3 D.C. line signalling ter and would not be conveyed by the line signalling system. 43.1.3.1 Short-haul d.c. line signalling systems 43.1.3.2 Long distance d.c. line signalling (LDDC) There are a number of different forms (Welch, 1979): loop discon- LDDC is applied on lines beyond the limits of short-haul d.c. nect d.c., loop reverse battery, single wire d.c., high/low resistance signalling when a metallic circuit is available (Welch, 1979). The d.c., battery and earth pulsing. Loop disconnect d.c. signalling on phantom of the 4-wire circuit is used for signalling. The method 2-wire lines is the simplest and most widely used. extends the signalling limit by the use of sensitive polarised relays The basic signal repertoire is: and the decadic address pulsing limit by a more efficient pulsing technique than loop disconnect. 1. Forward; seizure, address information (when direct control With loop disconnect pulsing, the send end impedance varies switching), clear forward. during the decadic pulsing process (substantially zero on pulse 2. Backward; answer, clear back. make and infinite on pulse break). This variation results in asymme- Exchange A Calling 2 wire Exchange B Called Figure 43.3 Principle of loop disconnect d.c. signalling Analogue network signalling 43/5 tric waveforms and thus to decadic pulse distortion on repetition. signals usually being pulsed since only one transmission path is Further, the single current pulsing receive relay, with fixed operate available. The technique has declined in importance. and release current values, gives varying pulse distortion when operating on asymmetric waveforms. LDDC necessitates symmetrical pulsing waveforms. To achieve 43.1.4.2 Voice frequency (v.f.) line signalling this: The signalling frequency is inband, within the speech range of 1. The send end impedance is equal during the pulse make and 300Hz to 3400Hz of f.d.m. transmission. The system is completely break periods. flexible in that it can operate on any circuit affording speech trans- 2. The pulsing battery