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11.4 CALLS INVOLVING ANALOG SUBSCRIBERS 303When a call setup arrives at the terminating exchange on an ISUP trunk, theexchange always checks whether the called party is compatible with the service indicatedin Par.25 (transmission medium requirement). In this example the called partyis a subscriber and is compatible with the “3.1-kHz audio service.” Exchange Rtherefore proceeds with the call setup.Assuming that S 2 is idle, it rings the subscriber line, connects trunk T 2 to aringing-tone source, and sends an ACM message to exchange Q, which repeats itto exchange P.In the ACM, bits DC, FE, and M of Par.4 (backward call indicators) are setto “subscriber free,” “subscriber,” and “called party has non-ISDN access.” Bits Jand K are copied from bits D and F in Par.14 (forward call indicators) of the receivedIAM and indicate “no interworking encountered” and “ISUP signaling all the way.”Exchange P is therefore aware that signaling is ISUP all the way and that the calledparty is non-ISDN (it may need this information for later call-control actions).Since exchange R receives the calling party number (Par.8) in the IAM, it canprovide CLASS services to the called subscriber (Section 3.5.2).When exchange R receives an answer signal from S 2 , it cuts through the pathbetween T 2 and the called subscriber line and sends an ANM message. When themessage reaches exchange P, it cuts through its forward transmission path and theconversation or data transmission can start.Release of the Connection. Figure 11.4-1 assumes that the called subscriberdisconnects first. Called subscribers have a grace period that allows them to disconnectthe telephone on which they answered the call, and to pick up another telephone,without causing the release of the connection (Section 3.1.1). Therefore,when S 2 disconnects, exchange R sends a SUS message (a REL message wouldinitiate the release of the network connection). On receipt of the message, exchangeP starts a timer with a timeout of, say, 30 s. If S 2 re-answers before the timer expires,exchange R sends a RES (Resume) message, exchange P stops the timer, and the callcontinues. In the example, exchange P has not received a RES message when thetimer expires and initiates the release of the connection.11.4.2 Calls Between a User and a SubscriberWe now consider calls between a user and a subscriber. The call configuration is asin Fig. 11.4-1, except that one party is a subscriber and the other party is an ISDNuser. Since the connection has ISUP signaling all the way, terminating exchange Ralways knows whether the remote (calling) party is a subscriber or an ISDN userfrom bit I of Par.14 (forward call indicators) in the IAM message. Likewise, originatingexchange P knows the type of the remote (called) party from bit M ofPar.4 (backward call indicators) in the ACM message.Since some user actions depend on whether the distant party is a user or a subscriber,a local exchange always informs its user if the distant party is non-ISDN.This is done by including an IE.11 (progress indicator) in the CONN message to