ssion system Figure 43.3 shows the principle of loop disconnect d.c. line signalling. Its operation is as follows: Analogue network transmission plant varies in type: audio, FDM, point-to-point PCM. As analogue network signalling is on the call 1. Seizure. Relay A at A operates to the caller's off-hook loop. speech path, the type of transmission plant has influence on the 2. Address. (Assuming direct control switching). Relay A at A signalling method. responds to the caller's dialled pulses. When switched through exchange A, contact A1 loops the line to operate relay A at B. A.C. signalling within the voice range (300Hz to 3400Hz) can be A1 loop disconnects the line, so transmitting the remaining applied to any type of transmission media, but this application dialled pulses to relay A at B. rationalisation is not economic since supervisory line signalling Answer. Relay D at B operates to the called subscriber's off- systems are equipped per line. Line signalling is of type (d.c., v.f. hook loop. D1 and D2 reverse the line d.c. polarity to operate etc.) according to the type of line transmission. the rectified relay D at A (or a polarised relay as in the loop A connection switched through an analogue exchange is super- reverse battery d.c. system). vised. A supervisory unit is equipped per line switched for this Clear back. The called subscriber's on-hook disconnect re- purpose. The unit incorporates a transmission bridge and detects leases relay D at B, contacts D1 and D2 restoring releases relay supervisory signal conditions on the calling and called sides inde- D at A. pendently. Line units associated with external lines enable electri- 5. Clear forward. The caller's on-hook disconnect releases relay cally independent signal conditions to be used in the exchanges and A at A. A1 releases relay A at B (assuming the more usual those on the external lines. The supervisory unit is then often calling party release). combined with such line units. With common control switching, address signalling is inter-regis- 43.1.