Hook flash

The inquiry key, often R button in telephony serves to private or public telecommunications networks to signal commands which go beyond the transfer of the number to be called. This can be for example a relaying a call to another participant.

Different signaling variants are distinguished:

  • Time Break for 70-90 ms: [NB 1] This function is often used in analog telephone systems that support two-wire cabling and DTMF commands. Ensuring that the system can recognize that it is commands to the telephone system, and not to DTMF commands eg for controlling a three-party conference in the public exchange, or to control an answering machine to the remote site in the transmitted DTMF tones, is the line connection for 70-90 ms [NB 1] interrupted and the telephone system goes into command mode. This function is referred to as flash function.
  • Time Break for 170-310 ms [NB 2]: This function signal analog telecommunications terminals of the switch that it is commands to the central office at the following DTMF tones (eg for the control of supplementary services such as three-way calling and Brokering ). This function is referred to as hook flash function, since it can also be achieved by a short press of the fork ( engl. hook = hook ). For very old phones with a button called Flackertaste could be realized by separating the bridge 2'- W2. The connection was interrupted for the duration of the key press.
  • Conjunction with a third line, the ground conductor: In this case one speaks of earth key. This type of signaling works exclusively in telecommunications networks with at least three wire cabling, so for example in telephone systems. The three-wire wiring with the yellow Erdader was mandatory; the protective earth of the socket could not be used for this. This function was used in telephone systems that use pulse dialing. In systems that use the multi-frequency dialing, use the grounding key is no longer used. Today's phones usually no longer have such a function. The ground key combines the potential of a wire with the Erdader.
Earthing system
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