1、 TECHNICAL REQUIREMENT T1.TRQ.9-2003 Technical Requirement on Support of Crankback in Both ISUP this subset is network specific. If the originating switch receives a release message (REL) from the succeeding switch, the switch shall perform the following procedure for re-routing: 1. If the switch re
2、ceives a REL from the succeeding switch with the ARR parameter indicating crankback (re-routing counter greater than or equal to one and re-routing inhibit indicator not 3The protocol limit is 63. 7 T1.TRQ.9-2003 set), the locally defined maximum number of re-routing attempts has not been exceeded,
3、and the received cause value is on the ARR Cause List, then the switch shall attempt to re-route the call automatically if an alternate route exists. If an alternate route is available, the switch shall include the ARR parameter in the IAM with the re-routing counter. If no alternate route is availa
4、ble, see . 2. Otherwise (i.e., if no alternate route is available or “do not crankback” is indicated in the ARR re-routing inhibit indicator field or no ARR parameter is included in the REL), the switch shall release the call according to the normal release procedures. If an intermediate switch cann
5、ot find an outgoing trunk to a succeeding switch, then the switch may perform regular overflow routing and attempt to route on the next available route. If no outgoing routes are available, then the switch shall perform the following procedure: 1. If the outgoing Trunk Group (TG) Crankback Treatment
6、, based on the setting of the last TG on the route list, is “crankback“ or “pass ARR or set crankback“ and the cause value generated is on the ARR Cause List, then the switch shall send a REL to the preceding switch including the ARR parameter with the re-route inhibit indicator not set. If an ARR p
7、arameter is received in the IAM, then the re-routing counter in the ARR parameter is incremented by one. Otherwise, the re-routing counter is set to one. 2. Otherwise, (i.e., if the cause value generated is not on the ARR list) the switch shall send a REL to the preceding switch with the ARR paramet
8、er populated as follows: a. If an ARR parameter was received in the IAM, then the switch shall include the ARR parameter with the re-route inhibit indicator set to “do not crankback.“ b. If no ARR parameter was received in the IAM, the switch shall use the regular backward release procedure for unsu
9、ccessful calls. If an intermediate switch receives a REL message from the succeeding switch without the ARR parameter, then the switch shall not attempt to re-route on any alternate routes. The switch shall also perform the following procedure: 1. If the outgoing TG Crankback Treatment is set to “cr
10、ankback“ and the cause value in the received REL message is on the ARR Cause List, then the switch shall send to the preceding switch a REL message that includes the ARR parameter with the re-route inhibit indicator not set. If an ARR parameter were received in the IAM, then the re-routing counter i
11、n the ARR parameter is incremented by one. Otherwise, the re-routing counter is set to one. 2. Otherwise, the switch shall send a REL message to the preceding switch. If an ARR parameter were received in the IAM, then the ARR parameter is included with the re-route inhibit indicator set to “do not c
12、rankback.” Otherwise, the regular backward release procedure for unsuccessful calls is used. 8 T1.TRQ.9-2003 If an intermediate switch receives a REL message from the succeeding switch with the ARR parameter, then the switch shall determine whether to re-route according to step 1 of . If no alternat
13、e routes are available, then the switch shall perform the following release procedures: 1. If the outgoing TG Crankback Treatment is set to “crankback“ and the cause value in the received REL is on the ARR Cause List, then the switch shall send to the preceding switch a REL message that includes the
14、 ARR parameter with the re-route inhibit indicator not set and the re-routing counter unchanged. 2. If the outgoing TG Crankback Treatment is set to “pass ARR or set crankback“ and the cause value in the received REL is on the ARR Cause List, then the switch shall pass unchanged the ARR parameter in
15、 the REL from the succeeding switch to the preceding switch. 3. Otherwise, the switch shall send a REL to the preceding switch including the ARR parameter with the re-route inhibit indicator set to “do not crankback.” If an intermediate switch sets up the bearer for a call in the backward direction
16、over a packet network, and the bearer setup fails, then the switch shall perform the following procedures: 1. If the incoming BICC Trunk Group Crankback Treatment is set to “crankback“ or “pass ARR or set crankback“ and the cause value generated by the switch to be included in the BICC REL message i
17、s on the ARR Cause List, then the switch shall send a BICC REL that includes the ARR parameter with the re-route inhibit indicator not set to the preceding switch. If an ARR parameter were received in the IAM, then the re-routing counter in the ARR parameter is incremented by one. Otherwise, the re-
