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11.4 CALLS INVOLVING ANALOG SUBSCRIBERS 301In Par.14 (forward call indicators), bits CB, D, F, and HG constitute the protocolcontrol indicator (PCI). Bits HG (ISUP preference indicators) specify whether ISUPsignaling is required all the way. Originating exchanges are usually programmed toindicate that all-the-way ISUP signaling is required for calls originated by ISDNusers and preferred for calls originated by subscribers. Indicators HG are not modifiedby later exchanges along the connection. Indicators D (interworking indicator)and F (ISUP indicator) indicate the actual signaling on the connection. If exchange Pin Fig. 11.3-1 has set HG, D, and F to “ISUP required all the way,” “no interworkingencountered,” and “ISUP signaling used all the way,” exchange Q selects an outgoingtrunk with ISUP signaling and does not change these indicators in its IAMto exchange R. The settings of these indicators in connections where interworkingoccurs between ISUP and other interexchange signaling systems are discussed inSection 11.4. The meaning of indicators CB is discussed in Section 11.5.Call Processing Information at the Originating and TerminatingExchange. In the call of Fig. 11.3-1, the local exchanges know that the signalingis ISUP all the way. Terminating exchange R receives this information in bit Fof Par.14 in the IAM, and originating exchange P is informed by bit K of Par.4(backward call indicators) of the ACM message. We shall see later that, if thesignaling is not ISUP all the way, both local exchanges are also made aware thatthis is the case. This information is sometimes needed for subsequent call-processingactions.In connections with ISUP signaling all the way, both local exchanges also knowwhether the distant party in the connection is an ISDN user or a subscriber from,respectively, bit I of Par.14 (in IAM) and bit M of Par.4 (in ACM). In the call ofFig. 11.3-1, both parties are ISDN users. In calls between an ISDN user and a subscriber,the ISDN user is informed by his local exchange that the distant party isnon-ISDN. This is done by including an IE.11 (progress indicator) in a Q.931message to the ISDN user (Section 11.4).Information Included in DISC and REL Messages. The DISC and RELmessages always include, respectively, an IE.6 (cause) and Par.9 (cause indicator).In the example of Fig. 11.3-2, the location and cause values indicate “public networkof the remote user” and “normal call clearing” (Section 10.3.5).11.4 CALLS INVOLVING ANALOG SUBSCRIBERSPresently, the number of ISDN users is small, and the majority of calls carried onISUP trunks are between two analog subscribers. This section explores ISUP signalingfor calls in which one or both parties are subscribers.11.4.1 Call Between Two SubscribersFigure 11.4-1 shows a call between subscribers in which trunks T 1 and T 2 have ISUPsignaling.