Page 41 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes
Figure 3.3: 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.

General Properties of Twisted Pair

Gauge: 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. The
gauge numbers are retrogressive. In other words, the larger is the number, the smaller is
the conductor.
Configuration: In a single pair configuration, the pair of wires is enclosed in a sheath or
jacket, made of polyethylene, polyvinyl chloride or Teflon. Usually, 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.
Bandwidth: 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 signals on other pairs in close proximity.
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 wire act as antennae and, thereby, absorbs such errant signals.
Potential sources of Electro Magnetic 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.

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