18、routing counter is set to one. 2. Otherwise, the switch shall send a BICC REL to the preceding switch. If an ARR parameter were received in the IAM, then the ARR parameter with the re-route inhibit indicator set to “do not crankback” is included Otherwise, the regular backward release procedure for
19、unsuccessful calls is used. If the destination switch cannot establish the call toward the user and the connection between the destination switch and the user is a trunk, then the switch shall perform the same procedure as specified in . If the destination switch cannot establish the call toward the
20、 user and the connection between the destination switch and the user is a line, then the switch shall perform regular release procedures for line busy. 9 T1.TRQ.9-2003 4.2 Signalling and Protocol Requirements 4.2.1 Automatic Re-routing parameter (crankback) The switch must be capable of receiving, a
21、nd formatting and transmitting the following ISUP/BICC parameter in either an ISUP/BICC IAM or REL message: Parameter name code: 1001 0110 same as for BISUP (Q.2763) The format of the Automatic Re-routing parameter is as follows: 8 7 6 5 4 3 2 1 1 Ext. 14Re-routing Inhibit Indicator Re-routing Count
22、er The following codes are used in the subfields of the Automatic Re-routing parameter field: a) Extension indicator (ext.): 0 octet continues through the next octet (e.g., octet 1 to 1a) 1 last octet b) Re-routing Inhibit Indicator: 0 no indication 1 do not crankback c) Re-routing Counter: 00 0000
23、not used 00 0001 1stcrankback attempt 00 0010 2ndcrankback attempt to 11 1111 63rdcrankback attempt NOTE - Octet 1a is not present if the extension bit in octet 1 is set to 1. 4.3 Operations, Administration and Provisioning Requirements , , and define three provisionable items: 1. provides for a spe
24、cification of the allowed Crankback Treatment on each outgoing Trunk Group. 4NOTE - The Re-Routing Reason is not used in ANSI networks; therefore, the extension indicator is always coded as 1. 10 T1.TRQ.9-2003 2. provides a switch-wide specification of the maximum number of Crankbacks for any given
25、call. 3. provides a provisionable list of Cause Values which may trigger crankback. 5 Signalling Procedures For Crankback 5.1 Introduction The crankback signalling procedure allows the call set-up to return to a preceding SN exchange5so that the call can be automatically re-routed from there. Crankb
26、ack is an optional signalling procedure that allows for sophisticated support of the ARR capability (refer to ITU-T Recommendation E.170). This procedure is an additional procedure to the unsuccessful call set-up procedures described in T1.673-2002, clause 9 T1.113.4-2000, clause 2.2. An SN An excha
27、nge invokes the crankback procedure when a call cannot be routed further from that SN exchange. There are three possible cases: 1. The process to select an outgoing route circuit from the SN exchange fails. 2. A backward REL is received during the outgoing call set-up. The cause value received is ei
28、ther specific for the route chosen (e.g., bearer capability not implemented) or is temporary in nature (e.g., congestion) 3. The call cannot be established to the user at the destination local SN exchange. The number of attempts to re-route a call is limited. This limit is a network specific value,
29、not exceeding 63. It needs to be emphasized that the crankback signalling procedure can only be effective when introduced on a network wide basis. 5.2 Actions at an intermediate SN exchange 5.2.1 Sending a REL with the possible invocation of crankback Crankback may or may not be invoked when the cal
30、l cannot be routed further from an intermediate SN exchange as described in the cases 1 and 2 in clause 5.1. Invocation of crankback involves the setting or updating of the Re-routing Counter, which keeps track of the number of re-routing attempts. A reason for not invoking crankback is when the cou
31、nter has reached its upper limit. Four cases can be distinguished in an intermediate SN exchange: a) Crankback is invoked and the Automatic Re-routing parameter has not been received in the IAM for the incoming call: In this case, the intermediate SN exchange sends a REL towards the preceding SN exc
32、hange including the Automatic Re-routing parameter with the Re-routing Counter set to “one“ and the Re-routing Inhibit Indicator set to “no indication.” b) Crankback is invoked and the Automatic Re-routing parameter has been received in the IAM for the incoming call: In this case, the intermediate S
33、N exchange sends a REL towards the preceding SN exchange including the Automatic Re-routing parameter with the Re-routing Counter incremented by one and the Re-routing Inhibit Indicator set to “no indication.” 5Text specific for ISUP is shown in square brackets, e.g., text. It replaces preceding BIC
