ing a tone high enough in the speech band tween PABXs. The signalling system requires a supporting line such that the power within the voice signal was at its lowest level. signalling system on each link, such as E&M or SSAC15, to control The tone selected was 2280Hz. seizing, answering and clearing signals. Transmitting Tie line Receiving Polarised Pulsing contact receiving relay Figure 34.7 Double current signalling Private networking 34/9 2280Hz Transmit To PABX To 4-wire amplified circuits Receive To PABX Buffer amplifier Line splitting control 2280Hz receiver To PABX Digit reception Figure 34.8 Single frequency signalling The signalling system operates on an end to end basis with the Fast call set up over AC15 tie lines was then possible by using the signalling path being progressively extended as the call progresses AC15 for line signalling seize, release, and answer. The AC15 through the private network tandem switches. It is a compelled 10pps address signalling was then suppressed and the sending system i.e. a forward signal once sent remains on the line until a PABX connected a DTMF sender to the tie line; the distant end on response is received from the distant end. Once the forward removal seizure connected a DTMF receiver to detect the incoming DTMF signal has been detected at the distant end this causes the removal signals. of the acknowledgement signal and completes the compelled sig- On the simpler PABXs where no restrictions were placed on nalling cycle. dialling or where the routing digits required by the private network Twelve frequencies are used, divided into upper and lower bands were minimal, the PABX user's telephone was directly connected for signalling in the forward and reverse direction respectively. to the tie line and the DTMF pulses dialled directly into the distant Each frequency is spaced 120Hz apart starting at 540Hz and going PABX. The frequencies used in this form of signalling are identical through the range to 1980Hz. The frequencies are sent as 2 out of 6 to those of the basi