Twisted Pair

The wires in twisted pair cabling are twisted together in pairs. Each pair consists of a wire used for the +ve data signal and wire used for the negative data signal. Any noise that appears on 1 wire of the pair will also 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).

1. Advantages of Twisted Pair (Phone Line):
   • Easy to string
   • Cheap
2. Disadvantages of Twisted Pair (Phone Line):
   • Subject to interference =static and garble.

1. Unshielded Twisted Pair

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).

2. Shielded Twisted Pair

Cables with a shield are called shielded twisted pair and are 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 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.