Page 39 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes conduct light or optical energy, generally using a glass conductor. The term bounded or guided
media means that the signal is contained within an enclosed physical path. It also refers to the
fact that some form of shield, cladding, and/or insulation is employed to bind the signal within
the core medium, thereby improving signal strength over a distance and enhancing the
performance of the transmission system in the process. Twisted pair (both unshielded and
shielded), coaxial and fiber optic cable systems fall into this category.

3.2.1 Twisted Pair (Copper Conductors)

A twisted pair as shown in Figure 3.1 is a pair of copper wires, with diameters of 0.4-0.8 mm,
twisted together and wrapped with a plastic coating. The twisting increases the electrical noise
immunity, and reduces the error rate of the data transmission. Each conductor is separately
insulated by some low smoke and fires retardant substance. Polyethylene, polyvinyl chloride,
flouropolymer resin and Teflon are some substances that are used for insulation purposes.

Figure 3.1: Two Wires Open Lines

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 multiple 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.
These are popular for telephone network. The energy flow is in guided media. Metallic wires
were used almost exclusively in telecommunications networks for the last 90 years, until the
development of microwave and satellite radio communications systems. Therefore, copper
wire is now a mature technology, rugged and inexpensive. In certain applications, copper-
covered steel, copper alloy, nickel- and/or gold-plated copper and even aluminum metallic
conductors are employed.

The maximum transmission speed is limited in this case. The copper conductor that carries
analog data can be used to carry digital data also in association with the modem. Modem is a
device to convert digital signal into analog signal and vice versa. The data rate in this category
is limited to around 28 Kbps. The introduction of the Integrated Services Digital Network
(ISDN) led to the use of improved modulation and coding schemes and data rate up to 128 Kbps.
Local Area Networks (LANs) also use twisted pairs. These networks also upgraded to support
for high bit rate real time multimedia. The Asymmetric Digital Subscriber Lines (ADSL)
technology is aimed at using two wire copper loops at data rates of 1.544 Mbps in the network
to user direction and about 600 Kbps from the user to network.
The twisted pair cable may be defined in two categories based upon the shielding and without
shielding.

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