0Hz frequency (tone on) or removal of the frequency (tone off). With double current signalling a polarised relay is used which is The system operates on a link by link basis and signals are very sensitive and the signals are transmitted as opposite directions contained within each link. Forward and backward signals may be of current flow as in Figure 34.7. The use of double current signall- passed simultaneously and in some cases when speech is present but ing also minimises the effect of line capacitance by continually the primary use is for call set up and clear down. The system allows reversing the current in the line. The limitation of double current for address signals to be sent at 10pps. signalling is that the address signalling can only be sent at 10pps and Like E&M signalling on physical wires this signalling system a physical pair is required between the PABXs. allows the use of multifrequency inter-register signalling for ad- dress signalling and the 2280Hz for the line signalling. 34.4.3 Single frequency signalling 34.4.4 Inter-register signalling Single frequency a.c. signalling has been developed for use on 4-wire amplified circuits between PABXs enabling line and address Inter-register signalling, like its counterpart in the public network, signalling to be sent over the amplified circuits on unlimited dis- is intended to provide the PABX private network user with fast call tances for the PABX user. The signalling system was derived from set up and the use of a set of supplementary services across the the signalling circuits used in the 4-wire amplified circuits between network, similar to those available between the the telephone users the public exchange and the long distance transmission systems. on the same PABX. An important requirement was the choice of tone for line signall- The signalling system is based on CCITT R2 signalling using ing to avoid imitation of the signalling tone by the voice signal. This multifrequency signalling on either 2-wire or 4-wire circuits be- was achieved by select