pulsedialing

Pulse dialing, also known as decadic dialing, is a signaling method used by early telephone networks and rotary-dial telephones. In a pulse dialing system, each digit of a telephone number is encoded as a sequence of electrical pulses produced by the dialing mechanism. The digits 1 through 9 are represented by 1 to 9 pulses, while 0 is represented by ten pulses. When a dial is turned to a digit and released, the local line experiences a rapid series of on-off interruptions, or breaks, that the exchange interprets as the corresponding digit. The total rate is commonly around 10 pulses per second, with the exact make/break timing varying by equipment and country. There is an inter-digit gap between successive digits, typically several hundred milliseconds, during which the line remains idle.

In the central office, the pulses are counted by the switching equipment to reconstruct the dialed number. Pulse dialing relies on analog signaling and historical electromechanical switches; as such, it is closely associated with rotary dial telephones. With the advent of tone dialing (DTMF), most new installations used dual-tone signals; however, many telephone networks and devices still maintain support for pulse dialing, and some modern systems automatically convert pulse input to the equivalent digits for compatibility.

Today, pulse dialing is largely obsolete for new designs but remains provisioned on many legacy systems and rural lines, and is often available as a compatibility feature in VoIP gateways and some telephony adapters. See also: Rotary dial, DTMF.