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9.5 OTHER TUP PROCEDURES, MESSAGES, AND SIGNALS 223If the record at V shows that its call on T is an outgoing call (to W), it sets trunk T to“idle,” sends a CLB signal to the previous exchange in the connection, and sends aCLF signal to W, which then responds with a RLG signal. If exchange V receives aRSC signal when the affected trunk T is idle, it responds with a RLG signal.9.5.4 Circuit Group Blocking, Unblocking, and ResettingTUP signaling includes a group of circuit group supervision messages that are usedto block, unblock, and reset up to 256 trunks in a trunk group simultaneously. Themessage acronyms and heading codes are listed in Table 9.5-2.The structure of a circuit group supervision message is shown in Fig. 9.5-2. Thevalues of OPC and DPC (originating and destination point codes) identify the trunkgroup TG. The particular set of up to 256 trunks that are covered by the message isidentified by the values of parameters CIC (circuit identification code) and R(range). For example, the combination (CIC ¼ c, R ¼ r) identifies the set oftrunks in group TG that have the CIC values c, c þ 1, c þ 2, ..., and c þ r.In Fig. 9.5-2, parameter R is followed by up to 32 octets of status indicator bitsfor the trunks in the set. Bits 1 through 8 in octet 3 represent the trunks with CICvalues (c þ 0) through (c þ 7), bits 1 through 8 in the next octet represent trunkswith CIC codes (c þ 8) through (c þ 15), and so on.In the request messages, the indicator value is 1 if the request is made for theparticular trunk, and 0 otherwise. These indicator values are copied in the groupblocking and unblocking acknowledgment messages.In the Group Reset Acknowledgment (GRA) message, a status indicator value 1or 0 indicates that the corresponding trunk is blocked, or not blocked, by theexchange that sends the GRA message.Maintenance and Failure Group Blocking. A Maintenance Group Blocking(MGB) message is sent by the exchange (V or W) that wants to test a number oftrunks in trunk group TG. A Hardware Failure Group Blocking (HGB) message issent by an exchange when it detects a failure on a number of trunks.TABLE 9.5-2 Circuit Group Supervision Messages (H0 5 1000)H1 Name Acronym0001 Maintenance Group Blocking Request MGB0010 Maintenance Group Blocking Acknowledgment MGA0011 Maintenance Group Unblocking Request MGU0100 Maintenance Group Unblocking Acknowledgment MUA0101 Hardware Failure Group Blocking Request HGB0110 Hardware Failure Group Blocking Acknowledgment HBA0111 Hardware Failure Group Unblocking Request HGU1000 HGU Acknowledgment HUA1001 Group Reset Request GRS1010 Group Reset Acknowledgment GRASource: Rec. Q.723. Courtesy of ITU-T.