Telephone Signalling Systems

Signalling in a telephone network is the exchange of control information that sets up, manages and ends a call, as distinct from the speech itself. Before two parties can talk, the network must learn that a call is wanted, where it should go, and when each side answers or hangs up. Signalling carries this information between the telephone and the exchange and between exchanges, and a set of standardised tones and electrical states make the process work consistently.

The sequence begins when a telephone goes off-hook, drawing current that tells the exchange a call is being requested. The exchange responds with a dial tone, signalling that it is ready to receive the number. Digits are sent either by pulse dialling, which interrupts the line current a number of times for each digit, or by dual-tone multi-frequency (DTMF) signalling. In DTMF each digit corresponds to a unique combination of two simultaneous tones, one from a low group and one from a high group of frequencies, which the exchange decodes reliably even over a noisy line.

Once the number is received, the exchange routes the call and applies a ringing current to the called line to alert that party, while sending a ringing tone back to the caller. Other supervisory tones, such as the busy tone and the number-unobtainable tone, inform the caller of the call's progress. Within an organisation a private branch exchange (PBX) performs comparable signalling for internal extensions. These signalling conventions allow telephones from different makers to interoperate across the network.

Frequently asked questions

What is the difference between pulse and DTMF dialling?

Pulse dialling interrupts the line current a set number of times per digit, while DTMF sends each digit as a unique pair of audible tones that the exchange decodes.

What does the dial tone signal?

It tells the caller that the exchange has detected the off-hook condition and is ready to receive the dialled number.

Why are two tones used for each DTMF digit?

Combining one tone from a low group and one from a high group makes each digit a distinct, easily detected signal that is robust against noise on the line.