Fiber versus copper in local networks
WILLIAM B. GARDNER, AT&T
Although fiber has prevailed over copper in long-haul and interexchange networks, the competition continues in local area networks. Several members of the Telecommunications Industry Association claim that some of the arguments used against fiber-optic cable for local area network applications are, in fact, misconceptions.
Consequently, the TIA chartered a Fiber Optics LAN Section to educate end users about the technical and economic benefits that optical transmission brings to local area network and fiber-to-the-desk applications (see Lightwave, November 1994, page 8).
Led by Glenn Moore of AMP Inc., who chairs the Fiber Optics LAN Section, and Steve Swanson of Corning Inc., who chairs the standards subcommittee, the group has responded to the principal arguments used to advocate Category 5 unshielded twisted-pair copper wire systems.
One of the arguments concerns price. Fiber-optic technology is not too expensive. There is no longer a significant price premium associated with fiber-optic networks operating above 100 megabits per second, and there is only a modest premium for lower-bit-rate Ethernet or token ring systems. Any small premium in initial cost is outweighed by reduced operating and maintenance costs over the life of the network.
Another argument relates to the number of outages. Several studies have shown that copper-based systems average more than two outages--caused by electromagnetic interference, radio-frequency interference, crosstalk and impedance mismatches--per year. Optical fiber`s immunity to these parameters increases network reliability by at least 60%. Optical transceivers and electronics have historically cost as much as twice that of their copper-based counterparts; however, this differential has decreased markedly with the increased use of low-cost light-emitting diodes and compact-disc lasers.
Installation and testing are other concerns. Fiber-optic cable is not difficult to install and test (see page 46). Two-fiber cable is 25% to 40% lighter and occupies 15% less space than four-pair Category 5 copper cable. Despite its size and weight, fiber-optic cable can typically withstand 150 pounds of pulling force, compared to 25 pounds for Category 5 copper cable. Furthermore, because crosstalk is a problem for copper but not fiber, testing is generally simpler and less expensive for fiber.
Contrary to general perception, copper is not adequate for current and future local area network requirements. Even though copper cable is now available to support higher data rates, this advance involves tradeoffs in link lengths, bit errors and reliability.
Because optical fiber is immune to interference and crosstalk, error rates are reduced from a few per minute with copper to a few per month with fiber, even when data rates exceed 1 gigabit per second. This capability reduces the need for cumbersome error detection and correction protocols.
Fiber`s low attenuation and high bandwidth also support increased transmission distances, which in turn increase network design flexibility. For example, longer link lengths can reduce the number of wiring closets required and allow centralized local area network electronics for increased security and reduced costs.
In addition to providing literature about fiber optics technology, the Fiber Optics LAN Section also offers advice to national and international committees to improve existing fiber local area network standards. The group points out the need to draft cost-effective fiber standards for building-cable distances of less than 300 meters. These standards would supplement the plethora of campus backbone standards for distances to 2 kilometers. Standards for 300-meter distances would provide users with a migration path based on current and proposed source technology that supports higher network speeds for all applications. q