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 Originating / Terminating Points in Session Plan Network Operator Data Subscriber Data Stateful or Stateless POJO Feature or SBB Feature Feature FSM



SipAccessPartyRequest, SipAccessPartyResponse, and SipMidSession_PartyResponse







SCCTerminatingPreAnswerSessionTransfer statistics are tracked by the volte.sentinel.sip SBB and can be found under the following parameter set in REM:
SLEE-Usage → volte.sentinel.sip service → volte.sentinel.sip SBB → feature → SCCTerminatingPreAnswerSessionTransfer
or with rhino-stats:

Name Description

Incremented each time the feature runs


Incremented when a fatal error occurs before feature execution


Incremented when a non-fatal error occurs during feature execution


Incremented when a fatal error occurs during feature execution


Incremented when feature execution does not complete within a reasonable time frame


Incremented when the ACK to the 2xx-INVITE due to ATU-STI or STN-SR is received by the feature


Incremented when the access transfer procedures are aborted before the transfer is complete


Incremented when a 183 is sent to the MSC after a 405-UPDATE is received


Incremented when a remote reINVITE occurs


Incremented when a remote UPDATE occurs


Incremented when a 183 is sent to the MSC following an UPDATE-200 to the originating party


Incremented when an INFO is sent to the MSC as part of an Access Transfer


Incremented when a forked 183 response follows an UPDATE-200 with different SDP sent and received


Incremented when an access transfer does not trigger an UPDATE to the originating party and a 183 is immediately sent to the MSC


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 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:

  1. the speech media component of the session to transfer is not equal to that in the Handover INVITE, and

  2. 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:

  1. with c-line set to the unspecified address ( 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

  2. 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

Error responses other than 405-UPDATE

The specification does not describe behaviour for error responses other than 405-UPDATE. If a necessary SIP procedure fails due to an error response, the feature terminates:

  1. the old access leg

  2. the new access leg

  3. the remote leg

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.

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