circuit. Trunk circuits 34/5 Trunk Switch Control lines circuits matrix processor Control bus Modem System circuits administration (remote Control bus access) Private Tie line Network circuits Control bus Control bus Figure 34.3 PABX block diagram Tie line circuits provide access to the private network intercon- Trunk circuits necting a number of PABXs directly or indirectly via tandem private exchanges. The signalling on these circuits range from basic E&M Trunk circuits provide the interface to the PSTN and use a number through the spectrum to common channel signalling carrying of different signalling systems to interact with the network. DPNSS or Qsig signalling, with feature rich inter-PABX functions transferred over the signalling channels during call set up and during the conversation phase of the call. Line circuits provide the basic 34.3.1 Loop calling BORSCHT functions for analogue telephones. Special proprietary analogue line circuits are in use for multifunc- This form of trunk circuit is the simplest interface which mimics tional terminals and digital multifunctional telephones, with data that of the basic telephone interface to the public network. The transmission on either the D-channel or the 2nd B-channel. The circuit is a 2-wire connection to the public exchange with the basic switching matrix can be either CMOS analogue switches for the elements shown in Figure 34.4. In the idle condition the public smaller systems and digital switching is the most cost effective on exchange presents a negative d.c. voltage on one wire and either an nearly all other designs. The system control provides all the oper- exchange earth on the other or the positive side of the exchange ational software/control functions to interconnect the various func- battery on the other wire, depending on whether the exchange has a tions. floating battery or a battery connected to ground. Pulse Address receiving signalling relay Seize detect To control Figure 34.4 Loop calling 34/6 PABX and key systems The PABX circuit presents th