Scott Haines, of King's High School, asks :-

How many phone calls can a telephone line transport at the same time?

Phil Carlisle, a telecommunications engineer with Telecom, responded.

A standard telephone line consists of a pair of copper wires (cable pair) running between an exchange or roadside cabinet and a customer's premises. The copper wires are contained within cables ranging in size from two pairs to over 2000 pairs.

Using equipment called PCM (pulse coded modulation) it is possible to carry thirty telephone circuits over two cable pairs. One cable pair is used for each direction ie towards the customer and away from the customer. The thirty telephone circuits are combined together using a method called multiplexing. This technology has been in use in New Zealand for more than fifteen years and is still used today.

More recently, equipment has been developed which can transport television signals over copper cable pairs. This is being trialed in some countries to provide cable TV service.

Nowadays, fibre optic cables are used to carry most telecommunication traffic. These are used within cities, between cities and between countries. The glass fibres within these cables are not much bigger than a human hair.

With fibre optic cable, the number of telephone calls that can be carried is currently limited by the technology available. The latest fibre optic transmission equipment is capable of transporting 30,720 telephone calls over two glass fibres. This equipment sends about 2400 million pulses of light per second down each fibre. Researchers have successfully transmitted 400,000 million pulses of light per second through 100 kilometres of fibre optic cable.