This feature implements SCC-AS procedures for access transfer of an originating call in the alerting or pre-alerting phases 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

Originating

SipAccessPartyRequest, SipAccessPartyResponse, and SipMidSession_PartyResponse.

No

No

Stateless

POJO

Yes

Statistics

SCCOriginatingPreAnswerSessionTransfer 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 → SCCOriginatingPreAnswerSessionTransfer
or with rhino-stats:
"SLEE-Usage.Services.ServiceID[name=sentinel.volte.sip,vendor=OpenCloud,version=3.1.0].SbbID[name=sentinel.volte.sip,vendor=OpenCloud,version=3.1.0].feature.SCCOriginatingPreAnswerSessionTransfer"

Parameter Type Description
Started

Counter

Incremented each time the feature runs

FailedToStart

Counter

Incremented when a fatal error occurs before feature execution

IssuedWarning

Counter

Incremented when a non-fatal error occurs during feature execution

FailedDuringExecution

Counter

Incremented when a fatal error occurs during feature execution

TimedOut

Counter

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

AccessTransferSuccessful

Counter

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

AccessTransferAborted

Counter

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

SentRemoteUpdateUsingUpdate

Counter

Incremented when a remote update occurs using the UPDATE method (prior to 2xx-INVITE on the remote leg)

SentRemoteUpdateUsingPrack

Counter

Incremented when a remote update should occur prior to session establishment, but the remote UE does not support the UPDATE method

SentRemoteUpdateUsingInvite

Counter

Incremented when a remote update occurs using the INVITE (reINVITE) method. Post 2xx-INVITE on the remote leg.

MaskedInviteEvent

Counter

Incremented when the feature masks delivery of further INVITE events on the new access leg.

UnmaskedInviteEvent

Counter

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 an originating call in the alerting or pre-alerting phases using PS to CS SRVCC. It includes support for media anchored in the ATGW and media non-anchored, forks of the remote leg, and various race conditions between processing the Handover INVITE and the remote leg answering the call.

The behaviour is best described by reading section 12.3.4.3 in 3GPP TS 24.237. The feature implements the full text. Several call flows showing different success cases are shown in the Example call flows 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 (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

  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

It then waits for the 2xx-INVITE from the remote leg and does not forward that on to the MSC. It initiates a re-INVITE to provide the new SDP offer to the remote UE, waits for the 2xx response and forwards it to the MSC.

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

Receipt of non reliable provisional responses from the remote UE

The specification does not cover the remote UE sending non-reliable provisional responses. The feature deals with this case by:

  1. making all provisional responses that it sends to the MSC be reliable,

  2. when it receives a PRACK from the MSC, it waits for 2xx-INVITE from the remote leg (and does not forward it onwards to the MSC. It then sends a re-INVITE with SDP from the Handover INVITE. When it receives a 2xx to the re-INVITE it forwards that onwards to the MSC.

Call flows for session transfer of an outgoing call in the alerting phases

Session Transfer for an outgoing call in the alerting phase

Please refer to 3GPP TS 24.237 appendix A.17.3. It is a single "successful case" for a originating alerting access transfer where media is not anchored in the ATGW.

Session Transfer for an outgoing call in the alerting phase with forks

Please refer to 3GPP TS 24.237 appendix A.17.6. It is a single "successful case" for a originating alerting access transfer where media is not anchored in the ATGW. Additionally the SC UE has received several forked responses prior to access transfer.

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Sentinel VoLTE Version 3.1.0