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314 ISDN USER PARTBits 2,1 ¼ 0,0Bits 2,1 ¼ 0.1Blocking for maintenance reasons. No trunk should be seizedfor new calls, but existing calls are allowed to continue.Blocking because of a hardware failure. No new calls areallowed, and existing calls have to be released immediately.The “hardware failure” blocking and reset requests for a circuit group usuallyinterrupt a number of calls. Therefore, as a safety measure against an undetectederror in a CGB or GRS message, an exchange repeats a sent CGB or GRSmessage within 5 s. The other exchange only carries out the blocking or reset procedureif it has received two identical messages within 5 s.11.6.9 Signaling Route Set Congested or UnavailableThese procedures are the same as in TUP signaling (Section 9.5.5).Signaling Route Set Congested. When the ISUP at an exchange receives aMTP-status indication for a destination (Section 8.9.3), it temporarily reduces thenumber of seizures of outgoing trunks in the routes to that destination.Signaling Route Set Unavailable. On receipt of a MTP-pause indication for adestination (Section 8.9.5), ISUP clears all ISUP trunks to that destination and suspendsseizing those trunks for new calls until it receives a MTP-resume indication.11.6.10 Application Transport Mechanism (APM)The application transport mechanism [7] is an ISUP procedure that can be used bytwo peer applications, residing in separate exchanges, to send application-specificdata sequences to each other. The signaling relationship between applications establishedby the APM procedure is similar to what is provided by TCAP (Chapter 16).The data is encapsulated in the application transport parameter (Par.2—ATP) andtransported transparently in the appropriate ISUP message. The APM procedureallows for segmentation and reassembly of data sequences that do not fit into asingle message. The maximum length allowed for a data sequence is 10 segmentsand 2048 octets. It should be pointed out that the APM acronym is used to denoteboth the “mechanism” (procedure) and one of the message types that may beused to invoke it. The procedure may be invoked with any of the messages thatcan carry the ATP parameter (Table 11.2-2): it may use an established connection(e.g., by inserting the ATP parameter in the IAM message or by sending an APMmessage at any time during the call), or the application needing the procedure canstart the connection.Each message involved in the APM procedure can carry more than one instanceof the ATP parameter (Par.2), each related to a different application, as indicated bythe ACI field value.