Centralized optical-fiber networks revolutionize LAN cabling design

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By Preston Buck

The all-optical-fiber centralized local area network (LAN)-a novelty not so long ago-is here to stay. In increasing numbers, network planners are taking advantage of optical fiber's extremely high bandwidth and low signal loss over distances to implement the centralized design: With all data electronics housed in a single location, optical-fiber cables provide direct connections to every workstation outlet in the network.

The centralized optical-fiber design offers many benefits, including improv ed security, fewer points of failure, and reduced telecommunications-closet (TC) build-out costs. Con sol idating network electronics, analyzers, uninterruptable power sources (UPSs), crossconnects, and servers in fewer communications closets greatly simplifies LAN management, provides more efficient use of hub ports, and allows for simple implementation of various network applications. Perhaps most significantly, the centralized optical-fiber cabling design provides a cost-effective alternative to the traditional design, because using fewer active components streamlines installation, simplifies maintenance, and lowers overall costs.

In the conventional, decentralized premises data network, backbone cables travel from a main crossconnect (or, in an interbuilding network, an intermediate crossconnect) to one or more horizontal crossconnects (HC) within TCs on each floor of a building. The HC typically includes active electronics equipment such as a hub, concentrator, or switch. Individual outlets for each user are located within 100 m of the TC and connected to the HC using a single cable per user in a physical star configuration.

In the conventional design, most interbuilding and intrabuilding backbone cable is optical fiber; the horizontal segment of the network typically comprises unshielded twisted-pair (UTP) copper cable. The transmission-distance limitations inherent in copper cabling make the distributed design a necessity, in that using copper in the horizontal requires that data electronics be located no more than 100 m from workstations.

The traditional cabling infrastructure was designed to provide maximum flexibility in the deployment of distributed electronics. However, TCs take up valuable real estate, and, because of the active electronics, they require power, air conditioning, and grounding. Decentralization increases complexity and presents many potential points of failure.

Moreover, the use of UTP copper cables in the conventional design places bandwidth limitations on the network. And because of its inherent electrical properties, UTP is vulnerable to electromagnetic interference (EMI), radio-frequency interference (RFI), crosstalk, and breaches in data security.

Today, after years of the decentralized network's popularity, managers are turning to a more elegant, efficient, and cost-effective design: the optical fiber centralized network. The centralized design provides direct connections between hundreds, even thousands of workstations and a single main crossconnect by using pull-through cables or a splice or interconnect in the TC.

With network electronics, analyzers, UPSs, crossconnects, and servers consolidated in the main crossconnect, the centralized design is a vehicle for reducing the number of TCs. The long cabling runs typical of these centralized designs, often exceeding 100 m, are perfect for fiber but impractical for copper.

Any fiber-to-the-desk design offers significant networking advantages. Most important, an all-fiber cabling infrastructure provides very high bandwidth, which has become critical for organizations that require "bandwidth-hungry" applications such as those for graphics, multimedia, and real-time video. An optical-fiber centralized network is futureproofed against growing bandwidth demands from users. Also, the fiber infrastructure is protocol-independent, able to accommodate all current and future transmission protocols-Fiber Distributed Data Interface, Asynchronous Transfer Mode, Gigabit Ethernet, 100Base-FX, 100VG-AnyLAN, Fibre Channel-without disruptive and expensive recabling. Finally, optical fiber's immunity to EMI/RFI, impedance mismatches, and ground loops improves link performance and virtually eliminates maintenance.

In addition to these performance advantages, the optical-fiber centralized design offers numerous cost-saving benefits. With direct connections between network hardware and desktops, maintenance and network management are simplified. There are fewer electronics to maintain in fewer locations, thus reducing downtime and maintenance costs. Also, network reconfiguration is simplified. A network manager can establish workgroup networks very quickly, because all cables terminate in a single location.

The centralized design also is cost-effective, because it eliminates the need for multiple TCs with active electronics, which require power and air-conditioning, as well as devices for fire detection and security. Keep in mind that the average cost of "owning" a single TC-excluding the costs of labor, power, and cooling-is approximately $355 per square foot, according to the Gartner Group.

Eliminating the costs associated with housing multiple TCs in a network translates into considerable first-installed savings. For example, Sellard Communications (Horseheads, NY) designed an optical-fiber centralized network for the Erwin, NY, manufacturing facility, a part of Corning Inc. At the Erwin plant, four TCs, required for the previous network, were replaced with one closet using the centralized design. The resultant savings in first-installed costs amounted to approximately $24,000 per closet.

In another centralized design, at the Getty Museum in Los Angeles, 55 TCs were replaced with one, at a savings of $73,000 per closet. These numbers are significant especially considering that approximately 45% of corporations have three or more TCs per floor, according to Digital Equipment Corp. (now part of Compaq).

The centralized design also is an attractive option when cabling or recabling existing buildings in which closets either do not exist or are not suitable to house active network hardware. In these situations, running optical fiber from the main crossconnect directly to workstations often is the most cost-effective installation method.

Another benefit of the optical-fiber centralized design is improved port and chassis utilization. Centralizing all electronics in the main crossconnect reduces the number of ports and chassis required by a network, resulting in cost savings. On average, only 70% of hub ports are used in the conventional decentralized design due to the varying number of users per TC. The centralized design is much more efficient-typically hub port usage is 90%. That 20% differential equals real and immediate savings.

Extending optical fiber all the way to the desktop over centralized networks will simplify maintenance and help avoid bandwidth bottlenecks. By taking advantage of fiber's superior distance performance, network designers can reduce the number of electronic components, increase ease of network administration, and most important of all, save money.

Member companies of the FOLS include 3M, AMP, Allied Telesyn, Berk-Tek, Belden Wire & Cable, CommScope, Corning, LANCAST, Lucent Technologies, Ortronics, Siecor, Siemon Co., SpecTran, Sumitomo Electric Lightwave, and Transition Networks. For more information from the FOLS visit www.fols.org.Th 0003lwe Premise

Preston Buck is the market manager for premises applications in the Telecommunications Products Division of Corning Inc. (Corning, NY). He wrote this column on behalf of the Telecommunication Industry Association's Fiber Optic LAN Section.

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