labor charges required by the consolidation points.

A third zone cabling approach has been developed specifically for open- office environments, which have grown increasingly common in today`s workplace. This is the multiuser outlet option. Here, a high-count cable runs from the closet to a shared outlet mounted on a permanent structure (such as a support column or wall) within the open office. The outlet is sized to hold enough connections for several work areas. Patch cords extend from the multiuser outlet through the raceways of the modular furniture and connect directly to the work-area electronics.

Because there are no individual outlets on the furniture, the multiuser outlet approach allows users to rearrange furniture without disturbing permanent building cabling. The use of high-count horizontal cables, shared outlets and a single patch cord for each work area minimizes installation costs. When used in combination with home-run centralized cabling architecture, this option also reduces the need for vertical pathway space. The consolidation point and multiuser outlet approaches are supported by the TIA in TSB-75.

Choosing the right architecture is a matter of understanding your building environment and your specific needs for both today and the future. You must consider the structure of the building and the way it is occupied--whether offices and workspaces are opened or closed. And you must decide which of the benefits mentioned--such as initial cost, greater flexibility, low maintenance--are most important. These new options allow network designs optimized to your needs, while using optical fiber to its best advantage.

Paul Kolesar is a member of technical staff at Lucent Technologies (Middletown, NJ) and a member of the TIA`s Fiber Optics LAN Section, whose other member companies include 3M, AMP, Berk-Tek, Belden, CommScope General Instrument, Corning, Siecor, SpecTran and Sumitomo Electric.

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