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78 CHANNEL-ASSOCIATED INTEREXCHANGE SIGNALINGFigure 4.1-2. Exchange equipment for channel-associated interexchange signaling.All trunks have CAS. The attachment of analog and digital (PCM) trunks to theswitchblock is as described in Section 1.7.2. First-order TDM multiplexes carryingPCM trunks (b) are directly connected to DMPs. Analog trunks on first-order FDMtransmission systems are first demultiplexed into individual four-wire analog circuits(d), which pass through trunk circuits (TC4) and then enter a first-orderPCM multiplexer, where they are converted into TDM digital circuits (b).Supervision Signaling. The TC4 circuits of analog four-wire trunks inject outgoingsupervision signals into the trunk and detect supervision signals received onthe trunk. Each TC4 has a control channel (CC). It is used by the processor toorder the TC4 to send signals, and by the TC4 to report received signals.The supervision signaling information for the trunks in first-order digital trunkmultiplexes is in certain bits of the bit streams (b). Control channel (CC) to theswitchblock has two functions. In the first place, it transfers processor commandsto set up and release switchblock paths. In the second place, it is used for communicationsbetween the processor and those DMPs that serve groups of multiplexeddigital trunks. The processor can order a DMP to send out a supervision signal ona specified trunk in the multiplex, and a DMP reports the supervision signalsreceived from the trunks in its multiplex to the processor.Service Circuits. Address signaling and the sending of tones/announcements aredone with service circuits. The exchange has pools of digital multiplexed service circuitsthat are attached to DMPs on the switchblock. Tone and announcement circuits(TACs) send tones and announcements. Digit-senders (SND) and digit-receivers(RCV) send and receive MF address signals to and from the trunks. These circuitsare signaling system specific. If an exchange has trunks with two CAS signaling