Information Technology Reference
In-Depth Information
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Tolerance to harsh conditions.
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Expense and general availability of the cable.
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Ease of connection and maintenance.
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Ease of running cables, and so on.
The main types of cables used are standard copper cable, unshielded twisted-pair copper
(UTP), shielded twisted-pair cable (STP), coaxial and fibre optic. Twisted-pair and coaxial
cables transmit electric signals, whereas fibre optic cables transmit light pulses. Twisted-pair
cables are not shielded and thus interfere with nearby cables. Public telephone lines generally
use twisted-pair cables. In LANs they are generally used up to bit rates of 10 Mbps and with
maximum lengths of 100 m.
Coaxial cable has a grounded metal sheath around the signal conductor. This limits the
amount of interference between cables and thus allows higher data rates. Typically, they are
used at bit rates of 100 Mbps for maximum lengths of 1 km.
The highest specification of the three cables is fibre optic. This type of cable allows ex-
tremely high bit rates over long distances. Fibre optic cables do not interfere with nearby
cables and give greater security, give more protection from electrical damage by external
equipment and greater resistance to harsh environments; they are also safer in hazardous en-
vironments.
Cable characteristics
The main characteristics of cables are attenuation, cross-talk and characteristic impedance.
Attenuation defines the reduction in the signal strength at a given frequency for a defined
distance. It is normally defined in dB/100 m, which is the attenuation (in dB) for 100 m. An
attenuation of 3 dB/100 m gives a signal voltage reduction of 0.5 for every 100 m. Table 2.2
lists some attenuation rates and equivalent voltage ratios; they are illustrated in Figure 2.6.
Attenuation is given by
Calculation of attenuation from
input and output voltages
V
Ł
in
ł
Attenuatio
n
=
20
log
dB
10
V
out
For example if the input voltage to a cable is 10 V and the voltage at the other end is only
7 V, then the attenuation is calculated as
10
Attenuatio
n
=
20
log
Ł
ł
=
3.1
dB
10
7
Coaxial cables have an inner core separated from an outer shield by a dielectric. They have
an accurate characteristic impedance (which reduces reflections), and because they are
shielded they have very low cross-talk levels. They tend also to have very low attenuation,
(such as 1.2 dB at 4 MHz), with a relatively flat response. UTPs (unshielded twisted-pair
cables) have either solid cores (for long cable runs) or are stranded patch cables (for shorts
run, such as connecting to workstations, patch panels, and so on). Solid cables should not be
flexed, bent or twisted repeatedly, whereas stranded cable can be flexed without damaging
the cable. Coaxial cables use BNC connectors while UTP cables use either the RJ-11 (small
