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68 SUBSCRIBER SIGNALINGCall Waiting Service. Suppose that subscriber S 1 is marked at his local exchangefor call waiting service. When S 1 is engaged in a call with subscriber S 2 and is calledby another subscriber S 3 , he is alerted by his local exchange with a call waiting tone.By sending a hookswitch flash, S 1 can put subscriber S 2 on hold and be connected toS 3. From this point on, S 1 can alternate between S 2 and S 3 with subsequent flashes.Call waiting tone in the United States is a 440-Hz tone with a cadence of 0.3 s on–10 s off.Call Forwarding Service. When S 1 is marked at the local exchange for callforwarding service, he can activate this service by dialing a feature access code.The exchange responds with dial-tone, after which S 1 enters the number of subscriberS 2 to whom his incoming calls are to be forwarded. The exchange acknowledgesreceipt of the number with two short “beeps” and then sets up a call from S 1 to S 2 .Subscriber S 1 then hears ringing-tone. When S 2 answers, the call forwarding for S 1is activated. If S 2 is busy or does not answer, S 1 disconnects and repeats the activationrequest, which is then accepted by the exchange without setting up a callto S 2 . When call forwarding has been activated, S 1 can still make outgoing calls,and his telephone still rings for incoming calls. However, these calls are forwardeduntil he deactivates the service, by dialing.Three-Way Calling. When S 1 is registered for three-way calling service, he cancall S 2 and then add a call to S 3 . Having established the call to S 2 , subscriber S 1flashes. The local exchange returns dial tone, and S 1 enters S 3 ’s number. Theexchange then sets up the call to S 3 while maintaining the call to S 2 .IfS 3 is busyor does not answer, S 1 can end the new call (but remain in conversation with S 2 ),by flashing twice.3.5.2 Custom Local Area Subscriber ServicesCLASS services are new supplementary services that have been introduced in theUnited States [8]. They depend on the availability of common-channel signalingsystem No. 7 (see Chapter 11), which can transfer the calling number from thecalling to the called exchange. Some CLASS also require additions to subscribersignaling at the local exchanges, such as recognition of feature access codes,TACs that include the required tones and announcements, and distinctive ringing.A few examples are outlined below.Distinctive Ringing. A subscriber registered for this service is alerted by aringing signal with a distinctive cadence when called by certain specified lines.The subscriber can list up to ten such calling numbers. To make a change in thelist, the subscriber dials a feature access code. This initiates a dialogue with theexchange during which the subscriber is prompted by recorded messages andenters the changes by sending digit strings that represent the special callingnumbers. For this service, the local exchanges have to be able to generate the distinctiveringing signal and to send the necessary recorded messages.