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10.4 INTRODUCTION TO CALL-CONTROL SIGNALING 26110.4.1 Typical Signaling SequenceWe first examine the Q.931 messages in a typical signaling sequence for an intraexchangecall from user P to user Q (see Fig. 10.4-1). In this section, the focus isprimarily on the messages, and the IEs are not discussed in detail. The messagesequence is indicated by letters in parentheses, (a), (b), and so on.Call Setup. User P starts the call by sending a SETUP message (a) from his terminalTE-P to the exchange. We assume that the message includes the complete callednumber (en-bloc signaling). The exchange then sends a SETUP message (b) onDSL-Q. It also assigns B-channels on DSL-P and DSL-Q and informs TE-P thatthe call setup has started, with a CALPRC message (c).We also assume that TE-Q accepts the incoming call. It alerts the called user witha visual or audible signal and returns an ALERT message (d) to the exchange. Theexchange then sends an ALERT message (e) to TE-P. If the bearer service for thecall is “speech” or “3.1-kHz audio,” the exchange also connects a ringing-tonesource to the B-channel of the calling user (f).When user Q answers, TE-Q sends a CONN message (g) to the exchange, whichthen connects the B-channels and sends a CONN message (h) to TE-P, indicatingthat the call has been answered. The exchange always acknowledges a receivedCONN message with a CONACK message (i). The figure also shows that TE-Packnowledges its CONN message with a CONACK message (j). This is notalways the case: some TEs do not provide this message, and exchanges thereforedo not require a CONACK response from a calling TE.At this point, the users can begin to talk or to send data.Figure 10.4-1. Signaling sequence for an intraexchange between ISDN users.