It's easy to see why optical fiber has long dominated the market as the backbone data network. Fiber's inherent advantages of superior bandwidth, low-signal attenuation, and immunity to EMI problems make it a hard technology to beat. At the same time, copper's lower price and improved Cat 5 bandwidth allowed it to dominate the horizontal-cabling portion of a building's local area network (LAN). But, now, with falling optical fiber prices and easier installation methods, copper's stronghold on horizontal datacom cabling seems to be loosening.

Up to now, the price of associated electronics, particularly transceivers, made fiber-based LAN more expensive. But with the cost of fiber-optic electronics shrinking because of price pressure and economies of scale, both users and installers tend to comparison shop more between optical fiber and copper media.

In the final analysis, fiber's advantage over copper for network horizontal cable may be long-term performance rather than first cost. Essentially fiber has greater bandwidth than copper. Therefore, optical fiber LANs can meet future applications. As new applications emerge, the electronics can be upgraded gradually without replacing the existing fiber optic cabling system.

Is fiber's bandwidth unlimited? In fiber-optic network design, the industry standard 62.5-micron multimode fiber has served as the network backbone in a building. Until now, the industry thought of the 62.5-micro multimode fiber as being able to handle ever-increasing amounts of data at higher and higher speeds. However, as protocols for higher-speed applications are developed, it seems that the optical characteristics of Fiber Distributed Data Interface (FDDI) grade 62.5-micron fiber may not provide "virtually unlimited bandwidth." Let's look at why that condition exists.

The 62.5-micron optical fiber has been used in LANs for more than a decade, and the fiber, its connectors, and its installation practices are well understood. However, the bandwidth in the 850-nm window of the LED transmitter it uses is not adequate to transmit gigabit signals along distances needed for most LAN installations. The industry is looking at new types of fiber construction and newer light sources. Designers and installers should pay attention to these developments.

Here's why. Recently, the 802.z committee of the Institute of Electrical and Electronics Engineers (IEEE) created a Gigabit Ethernet specification that calls for the transmission of 1.25 G/sec over 300 meters of FDDI-grade 62.5-micron multimode fiber. As this specification was developed, the bandwidth requirement, rather than attenuation, emerged as the limiting factor in the system. If distances are short (less than 200 meters), the standard 62.5-micron multimode fiber is satisfactory for Gigabit Ethernet, and for most other newer-term applications.

If a cabling run will be longer than 200 meters, the installation of single-mode fiber-optic cable or a hybrid cable containing single mode and multimode fiber could be the answer. Single mode fiber has an order of magnitude more bandwidth capacity than multimode fiber, is readily available and is cost-effective. But keep in mind that the cost of single-mode fiber's connector hardware and the installation is higher than multimode due to the need for tighter tolerances. Because a single-mode fiber system uses a 1300-nm laser as the transmitter, it can cost up to five times more than a multimode system, which, up to now, has used LEDs.

Another choice under the Gigabit Ethernet standard, is 50-micron multimode fiber. It offers a modal bandwidth of 500 MHz-km in both the 850- and 1300-nm windows, so it sup ports gigabit transmission over a 550-meter run. In addition, it generally costs less than 62.5-micron fiber, and it is compatible with field-installable and factory-terminated connectors.

However, 50-micron multimode fiber has some disadvantages. For example, it is more bend-sensitive than 62.5-micron fiber. In addition, installers who test single and multimode fiber with a optical time domain reflectometer (OTDR), would have to purchase an additional module for the OTDR in order to test the 50-micron multimode type. So, if 50-micron fiber is used in the network, another module for the OTDR will be required, increasing a contractor's capital outlay.

In addition, the 50-micron cable is more sensitive to bending than 62.5-micron cable, so if the cabling is not properly handled during installation, increased attenuation in the cable could result. (Note that at this time, 50-micron multimode fiber is not included in the TIA/EIA-568A standard).

