Few components of a local area network elicit more consternation than cabling. It can be expensive to install and difficult to manage. Coaxial cable has been the principal transmission media for most LANs, but other types of cabling, including shielded and unshielded twisted pair, are gaining acceptance. Even skeptics acknowledge that the cable originally specified for Ethernet, the most popular type of LAN, was over-engineered. It contained more shielding than necessary and was thus thicker and bulkier (and more expensive) than needed. In fact, it provided so much shielding that it could be used in an electrical transformer room.
However, it wasn't until 1985 that a smaller, single-shielded coaxial cable was certified for use with Ethernet. This cable, known as 'thin Ethernet' because of its small diameter (0.193 in.), was the central element in a LAN architecture pioneered in 1982 by 3COM--now the leading vendor of twisted-pair wire LANs. "Thin Ethernet" significantly lowered the costs of LANs. This cable is not only small in size, but highly flexible, readily fitting around tight corners and into cable trays, along baseboards, and inside partitions.
As sales of LANs have grown in recent years, so has the need to use alternate cable types for Ethernet. Although thick and thin coaxial cables provide ideal transmission media, they're not appropriate in some situations because of high installation costs or lack of a suitable environment.
The most viable of the emerging transmission alternatives have been fiber optics, various other shielded cabling (most notably, IBM's shielded twisted pair wiring), and standard telephone twisted pair.
Fiber optic cables offer clear, uniform transmission characteristics that are well suited for Ethernet application. Fiber optics employs an optical transmission scheme that differs dramatically from the one used by metal wires, which carry an electrical charge. At the center of a fiber optic cable is an extremely small glass or plastic shaft that conducts light. Fiber optics provides substantially more than the necessary bandwidth required by Ethernet and can carry Ethernet data signals about twice the distance of standard coaxial cable (about 3 miles instead of 1.5) . Furthermore, fiber optic cable is completely immune to electromagnetic and radio- frequency signals.
Numerous factors have limited fiber optic cable's widespread installation. For example, connecting two fiber optic cable segments requires making smooth, even cuts that must be polished. The two opposing glass fibers then must be aligned perfectly to ensure the integrity of the connection. This is a specialized process, and installation costs are correspondingly high, and the cost of the cable itself is comparable to that of the over-engineered "thick" coaxial cable.
IBM-pioneered products dominate the rest of shielded-cable category, principally two twisted-pair combinations designated by IBM as Type 2 (with both voice and data-grade wires) and Type 9 (with data-grade only). The material is only slightly less expensive to purchase than the coaxial and fiber optic options and it is necessary to have it installed.
The third and least expensive option, standard telephone wire, has the potential to make Ethernet easily accessible to personal computer users whose workstations, because of location or expense, are difficult or impossible to reach with the coaxial cable now generally specified for data transmission. With two or three unused pairs of unshielded wire running between each telephone connection. and a central wiring closet, unshielded twisted-pair wiring offers a widely available, ready-made means of incorporating desktop- personal computers into a LAN.
Because unshielded twisted-pair wiring is readily available in existing telephone connections, it holds tremendous economic potential for use with LANs. From a technical standpoint, the primary obstacles that must be overcome are related to its lack of shielding and its small conductive area. Specifically, these include varying impedance between the wires in the pair, attenuation on the pairs, meeting Federal Communications Commission RFI/EMI limitations and, if the user is running data and voice signals in the same jacket, overcoming the crosstalk pairs within the single jacket.
Using unshielded twisted-pair wiring will limit the distance an Ethernet signal can be transmitted. However, since approximately 80 percent of all phone installations are believed to be located within 300 feet of a wiring closet, this need not be a serious limitation.
If trouble-free operation can be achieved with unshielded twisted-pair wiring, it will not only save the cost of the wire and the labor to install it, but also facilitate the addition and removal of network nodes without disruption.
