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300 ISDN USER PARTFigure 11.3-5. Mapping of Q.931 DISC messages and ISUP Release message (REL).Mapping of DISC and REL Messages (Fig. 11.3-5 ). DISC and REL messagesare general-purpose requests to release a B-channel or a trunk. They alwaysinclude, respectively, an IE.6 (cause indicators) and Par.9 (cause indicators), to indicatethe reason for the release. Mappings between IE.6 and Par.9 take place atexchanges P and R.The coding of the value fields of the information elements and parameters can befound in Sections 10.3.4 and 11.2.4. Some mappings require format changes. Forexample, the digits of the called and calling numbers have a seven-bit format inIE.2 and IE.3, and a four-bit format in Par.6 and Par.8. Other mappings, forexample, between IE.6 (cause) and Par.9 (cause indicators), do not requirereformatting.11.3.4 Use of Message ParametersThis section revisits the call example described in Sections 11.3.1 and 11.3.2, focusingthis time on the ISUP message parameters and their use. It is helpful to refer tothe parameter descriptions of Section 11.2.4 when reading this material.Routing Parameters in IAM. The selection of outgoing trunk groups byexchanges P and Q is influenced by Par.6, Par.25, Par.17, and Par.14 of the IAM(Fig. 11.3-3).Par.6 (called party number) is the primary routing parameter, from which theexchange derives the route set for the call. The other parameters can disqualifysome outgoing trunk groups in the route set.If Par.25 (transmission medium requirements) indicates that the transmissionmedium should accommodate a 64-kb/s data call, the connection is restricted toexchanges with digital switchblocks and to digital trunk groups without circuit multiplicationequipment.Par.17 (nature of connection indicators) is used to limit the number of satellitetrunks in the connection, to control the inclusion (enabling) of echo-controldevices, and to indicate whether a continuity check will be made on the trunk.