100-Ohm Balanced Twisted-Pair Cabling

Ohm Balanced Twisted Pair Cabling

This Chapter uses the TIA/EIA nomenclature for twisted-pair cabling. TIA/EIA-568-B [70] , [71] , [72] calls out both unshielded and screened versions of 100- W balanced twisted-pair cabling.

Unshielded 100- W twisted-pair cabling (UTP) is composed of a number of balanced, twisted pairs. Each pair of conductors is wound tightly together. As defined for horizontal building wiring (see Chapter 7), UTP contains four pairs, for a total count of four or eight conductors respectively. Typical construction for four-pair UTP appears in Figure 8.1.

Figure 8.1. Construction of typical four-pair unshielded twisted-pair (UTP) LAN cable.

UTP is cheap, ubiquitous, and easy to handle. A 1,000- foot roll of four-pair UTP can be carried in one hand up a ladder by a lone technician. Judiciously hurling the reel through the space above the false-ceiling tiles in a modern high-rise, one technician can quickly distribute UTP cabling among large number of offices. [64] The conductor is color -coded according to a well-recognized standard system. It may include a tough rip cord inside every jacket. Installation people yank the rip cord to quickly strip back the outer cable jacket prior to flaying out and terminating the individual conductors. An experienced technician can terminate four pairs of UTP in only a few seconds. These physical features contribute to the enormous popularity of UTP.

[64] The wire unrolls as it flies thorugh the false-ceiling space. I've done this, but if you break any ceiling tiles, please don't blame me.

Screened 100- W twisted-pair cabling (ScTP) incorporates an electrically continuous shield just under the thick plastic jacket, enveloping the core of the cable. According to the standard, the shield consists of "plastic and metal laminated tape with one or more longitudinal, helical, or braided noninsulated solid tin-coated copper conductors(s) [drain wire(s)] of 26 AWG equivalent or larger that are in contact with the metal side of the tape." ScTP cables have the same interior construction as UTP and meet the same electrical specifications for categories 3, 5e, and 6. Since the signal propagation issues are the same in both cables, I shall refer subsequently to 100- W balanced twisted-pair cabling as UTP.

Operating in a parallel but slightly different universe, ISO/IEC standards for generic building wiring [73] , [74] also specify category 3 horizontal wiring [75] , [76] , [77] , and categories 5e and 6 horizontal wiring [78] , [79] with performance essentially the same as that called out in TIA/EIA-568-B.2. In addition, [78] and [79] define another version of horizontal wiring called category 7 with performance slightly superior to category 6 and parameters guaranteed to 600 MHz. Although the North American TIA/EIA committees and the international ISO/IEC committees have worked hard to harmonize their standards, many differences still apply.

Beware that cable definitions are in constant flux. Cable manufacturers have determined it is to their advantage to promulgate new standards as often as possible, thus driving up customer demand for building rewiring . This works counter to the interests of communications systems vendors , who seek a consistent and reliable market for their products. The natural tension between these two groups produced this amusing statement in the forward to IEC 61156-5 [78] , released in March 2002: "The committee has decided that the contents of this publication will remain unchanged until 2004." When first penned, that promise was good for only 21 months. It hints that some members of the committee would like the standard to change even sooner. If you expect to keep pace with the latest developments in the cabling industry, you must read all the latest standards, read all the amendments , and then check with the relevant committees to obtain copies of upcoming drafts.

POINT TO REMEMBER

• Cabling standards proliferate faster than bunnies.