Changes

===Telephone Networks===

===Telephone Networks===

The actual methodology behind the telephone network is slightly more complicated. Each line (or telephone circuit) from a building consists of a pair of thin copper wires. A number of these wires (eg all the wires on a street or in a region) are connected at an entrance bridge (typically a box on the side of a road) where they are packed into a thick wire. For small networks this wire may run directly to the central exchange, for larger networks several of this wires may meet at a local exchange, where their signals are converted into a digital signal which is transmitted to the main exchange.

The actual methodology behind the telephone network is slightly more complicated. Each line (or telephone circuit) from a building consists of a pair of thin copper wires. A number of these wires (eg all the wires on a street or in a region) are connected at an entrance bridge (typically a box on the side of a road) where they are packed into a thick wire. For small networks this wire may run directly to the central exchange, for larger networks several of these wires may meet at a local exchange, where their signals are converted into a digital signal which is transmitted to the main exchange.

At the exchange a call is connected to its destination phone. Originally this was done manually with human operators. When a phone was taken off the hook the circuit would be completed and an indicator light would switch on. The worker at the exchange would speak down the line and determine the intended destination phone before manually connecting the incoming call to a jack corresponding to the target phone, effectively completing the circuit. More recently this is achieved automatically using dial tones. The typical phone has twelve dials powered by the electrical circuit, each consisting of a different pair of fluctuating frequencies (each row has a sound frequency, each column has a sound frequency, these are added together). When the phone is taken off the hook the computer at the exchange "listens" to these dial tones and determines the target phone (hence a telephone number), and subsequently completes the circuit between the two phones.

At the exchange a call is connected to its destination phone. Originally this was done manually with human operators. When a phone was taken off the hook the circuit would be completed and an indicator light would switch on. The worker at the exchange would speak down the line and determine the intended destination phone before manually connecting the incoming call to a jack corresponding to the target phone, effectively completing the circuit. More recently this is achieved automatically using dial tones. The typical phone has twelve dials powered by the electrical circuit, each consisting of a different pair of fluctuating frequencies (each row has a sound frequency, each column has a sound frequency, these are added together). When the phone is taken off the hook the computer at the exchange "listens" to these dial tones and determines the target phone (hence a telephone number), and subsequently completes the circuit between the two phones.