34、C-specific text. 11 T1.TRQ.9-2003 c) Crankback is not invoked and the Automatic Re-routing parameter has or has not been received in the IAM for the incoming call: In this case, the intermediate SN exchange sends a REL towards the preceding SN exchange including the Automatic Re-routing parameter wi
35、th the Re-routing Inhibit Indicator set to “do not crankback.” The Re-routing Counter is not incremented if it was received in the incoming IAM. d) If the intermediate SN exchange does not support the crankback signalling procedure, no Automatic Re-routing parameter is sent in the REL message and th
36、us a regular backward release according to T1.673-2002, clause 9.3 T1.113.4-2000, clause 2.2.2 or 2.2.3 takes place. 5.2.2 Receiving a REL with the Automatic Re-routing parameter An intermediate SN exchange can take four possible actions when it receives a REL from the succeeding SN exchange with th
37、e Automatic Re-routing parameter: a) It attempts to re-route the call to an alternative route if: Crankback has been invoked (re-routing counter greater or equal to one and Re-routing Inhibit Indicator coded as “no indication“) in the received ARR; and The procedures in Sec. 4.1in the SN exchange co
38、ncludes that crankback should be applied in this SN exchange. If an alternate route is available and the maximum number of re-routing attempts has not been exceeded, the SN exchange includes the Automatic Re-routing parameter into the IAM, to indicate how many crankback attempts have already occurre
39、d. If no alternative route is available, or the re-routing counter exceeds the maximum number of re-routing attempts allowed by the network, the received REL shall be passed towards the preceding SN exchange with the inclusion of the Automatic Re-routing parameter as received. NOTE - The maximum num
40、ber of re-routing attempts is network specific. b) It does not attempt re-routing, but passes the received REL towards the preceding SN exchange with the inclusion of the Automatic Re-routing parameter as received if the Re-routing Inhibit Indicator is coded as “do not crankback.” NOTE - The Re-rout
41、ing Inhibit Indicator is the means by which a succeeding SN exchange can explicitly prevent a preceding SN exchange from performing crankback. c) It does not attempt re-routing, but passes the received REL towards the preceding SN exchange with the inclusion of the Automatic Re-routing parameter as
42、received if the Re-routing Inhibit Indicator is coded as “no indication“ and the procedures in section 4.1 conclude that no re-routing should be applied in this SN exchange. If the intermediate SN exchange wants to inhibit re-routing, it includes the Re-routing Inhibit Indicator set to “do not crank
43、back“ in the Automatic Re-routing parameter. d) It handles the Automatic Re-routing parameter as an unrecognized parameter according to T1.673-2002, clause 13.4.4.2 T1.113.4-2000, clause 2.9.5.3.2 if the SN exchange does not support the crankback procedure and, as a result, does not recognize the pa
44、rameter. This may render the crankback mechanism ineffective. 5.2.3 Receiving an IAM with the Automatic Re-routing parameter The intermediate exchange may receive the Automatic Re-routing parameter in an IAM. This parameter is passed on if the call is routed to the succeeding SN exchange. If the cal
45、l cannot be routed to the succeeding exchange, clause 5.2.1 applies. 12 T1.TRQ.9-2003 Procedures for unrecognized parameters apply if the intermediate exchange does not support the crankback procedure and, as a result, does not recognize the parameter; see T1.673-2002, clause 13.4.4.2 T1.113.4-2000,
46、 clause 2.9.5.3.2. This may render the crankback mechanism ineffective. 5.3 Actions at an intermediate CMN BICC only 5.3.1 Sending a REL with the possible invocation of crankback The actions as described in 5.2.1 apply. 5.3.2 Receiving a REL with Automatic Re-routing parameter The CMN never initiate
47、s an attempt to re-route a call, as it has no control of the bearer. If the CMN determines that: 1. The locally defined maximum number of crankback attempts has been exceeded; or 2. Service logic at the CMN determines that no alternate route exists toward the destination, then the CMN shall pass the
48、 received REL toward the preceding SN with the Re-routing Inhibit Indicator in the ARR parameter set to “do not crankback.” Otherwise, the CMN shall pass the received REL towards the preceding SN with the inclusion of the ARR parameter as received. The CMN handles the Automatic Re-routing parameter
49、as an unrecognized parameter according to T1.673-2002, clause 13.4.4.2, if it does not support the crankback procedure and, as a result, does not recognize the parameter. This may render the crankback mechanism ineffective. 5.3.3 Receiving an IAM with the Automatic Re-routing parameter The actions as described in 5.2.3 apply. 5.4 Actions at a Gateway SN exchange The actions as described in 3.2 apply. However, passing of the ARR parameter in the IAM and REL messages between networks depends on bilateral agreement (e
