Horizontal network topologies for optical-fiber cable systems

Jan. 1, 1995
Advances in optical-fiber technology have made installing new data cable in the horizontal network easier and less costly.

Advances in optical-fiber technology have made installing new data cable in the horizontal network easier and less costly.

Tony E. Beam, Siecor Corp.

The benefits of fiber optics, specifically high bandwidth, low attenuation and system operating margin, allow greater flexibility in cabling design. Fiber-based systems can be implemented for today`s network requirements, such as Ethernet and token ring. These systems will also easily accommodate growth to higher-speed networks such as fiber distributed data interface, fast Ethernet, asynchronous transfer mode, Fibre Channel and beyond, without concern that crosstalk, electromagnetic/radio-frequency interference and Federal Communications Commission issues will be uncovered when frequencies are increased.

As contractors, installers and maintenance crews evaluate the requirements to make high-speed data a reality, they are quickly learning that fiber to the desk is a cost-effective and reliable solution. Let us consider three cable options: traditional network cabling, wiring with a multi-user outlet and collapsed cabling.

Traditional network cabling

The traditional network consists of an individual outlet for each user within 300 feet of the telecommunications closet. The network typically has data electronics equipment--hub, concentrator or switch--located in each telecommunications closet within the building.

This network is normally implemented with a small-fiber-count backbone cable, such as 12 or 24 fibers, because the electronics are located at each closet. While optical-fiber cabling provides high performance in a traditional horizontal cabling network--that is, from the outlet to the closet--when using fiber, two alternate designs are available that are not viable for high-speed applications over Category 5 unshielded twisted-pair copper cable.

Wiring with a multi-user outlet

The multi-user outlet configuration is based on the fact that much of today`s office space consists of open-systems furniture that is often rearranged. The multi-user outlet requires a high-fiber-count cable placed from the closet in a grid-type wiring scheme to an area in the open office where there is a fairly permanent structure, such as a wiring or structural column, or cabinet. At this location, a multi-user outlet capable of supporting four to 12 open offices is installed, instead of depending on individual outlets. Optical-fiber patch cords are then installed through the furniture raceways from the multi-user outlet to the office area, allowing the user to rearrange the furniture without disruption to or relocation of the horizontal cabling. Category 5 UTP systems do not have this flexibility because of the three-meter functional limit on the work-area patch cord; however, this approach can be used for Category 3 UTP systems in support of voice.

Fiber-optic cabling offers significant benefits to users of open-systems furniture. Because of its immunity to EMI, optical-fiber cable can be included in older furniture with telecommunications raceways located beside power cable. Category 5 UTP cable requires the separation of telecommunications cable from power cables.

Collapsed cabling system

Another cabling configuration available to the optical-fiber cable user is the collapsed cabling network. While Category 5 horizontal cabling systems are limited to 100 meters in total length and require electronics in the closet for high-speed applications, optical-fiber systems do not require the use of electronics in closets on each floor and therefore allow for a totally collapsed cabling network. This design lets the end user place all the data electronics in one closet in the building instead of in multiple closets throughout the building, simplifying the management of optical-fiber networks. It provides for more efficient use of ports on electronic hubs and switches, and allows users to establish work-group networks when group members are located on different floors.

This design can be implemented in three ways: home-run cables from outlets or multi-user outlets to the single central closet in the building; a splice between the horizontal cabling and a high-fiber-count backbone cable in the closet; or a passive crossconnect in each closet.

While each of these options--traditional, multi-user and collapsed cabling--have their corresponding benefits, most users will, and should, choose collapsed cabling because it allows simpler cable placement and offers an acceptable degree of flexibility. By using a splice in the telecommunications closet, users can be added at a reasonable cost. The installer has the option to choose either fusion or mechanical splicing, or to use remateable connectors in an interconnect, depending on the desired degree of flexibility.

Bills of materials

After a topology has been selected, it is important to specify the materials required for a wiring scheme. Assume the following in developing a bill of materials for a fiber-to-the-desk infrastructure:

- Each user has only one data application; for example, a two-fiber requirement.

- Horizontal cabling is plenum-rated; intrabuildlng backbone is riser-rated.

- Multi-user outlets are required to serve eight users.

- Closet hardware is to be rack-mounted.

- The 568SC duplex SC connector interface is chosen for outlet and connecting hardware.

- Category 3 UTP voice cable requirements are not included.

Regardless of which topology is used, testing and certifying the optical-fiber cable system remains the same. Optical-fiber systems are easy to test with a simple power meter and source. The test equipment, which must be calibrated to a National Institute of Standards and Technology standard, is inexpensive and simple to use, and a number of manufacturers offer handheld test equipment to test, certify and maintain an optical-fiber network.

On the other hand, no approved field-test procedures exist for testing and certifying a Category 5 UTP cabling system. The Telecommunications Industry Association working group has only now begun to investigate this complicated problem. Certifying a Category 5 UTP cabling system would require bidirectional testing of the completed system for length, attenuation and near-end crosstalk across the entire frequency range to 100 megahertz.

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Horizontal Cabling Standards

Because of the distance limitations of copper cabling and the fact that there is little or no operating margin, Telecommunications Industry Association Standard 568A, commercial building telecommunications cabling, has placed stringent design requirements on horizontal cabling. The total system length from the outlet to the telecommunications closet must be no more than 90 meters, and the total patch cord and jumper length at the workstation and closet must not exceed 10 meters. Furthermore, because of these limitations, TIA-569, the commercial building standard for telecommunications pathways and spaces, specifies closets with a minimum of 10x7 feet of floor space on every floor and with the capability to house active electronics.

The present draft of TIA-568A requires optical-fiber patch cords to meet the combined transmission performance requirements of the horizontal cable and optical connector. The same document had to relax by 20% the attenuation requirement for Category 5 patch cords, and requires that the length of copper patch cords at the workstation should not exceed 3 meters. However, optical-fiber patch cords can be longer with no adverse functional effect on the network, which greatly increases cabling flexibility, especially in open-systems furniture applications.

Although TIA-568A places the same restrictions on optical-fiber cable systems, these restrictions are not based on the functional limitations of those systems. When possible, users should follow these recommendations; however, specific requirements, site conditions or budget constraints may prevent them from doing so. Many users have requirements that only alternate designs offered by fiber to the desk can meet, such as those open-systems furniture or centralized management. Special task groups of the TIA are studying these items for possible inclusion in future revisions of TIA-568A.

Tony E. Beam is Universal Transport System marketing manager at Siecor Corp., Hickory, NC.

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