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11.2 ISUP MESSAGES, FORMATS, AND PARAMETERS 287Figure 11.2-4. Format of parameters. Par.3: automatic congestion level. Par.7: calling partycategory. Par.13: event information. Par.25: transmission medium requirement.Par.3 Automatic Congestion Level. This is an optional parameter in REL messages(Fig. 11.2-4). It indicates congestion at the exchange and is coded as follows:0000 0001 Congestion level 1 exceeded (moderate congestion)0000 0010 Congestion level 2 exceeded (severe congestion)Par.4 Backward Call Indicators. This is a mandatory parameter in the ACMmessage and an optional parameter in CPG and ANM messages (Fig. 11.2-5). It containsa number of indicator bits with information for the originating exchange:BitsBitsBA Charge indicator DC Called party status00 No indication 00 No information01 Do not charge 01 Subscriber free10 Charge 11 Call setup delayFE Called party category HG End-to-end signaling00 Not known 00 Not available01 Subscriber 01 Pass-along method available10 Payphone 10 SCCP method available11 Pass-along and SCCP methodsavailableJ Interworking indicator K ISUP indicator0 TUP or ISUP signaling allthe way (no interworking)1 Interworking encounteredL Holding indicator M ISDN access indicator0 Holding not required1 Holding required0 Not ISUP signaling all the way1 ISUP signaling all the way0 Called party has non-ISDN access(analog subscriber)1 Called party has ISDN access(ISDN user)N Echo-control indicator PO SCCP method indicator0 Incoming half-echo control 00 No indicationdevice not included 01 Connectionless method available1 Incoming half-echo control 10 Connection oriented method availabledevice included11 Connectionless and connectionorientedmethods available