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While a voice grade circuit over twisted pair is guaranteed at 4 kHz, standard copper is capable of supporting much greater bandwidth. A single twisted pair, in a typical telephone installation, is capable of providing up to 250 kHz, or 1–4 Mbps compressed, assuming amplifier or repeater spacing every 2-3 km [3-1]. Additional examples follow:

  T1 connections (1.544 Mbps) are routinely provided over specially conditioned, four-wire twisted pair, with repeaters spaced at approximately 6,000 ft.
  Category 5 (CAT 5) copper, in a Local Area Network (LAN) environment, provides bandwidth of up to 100 Mbps over twisted pair at distances of [le]20 meters.
  Asymmetrical Digital Subscriber Loop (ADSL), a recently developed local loop technology, provides 6.144 Mbps in the downstream direction, as well as a 608 Kbps bidirectional channel—over a single two-wire, twisted pair local loop at distances [le]2 miles. Specifications also provide for even greater bandwidth, although over shorter distances.

Error Performance

Signal quality is always important, especially relative to data transmission. Twisted pair is especially susceptible to the impacts of outside interference, as the lightly insulated wires act as antennae and, thereby, absorb such errant signals. Potential sources of ElectroMagnetic Interference (EMI) include electric motors, radio transmissions and fluorescent light boxes. As transmission frequency increases, the error performance of copper degrades significantly with signal attenuation increasing approximately as the square root of frequency [3-8].

Distance

UTP is especially distance-limited. As distance between network elements increases, attenuation (signal loss) increases and quality decreases at a given frequency. Even low-speed (voice grade) analog voice transmissions require amplifiers spaced at least every 2 to 4 miles. As a result, local loops generally are 10,000 to 18,000 feet in length. As bandwidth increases, the carrier frequency increases, attenuation becomes more of a issue, and amplifiers/repeaters must be spaced more closely.

Security

UTP is inherently an insecure transmission medium. It is relatively simple to place physical taps on UTP. Additionally, the radiated energy is easily intercepted through the use of antennae or inductive coils, without the requirement for placement of a physical tap.

Cost

The acquisition, deployment and rearrangement costs of UTP are very low, at least in inside wire applications. In, high-capacity, long distance applications, such as inter-office trunking, however, the relative cost is very high, due to the requirements for trenching or boring, conduit placement, and splicing of large, multipair cables. Additionally, there are finite limits to the capacity and other performance characteristics of UTP, regardless of the inventiveness of technologists. Hence, the popularity of alternatives such as microwave and fiber-optic cable.

Applications

Generally speaking, UTP no longer is deployed in long-haul outside plant transmission systems, satellite, microwave and fiber optic cable are the media of choice in such applications. However, its low cost, coupled with recently developed methods of improving its performance, have increased its application in short-haul distribution systems. UTP is still the medium of choice in most inside wire applications. Current and continuing applications include the local loop, inside wire and cable, and terminal-to-LAN.

Shielded Copper

Shielded twisted pair (STP) differs from UTP in that a metallic shield or screen surrounds the pairs, which may or may not be twisted. As illustrated in Figure 3.2, the pairs can be individually shielded. A single shield can surround a cable containing multiple pairs or both techniques can be employed in tandem. The shield itself is made of aluminum, steel, or copper; is in the form of a metallic foil or woven mesh; and is electrically grounded. Although less effective, the shield sometimes is in the form of nickel and/or gold plating of the individual conductors.


Figure 3.2  Shielded Twisted Pair (STP) configuration.

Shielded copper offers the advantage of enhanced performance for reasons of reduced emissions and reduction of electromagnetic interference. Reduction of emissions offers the advantage of maintaining the strength of the signal through the confinement of the electromagnetic field within the conductor; in other words, signal loss is reduced. An additional benefit of this reduction of emissions is that high-frequency signals do not cause interference in adjacent pairs or cables. Immunity from interference is realized through the shielding process, which reflects electromagnetic noise from outside sources, such as electric motors, other cables and wires, and radio systems.

Shielded twisted pair, on the other hand, has several disadvantages. First, the raw cost of acquisition is greater as the medium is more expensive to produce. Second, the cost of deployment is greater as the additional weight of the shield makes it more difficult to deploy. Additionally, the electrical grounding of the shield requires more time and effort.

Applications

The additional cost of shielded copper limits its application to inside wire applications. Specifically, it generally is limited to application in high-noise environments. It also is deployed where high frequency signals are transmitted and there is concern about either distance performance or interference with adjacent pairs. Examples include LANs and image transmission.

Coaxial Cable

Coaxial cable (Figure 3.3) is a very robust shielded copper wire. The center conductor (much thicker than a twisted pair conductor) is surrounded by an outer shield/conductor which serves to greatly improve signal strength and integrity. The two conductors generally are separated by a layer of foam or solid insulation; the entire cable is then protected by a layer of dielectric (nonconductive) material, such as PVC or Teflon. The two conductors share a common axis, hence the term coaxial. Reportedly invented by AT&T Bell Telephone Laboratories in 1934, the first coaxial cable was placed into service in New York City in 1936. Such a cable was used in 1940 to televise in New York City the Republican National Convention in Philadelphia at which Wendell Wilke was nominated [3-6].


Figure 3.3  Coaxial cable configuration.


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