Introduction to Data Communications
15. Transmission Media - Guided (cont'd)
15b. Twisted Pair
The wires in Twisted Pair cabling are
twisted together in pairs. Each pair would consist of a
wire used for the +ve data signal and a wire used for
the -ve data signal. Any noise that appears on 1 wire of
the pair would occur on the other wire. Because the
wires are opposite polarities, they are 180 degrees out
of phase (180 degrees - phasor definition of opposite
polarity). When the noise appears on both wires, it
cancels or nulls itself out at the receiving end.
Twisted Pair cables are most effectively used in systems
that use a balanced line method of transmission: polar
line coding (Manchester Encoding) as opposed to unipolar
line coding (TTL logic).
The degree of reduction in noise
interference is determined specifically by the number of
turns per foot. Increasing the number of turns per foot
reduces the noise interference. To further improve noise
rejection, a foil or wire braid shield is woven around
the twisted pairs. This "shield" can be woven around
individual pairs or around a multi-pair conductor
(several pairs).
Cables with a shield are called Shielded
Twisted Pair and commonly abbreviated STP. Cables
without a shield are called Unshielded Twisted Pair or
UTP. Twisting the wires together results in a
characteristic impedance for the cable. A typical
impedance for UTP is 100 ohm for Ethernet 10BaseT cable.
UTP or Unshielded Twisted Pair cable is
used on Ethernet 10BaseT and can also be used with Token
Ring. It uses the RJ line of connectors (RJ45, RJ11,
etc..)
STP or Shielded Twisted Pair is used
with the traditional Token Ring cabling or ICS - IBM
Cabling System. It requires a custom connector. IBM STP
(Shielded Twisted Pair) has a characteristic impedance
of 150 ohms.
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