Academic journal article Library Technology Reports

Cabling, LANs, and Networks

Academic journal article Library Technology Reports

Cabling, LANs, and Networks

Article excerpt


Cabling, once a relatively simple proposition, is now one of the most rapidly changing areas of information technology. In the 1970s, second, and in the 1980s, 1200 baud capability was considered adequate. In the early l990s applications demanded cabling capable of handling 2400 to 9600 bits per second; in the mid-1990s higher speed connections are de rigueur for many applications. Cabling can have a significant impact on physical facilities.


Future cabling should anticipate the time when 100 Megabits per second (Mbps) data transfer will be necessary to support full-text, images, and multimedia in library systems. Until recently, coaxial cable and fiber optics were the only transmission media considered capable of handling such volume. Standards now have been developed for 100 Mbps transmission using Category 5 (or Level 5) unshielded twisted-pair (UTP) and shielded twisted-pair (STP). The standards for 100 Mbps over UTP, designated lOOVG-AnyLAN and 100 Base-T, have not yet been adopted by all manufacturers. Many proprietary solutions continue to be offered.

Category 5 UTP, consisting of four copper pairs of 100-ohm wire twisted together and enclosed in a thin PVC jacket to reduce interference, supports up to 100 Mbps transmission speeds. Category 4 UTP-also four copper pairs of 100-ohm wire twisted together-supports up to 16 Mbps and continues to be available. It is used primarily for 16 Mbps token ring LANs. Category 3 UTP-also four copper pairs of 100-ohm wire twisted together-supports transmission speeds of up to 10 Mbps and is the most widely installed cabling today. It is what telcos usually use for voice service. When used for data it supports 4 Mbps token ring LANs, 10 Mbps Ethernet LANs, and direct connections of terminals and PCs to host computers. With the emergence of enhanced data transmission capacity, UTP is expected to remain the most popular data cabling in libraries, but with Category 5 displacing categories 3 and 4. Whichever UTP is used, its use should be limited to a single application: voice or data. Mixed applications over the same circuits should be avoided. UTP usually costs $.15 to $.17 per foot when purchased in bulk.

Shielded twisted-pair (STP)-two copper pairs of 155-ohm wire individually wrapped in metal shielding then sheathed in an additional braided metal shield and a plastic outer jacket-can be used in the same conduit as telephone and electrical cabling. It can support up to 155 Mbps, but its major disadvantages compared to UTP are greater cost (at least six times as much for the cable and 25 percent more for the installation) and greater thickness (thus making it more difficult to pull). The amount of space required to pull STP is a half-inch; UTP requires a quarter-inch when 25 twisted-pairs are stranded into a single cable or a solid cable is used. The advantage of STP, however, is its greater protection against interference. It can be used for multiple applications but the type of cable used in this case is different from the regular type-it includes an additional voice circuit. Plenum-rated STP is needed when pulling horizontally between floors. It usually costs $1 per foot when purchased in bulk.

Coaxial cable-a single 75-ohm copper core wrapped in polyurethane insulation and sheathed in an additional braided copper wire and plastic jacket-while perceived by some as an old-fashioned technology for data transmission, remains an excellent choice, especially when data, video, and voice all are to be accommodated. A 50-ohm version, known as ThinNet, is used exclusively for 10 Mbps Ethernet LANs. Plenum-rated coax is usually required when pulling horizontally between floors. It costs $.60 (ThinNet) to $1.30 a foot when purchased in bulk.

Fiber optic cabling-a single strand of 62.5 or 125-micron optical fiber in a plastic jacket-is expected to remain the preferred choice for backbone networks serving a large organization where the amount of data being transferred exceeds the capacity of coaxial cable. …

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