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Metallic wires were used almost exclusively in telecommunications networks for the first 80 years, certainly until the development of microwave and satellite radio communications systems. Initially, uninsulated iron telegraph wires were used, although copper was soon found to be a much more appropriate medium. The early metallic electrical circuits were one-wire, supporting two-way communications with each telephone connected to ground in order to complete the circuit. In 1881, John J. Carty, a young American Bell technician and one of the original operators, suggested the use of a second wire to complete the circuit and, thereby, to avoid the emanation of electrical noise from the earth ground. In certain contemporary applications, copper-covered steel, copper alloy, nickel- and/or gold-plated copper, and even aluminum metallic conductors are employed. The most common form of copper wire used in communications is that of twisted pair [3-6].
A twisted pair (Figure 3.1) involves two copper conductors, which generally are solid core, although stranded wire is used occasionally in some applications. Each conductor is separately insulated by polyethylene, polyvinyl chloride, flouropolymer resin (Category 5, or Cat 5), Teflon®, or some other low-smoke, fire retardant substance. The insulation separates the conductors, so that the electrical circuit is not shorted. This is accomplished by virtue of the two conductors, and serves to reduce electromagnetic emissions. Both conductors serve for signal transmission and reception. Because each conductor carries a similar electrical signal, twisted pair is considered to be a balanced medium [3-1] and [3-3].
Figure 3.1 Unshielded Twisted Pair (UTP) configuration.
The separately insulated conductors are twisted 90º at routine, specified intervals, hence the term twisted pair. This twisting process serves to improve the performance of the medium by containing the electromagnetic field within the pair. Thereby, the radiation of electromagnetic energy is reduced and the strength of the signal within the wire is improved over a distance. Clearly, this reduction of radiated energy also serves to minimize the impact on adjacent pairs in a multipair cable configuration. This is especially important in high-bandwidth applications, as higher frequency signals tend to lose power more rapidly over distance. Additionally, the radiated electromagnetic field tends to be greater at higher frequencies, impacting adjacent pairs to a greater extent. Generally speaking, the more twists per foot, the better the performance of the wire [3-1].
Gauge is a measure of the thickness of the conductor. The thicker the wire, the less the resistance, the stronger the signal over a given distance, and the better the performance of the medium. Thicker wires also offer the advantage of greater break strength.
American Wire Gauge (AWG) is a commonly used standard measurement of gauge, although others are used outside the United States. The gauge numbers are retrogressive; in other words, the larger the number, the smaller the conductor. Originally known as Brown and Sharp (B&S) Gauge, the AWG number indicated the number of times the copper wire was drawn through the wire machine to reduce its diameter. As an example, a 24-gauge (AWG) wire has a diameter of .0201 in. (.511mm), a weight of 1.22 lbs./ft. (1.82 kg./km.), a maximum break strength of 12.69 lbs. (5.756 kg.) and D.C. resistance ohms of 25.7/1000ft. (84.2/km.). Twisted pair commonly employed in telephone company networks varies from 19 to 28-gauge, with the most common being 24-gauge. Table 3.2 provides an abbreviated comparison of the various UTP Categories [3-7].
Category | Gauge (AWG) | Performance | Data |
---|---|---|---|
Cat 1 | Various | Undetermined | No |
Cat 2 | 22 & 24 | Undetermined | No |
Cat 3 | 22 & 24 | 16 MHz10 Mbps | Yes |
Cat 4 | Various | 16 Mbps | Yes |
Cat 5 | Various | 100 Mbps | Yes |
In a single pair configuration, the pair of wires is enclosed in a sheath or jacket, also of polyethylene, polyvinyl chloride or Teflon. Oftentimes, multiple pairs are so bundled in order to minimize deployment costs associated with connecting multiple devices (e.g., electronic PBX or KTS telephone sets, data terminals, and modems) at a single workstation.
Larger number of pairs are bundled into large cables to serve departments, quadrants of a building, or floors of a high-rise office buildingsuch cables may contain 25, 50, 100, 500 or more pairs. While twisted pair cables of up to 3,600 pairs are still used in outside plant applications, such continuing use is decidedly uncommon. In large cables, pairs are grouped into binder groups of 25 pairs for ease of connectivity management. Each binder group is wrapped (bound) with some sort of tape in order to separate it from other groups. Each pair within a binder group is color-coded for further ease of connectivity management. with the color codes being repeated within each binder group [3-5].
The effective capacity of twisted pair cable depends on several factors, including the gauge of the conductor, the length of the circuit and the spacing of the amplifiers/repeaters. One must also recognize that a high-bandwidth (high frequency) application may cause interference with other conversations on other pairs in close proximity.
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