This feature implements SCC-AS procedures for access transfer of a terminating call in the alerting phase using PS to CS SRVCC .
This page refers to SIP INVITE due to STN-SR or SIP INVITE due to ATU-STI as a "Handover INVITE".
Feature cheat sheet
| B2BUA Instance | SAS Support | Originating / Terminating | Points in Session Plan | Network Operator Data | Subscriber Data | Stateful or Stateless | POJO Feature or SBB Feature | Feature FSM |
|---|---|---|---|---|---|---|---|---|
SCC |
Yes |
Terminating |
|
No |
No |
Stateless |
POJO |
Yes |
Statistics
SCCTerminatingPreAnswerSessionTransfer statistics are tracked by the sentinel.volte.sip SBB
and can be found under the following parameter set in REM:
SLEE-Usage → sentinel.volte.sip service → sentinel.volte.sip SBB → feature → SCCTerminatingPreAnswerSessionTransfer
or with rhino-stats:
"SLEE-Usage.Services.ServiceID[name=sentinel.volte.sip,vendor=OpenCloud,version=3.0.0].SbbID[name=sentinel.volte.sip,vendor=OpenCloud,version=3.0.0].feature.SCCTerminatingPreAnswerSessionTransfer"
| Name | Description |
|---|---|
Started |
Incremented each time the feature runs |
FailedToStart |
Incremented when a fatal error occurs before feature execution |
IssuedWarning |
Incremented when a non-fatal error occurs during feature execution |
FailedDuringExecution |
Incremented when a fatal error occurs during feature execution |
TimedOut |
Incremented when feature execution does not complete within a reasonable time frame |
AccessTransferSuccessful |
Incremented when the ACK to the 2xx-INVITE due to ATU-STI or STN-SR is received by the feature |
AccessTransferAborted |
Incremented when the access transfer procedures are aborted before the transfer is complete |
SentPost405Update183 |
Incremented when a 183 is sent to the MSC after a 405-UPDATE is received |
SentRemoteReInvite |
Incremented when a remote reINVITE occurs |
SentRemoteUpdate |
Incremented when a remote UPDATE occurs |
SentPostUpdate183 |
Incremented when a 183 is sent to the MSC following an UPDATE-200 to the originating party |
SentInfoOnNewAccessLeg |
Incremented when an INFO is sent to the MSC as part of an Access Transfer |
SentForked183Response |
Incremented when a forked 183 response follows an UPDATE-200 with different SDP sent and received |
SentImmediate183 |
Incremented when an access transfer does not trigger an UPDATE to the originating party and a 183 is immediately sent to the MSC |
MaskedInviteEvent |
Incremented when the feature masks delivery of further INVITE events on the new access leg. |
UnmaskedInviteEvent |
Incremented when the feature unmasks delivery of further INVITE events on the new access leg. |
Behaviour
This feature implements SCC-AS procedures for access transfer of a terminating call in the alerting phase using PS to CS SRVCC. It includes support for media anchored in the ATGW and media non-anchored.
The behaviour is best described by reading section 12.3.4.2 in 3GPP TS 24.237. The feature implements the full text. A call flow diagram showing the media anchored case is in the Example call flow section of this page.
Removal of superfluous sessions is implemented through the SCCCoordinateSignallingAnchors and SCCRemoveSuperfluousSession features.
The following sections define additional behaviour to cover specification gaps.
UEs with an Allow header that does not include UPDATE
TS 24.237 specification doesn’t explicitly describe procedures at the SCC-AS for the case where a UE has specified an Allows header without including an UPDATE as the value. This feature implements behaviour as though it has received a 405-UPDATE when it receives the handover INVITE if:
-
the speech media component of the session to transfer is not equal to that in the Handover INVITE, and
-
the session to transfer has an Allows header without including UPDATE
That is, it creates a new early dialog towards the MSC using a 183 response. The 183 response includes signalling elements described in subclause 6A.4.3A. It includes an SDP answer:
-
with c-line set to the unspecified address (0.0.0.0) if IPv4 or to a domain name within the ".invalid" DNS top-level domain in case of IPv6 as described in IETF RFC 6157; and
-
including media of media types received in SDP offer of the SIP INVITE request due to STN-SR, which are also offered in the SIP INVITE request from the served user; and
Example call flow
Please refer to 3GPP TS 24.237 appendix A.17.8. It is a single "successful case" for a terminating alerting access transfer where media is anchored in the ATGW.
