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3.3 SIGNALING EQUIPMENT AT THE LOCAL EXCHANGE 61with the subscriber line. These paths transfer PCM-coded tones, DTMF frequencies,and announcements.The switchblock and the line and service circuits have control channels (CCs) tothe exchange processor. This enables the processor to send commands to, andreceive information from, these entities.The implementation of subscriber-signaling functions is manufacturer-specific.In this example, we assume that the line circuits receive the supervision signalsand dial-pulse digits from their lines and send ringing signals to the lines.We also assume that there are two types of service circuits. Tone and announcementcircuits (TACs) have memories that store PCM sequences for all tones andannouncements that can be sent to a subscriber. When, for example, a busy-tonehas to be sent to a subscriber line, a switchblock path is set up between an availableTAC and the PCM channel associated with the line. The processor then orders thecircuit to send a busy-tone. The second type of service circuits are DTMF-digitreceiver circuits (DRC). These circuits can provide dial-tone and receive DTMFdigits.3.3.2 Reception of Supervision SignalsFigure 3.3-2 shows a line circuit (LC) in some detail. The circuit can be in two states,which are changed on command from the processor. In the figure, LC is in its“normal” state. Switch (S) connects transformer (T) to the subscriber line. Whenthe telephone is involved on a call, the transformer transfers the (analog) speechbetween the subscribers. The LC is set to the ringing state when the telephone hasto receive a ringing signal.Figure 3.3-2. Components in line circuit, shown in the normal state. T, Transformer; H, hybridcircuit; CD, current detector; CC, control channel; RS, ringing source; BAT, exchange battery;S, switch.