Information Technology Reference
In-Depth Information
Typical cables used are:
•
Coaxial cable - cables with an inner core and a conducting shield having characteristic
impedance of either 75 W for TV signal or 50 W for other types.
•
Cat-3 UTP cable - level 3 cables have non-twisted-pair cores with a characteristic im-
pedance of 100 W (-15 W) and a capacitance of 59 pF/m. Conductor resistance is around
9.2W/100 m.
•
Cat-5 UTP cable - level 5 cables have twisted-pair cores with a characteristic impedance
of 100 W (-15 W) and a capacitance of 45.9 pF/m. Conductor resistance is around
9 W/100 m.
The Electrical Industries Association (EIA) has defined five main types of cables. Levels 1
and 2 are used for voice and low-speed communications (up to 4 Mbps). Level 3 is designed
for LAN data transmission up to 16 Mbps and level 4 is designed for speeds up to 20 Mbps.
Level 5 cables, have the highest specification of the UTP cables and allow data speeds of up
to 100 Mbps. The main EIA specification on these types of cables is EIA/TIA568 and the
ISO standard is ISO/IEC11801.
Table 2.3 gives typical attenuation rates (dB/100 m) for Cat-3, Cat-4 and Cat-5 cables.
Notice that the attenuation rates for Cat-4 and Cat-5 are approximately the same. These two
types of cable have lower attenuation rates than equivalent Cat-3 cables. Notice that the at-
tenuation of the cable increases as the frequency increases. This is due to several factors,
such as the skin effect, where the electrical current in the conductors becomes concentrated
around the outside of the conductor, and the fact that the insulation (or dielectric) between
the conductors actually starts to conduct as the frequency increases.
The Cat-3 cable produces considerable attenuation over a distance of 100 m. The table
shows that the signal ratio of the output to the input at 1 MHz, will be 0.76 (2.39 dB), then, at
4 MHz it is 0.55 (5.24 dB), until at 16 MHz it is 0.26. This differing attenuation at different
frequencies produces not just a reduction in the signal strength but also distorts the signal
(because each frequency is affected differently by the cable. Cat-4 and Cat-5 cables also pro-
duce distortion but their effects will be lessened because attenuation characteristics have flat-
ter shapes.
Table 2.4 gives typical near-end cross-talk rates (dB/100 m) for Cat-3, Cat-4 and Cat-5
cables. The higher the figure, the smaller the cross-talk. Notice that Cat-3 cables have the
most cross-talk and Cat-5 have the least, for any given frequency. Notice also that the cross
talk increases as the frequency of the signal increases. Thus, high-frequency signals have
more cross-talk than lower-frequency signals.
Table 2.3
Attenuation rates (dB/100 m) for Cat-3, Cat-4 and Cat-5 cable
Frequency (MHz)
Attenuation rate (dB/100 m)
Cat-3
Cat-4
Cat-5
1
2.39
1.96
2.63
4
5.24
3.93
4.26
10
8.85
6.56
6.56
16
11.8
8.2
8.2
