Pulse dialing

Pulse dialing (PD ) is the common name today in the German language for the oldest signaling method of automatic telephone exchanges.

Previously it was the only dialing and therefore did not need a proper name. Today it is widely used for analog telephone lines from multi-frequency (DTMF ) or DTMF (dual - tone multi - frequency) called, has been replaced.

With the invention of the dial ( number button ) and the associated switching equipment, the operation and protocol of the dialing method is set.

By picking up the handset in the analog terminal a current loop is closed to the exchange and sent from the central office dial tone to the subscriber. Pressing the number button interrupts this loop according to the selected digit: When dialing number 1 once, at number 2 twice ... at digit 0 times. A single pulse lasts 100 ms. The most common pulse - pause ratio is 60/40, which means that the contact with a pulse duration of 100 ms, 60 ms and 40 ms open is closed. After expiry of the pulse train, the contact will remain closed.

The dialed digits are converted in this way into current pulses that were headed step the magnets of the rotary knobs in the exchange. The handset can be heard on some phones without short circuit on the audio portion during the election as a series of clicking noises.

Once a longer break follows, the telephone exchange waits for the next digit. Quickly pressing the hook switch generates pulses - are interpreted by the exchange as dial pulses - at the right timing. Some phones (eg the subscriber Mü 55b) therefore have an additional delay device ( mechanical or electrical) in the hook switch, to prevent " fork choice."

The pause between dialed digits is at least the time between two pulses, ie 200 ms. This pause is Inter-digit pause ( English), often also called spatium.

Older phones with keypads and ringer ( tone dialing ) could also be switched to pulse dialing usually. The dial pulses were generated not mechanically but electronically.

Since the signaling is done via the normal voice channel (and therefore can also be perceived in the telephone receiver ), one speaks of an in-band signaling. ISDN and other digital telephone networks, however, a separate channel is used ( in the ISDN D-channel ) for signaling. This process is also referred to as out-of- band signaling.