The telephone network is the world's most complicated machine. When a telephone call is made it may travel over many different types of channels, and complex switching facilities are needed to set up its path. Telecommunications systems can be divided into four main parts.

1. Instruments : The term instrument is used for the device the subscriber employs to originate and receive signals. The vast majority of instruments are telephone hand sets. Today, however, an endless array of other devices are being attached to telephone lines including computer terminals used for data transmission.

2. Local loops : These are the cables that enter the subscriber's premises. On a telephone network they connect the telephone handsets or ...

Get Consumer Electronics now with the O’Reilly learning platform.

O’Reilly members experience live online training, plus books, videos, and digital content from nearly 200 publishers.