- Many technicians are happier when using copper cables because they are familiar and unless they have taken the trouble to get to grips with optic fiber, they seem so much simpler.
- Connections are easier with hand tools and do not require expensive equipment and great precision.
- A simple metal detector can find a buried cable. This is only half an advantage because many optic fiber cables use metallic foil for moisture protection and metallic armoring.
- Electrical interference and crosstalk.
- Care has to be taken in high voltage environments.
- Reduced bandwidths.
- Higher losses.
- Size and weight
- Two wires are used to send a single signal.
The transmitter generates a voltage signal between the two wires at the input and this results in a current flowing along one wire, through the receiving circuitry and back along the other wire. Compared with fiber, this doubles the number of connections necessary.
One problem with any electrical signal is that whenever electric current flows, it causes a changing magnetic field. This is, in itself, not a worry unless there is another copper wire nearby. The changing magnetic field caused by the signal will cause a voltage to be induced into the copper wire causing a weak copy of the original signal. This effect is called 'crosstalk'.
The magnitude of the induced voltage decreases by the square of the distance away from the source of the interference so if we double the distance, we reduce the induced voltage by a factor of four. If two copper wires are running side by side, the one closest to the source of interference will pick up a larger signal than the one further away.
In circuit A, the upper wire is closer to the source and hence has a higher voltage induced. This larger voltage is indicated by the larger arrows and, at the moment shown, is trying to push the current clockwise round the circuit while the other wire is trying to push the current counterclockwise. The big arrows will win and the overall effect will be to produce a small current that will be a copy of the interference signal.
In a communication system the interference signal may be a conversation on a nearby telephone or a crackle from electrical machinery or lightning or any other source of interference.
In circuit B, each wire changes position so that the total value of the induced voltage is equal in each wire. This would give the happy result of no overall effect and hence no interference.
The equipment room will be something between a proper room and a cupboard where the brushes are kept. Somewhere between the manhole in the road and the equipment room outlets,
the responsibility for the cabling is transferred to us from the service provider.
These are governed by local regulations in each country but typical values are shown in the connection summary in Figure 18.9.
Kevin M Contreras H