24.5293GPPProtocol specificationPSTN/ISDN simulation services: Explicit Communication Transfer (ECT)Release 8TISPANTS
Figure A.2 signalling flow shows a consultative transfer scenario.
Figure A.2: Consultative transfer
1. A multimedia session exists between A-B and between B-C. B initiates transfer A to C, by sending REFER method To: UE-A GRUU with the Refer-To: UE-C?Replaces=dialog2&Require=replaces, Referred-By: UE-B. The REFER reuses the dialog that exists from A-B.
1.1 Upon reception of the REFER operation AS-B must check whether there is no outgoing call barring active from B to C. Because B is charged for the call from B-C when A is referred to C, when outgoing call barring is active from B-C the REFER is rejected.
AS-B checks whether B is allowed to transfer calls, if it is allowed to transfer the call then AS-B generates an ECT Session Identifier URI, addressed to itself, with the new destination information and billing information that will be needed for the new session. It replaces the Refer-To value with the ECT Session Identifier URI. This ensures that AS-B will remain in the loop.
2. The REFER to method is sent on to AS-A.
2.1 AS-A checks whether it is allowed to transfer A.
3. Refer is sent on to A by AS-A.
4.1, 7.1, 31.1 AS-A can use result messages and notifications caused by REFER to track success of REFER and take appropriate actions. The AS-A can ensure that header fields that where replaced with other content are recreated with the original content on the way back.
5.1, 8.1, 32.1 AS-B can use this to track success of REFER and take appropriate actions. The AS-B can ensure that header fields that where replaced with other content are recreated with the original content on the way back.
13. UE-A initiates a new session by sending an INVITE to AS-B’s ECT Session Identifier URI (which represents UE-C).
13.1 AS-A checks whether A is allowed to use the Replace extension and routes the INVITE to AS-B using the AS-B’s ECT Session Identifier URI using normal SIP routing procedures. Normal charging from A to B applies.
14.1 Upon receiving the INVITE to the ECT Session Identifier URI that was inserted by the AS-B, the AS-B replaces the Request URI and creates an INVITE targeted towards UE-C. Further AS-B inserts a Replaces header field with the value of the Replaces parameter of the stored transfer Target URI if it is available, which will allow the new session to take the place of the existing session between B and C.
In this scenario it can be assumed that there is no active outgoing call barring towards UE-C, because UE-B was able to setup a call to UE-C in the first place. However when there was no consultation call to UE-C, there is an issue but this is solved at the initial reception of the REFER from UE-C and not at this stage.
The INVITE is forwarded to UE-C using normal routing procedures.
15.1, 17.1 Normal terminating services apply for UE-C. The call will be treated as a call from A-C regarding call policies. AS-C checks whether the Replace mechanism is used.
19.1 AS-A. Normal response handling applies.
21.1 AS-A. Normal ACK handling applies.
22.1 AS-B replaces all codified values and the ECT Session Identifier URI with stored values.
25. UE-C terminates dialog 2 as consequence of normal Replace procedures according to RFC 3891 .
Annex B (informative):
Example of filter criteria