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11.6 OTHER SIGNALING PROCEDURES 313TABLE 11.6-2AcronymCircuit Group Supervision MessagesNameMessageType CodeCGB Circuit Group Blocking 0001 1000CGBA Circuit Croup Blocking Acknowledgment 0001 1010CGU Circuit Group Unblocking 0001 1001CGUA Circuit Group Unblocking Acknowledgment 0001 1011GRS Circuit Group Reset 0001 0111GRA Circuit Group Reset Acknowledgment 0010 1001Source: Rec. Q.763. Courtesy of ITU-T.first-order digital multiplex system (Section 1.5.2) can be blocked or unblockedsimultaneously.The status sub-field has variable length and has a status bit for the individual circuits.Bits 1, 2, ..., 8 in octet 2 represent circuits with CIC ¼ c, c þ 1, ...,cþ 7; bit1 in octet 3 represents the circuit with CIC ¼ c þ 8, and so on.In the Circuit Group Blocking and Unblocking messages (CGB and CGU), astatus bit with value “1” indicates that the corresponding trunk should be, respectively,blocked or unblocked. In Circuit Group Blocking and Unblocking Acknowledgmentmessages (CGBA and CGUA), a status bit with value “1” indicates that therequested blocking or unblocking of the corresponding trunk is now in effect.A Circuit Group Reset Request (GRS) message includes only the range octet ofthe range and status parameter. When exchange B receives a GRS message withCIC ¼ c and R ¼ r, it releases all calls on circuits with CIC values c through c þ r.A Circuit Group Reset Acknowledgment (GRA) message includes a completerange and status parameter; the bits in the status field represent individual trunksin the group specified by the values of CIC and R. Exchange B sets the status bitsof a trunk to “1” if it is currently blocking the trunk at its end. This informs exchangeA of which trunks cannot be seized for new calls.Circuit Group Supervision Message Type. This is a parameter that appears inthe messages CGB, CGU, CGBA, and CGUA (Circuit Group Blocking, Unblocking,and their acknowledgments) only. The parameter has a one-octet value field,in which bits 2 and 1 indicate the reason for the blocking or unblocking.Figure 11.6-3. Range and status parameter.