Another solution now available from a number of manufacturers is a higher-bandwidth 62.5-micron fiber at the 850-nm window, which could be called an "enhanced" fiber optic cable. This fiber provides increased bandwidth for use with the VCSEL transmitter light source needed to support Gigabit Ethernet.

The concept of a single optical fiber type can serve all network needs is a misconception, because application bandwidth requirements are continually increasing. You can think of fiber technology as paralleling the changes in UTP technology over the past few years. For example, Cat 5 UTP cabling has limitations at high speeds similar to what we are finding with the 160/500 MHz-km 62.5-micron fiber. And, it is clear that quality is an important factor in optical fiber selection, just as it is in selecting Cat 5 cabling. So, just as users are now thinking of upgrading to Cat 5E and Cat 6 cabling, many too are considering an optical fiber that can perform beyond 160/500 MHz-km. For example, although the Gigabit Ethernet standard is almost finalized, work is beginning on a 2.5 Mbp/s Ethernet standard. Designers must now choose among an array of new fiber types and performances, each with varying characteristics.

For example, Corning has introduced a now multimode fiber called InfiniCor, engineered specifically for Gigabit Ethernet. The new fiber is optimized for laser-based LAN protocols, is completely compatible with existing multimode fiber networks, and it will operate at significantly greater distances at slower protocols such as Fast Ethernet, FDDI and 155 Mb/s ATM.

Fiber-to-the-desk-a reality? Simon & Schuster, the world's largest educational and English-language book publisher, recently upgraded the network cabling at six sites. All of the long-lease buildings at these locations are designed to accommodate changing technology for the next 15 years. Optical fiber, along with Cat 5 copper cabling, is key to achieving this objective.

In 1994, the company began implementing ATM protocol (155Mbp/s Asynchronous Transfer Mode) on its fiber backbone in a network. In 1998, the company began planing for Gigabit Ethernet to run over the same fiber backbone.

Distances between server rooms and hubs at the various sites average 200 meters. Cat 5 copper cabling cannot extend beyond 100 meters from a wiring closet. Multimode fiber easily covers the 500 meters specified in the TIS/EIA-568-A Commercial Building Telecommunications Cabling Standard. But fiber's low attenuation over long distances allows users, when necessary, to connect directly to servers while bypassing local hubs. These longer passive data links are key to unlocking savings, because the active electronics requirements of copper are eliminated.

In each facility, a single server room is connected to numerous hubs with a cable containing 60 strands of 62.5/125micron multimode optical fiber and 12 strands of single-mode fiber. The largest of these sites contains 18 hub rooms. Backbones, which are configured in star topologies, operate using FDDI and Fast Ethernet protocols and use 100Mbps switched Ethernet, 100 Mbps switched FDDI, 10Mbps Ethernet hubs and 10BaseT shared hubs.

At locations where leases extend beyond five years, optical fiber is used in the horizontal segment of the network. In these cases, a cable with four strands of multimode fiber extends to each desktop, and a strand is terminated on demand. In the horizontal segment of the network, "fiber-to-the-desk" helps the company plan for applications that go beyond Cat 5 copper specs, since graphics and images are important components in a publishing enterprise.

Fiber-to-the desk network designs are also suited to large manufacturing facilities where numerous heavy electrical loads abound. A manufacturing plant often has long cabling distances and is plagued by electromagnetic interference, a principal cause of network downtime.

A new facility for American Video Glass Company (AVI), Mt Pleasant, Penn., which makes glass components for television picture tubes, is served by a network that also brings fiber to the desktop. The end user connections are made either by way of two fiber cabling running to a workstation, or 24 fiber riser cables extending to passive cross-connects and then on to work area outlets via two-fiber cables. This greatly simplifies maintenance and troubleshooting and allows upgrades to be done quickly. The passive cross-connect racks take up little space, and unlike intermediate distribution frames containing active electronics, they require no power, air conditioning or grounding.

>From a central computer room, optical fibers extend to PCs in offices and >on shop floors, supporting these software applications: Enterprise >Resource Planning, Manufacturing Execution Systems, word processing, >payroll, maintenance management, engineering, quality control, and others.