Telephone signalling system between main and PBX exchanges - inserts secondary signals between main signals marking beginning and end of dial pulses

摘要

The signalling system, is connected between a main telephone exchange and a PBX. It can not only distinguish between signalling signals of different lengths but can also transmit other signals denoting long or short signals. 50 Hz AC is used for the main signalling and AF multiple-freq. signals for secondary signalling. When a dialling key is operated by the calling party, a main signalling signal is transmitted at the initial depression of the key and a main signalling signal at the end of the depression. The secondary signalling signal giving the authorisation of the calling party immediately follows the first or initial main signalling. This can also allow additional information to be transmitted during dialling.