1. Configuration Guide
  2. IMS Support
  3. SRVCC PS-CS Access Transfer
  4. Early Dialogs
  5. SRVCC Handover Support in the Pre-Alerting Phase

SRVCC Handover Support in the Pre-Alerting Phase

In addition to other SRVCC support, the Oracle Communications Session Border Controller (SBC) supports procedures to manage the handover from 4G to 3G/2G of sessions in pre-alerting phase. The conditions by which a session is defined as in the pre-alerting phase include the calling party has not yet received a 180 RINGING message.

Refer to TS 24.237 to review the anchoring endpoints' behavior.

To ensure that calls in a pre-alerting phase are transferred between PS and CS networks, set the sip-feature-caps value as follows:

  • Set state to enabled
  • Set atcf-management-uri to management or psi
  • Set atcf-pre-alerting to enabled

For information on the results of these settings, refer to the SIP Feature Capabilities section.

The SBC, as Proxy Call Session Control Function (P-CSCF) or Active Transfer Control Function (ATCF), selects any of the early dialogs as the session being transferred when:

  • There are no confirmed dialogs supporting a session with an inactive speech media component ("sendonly" or "inactive" directionality) in the transferable session set, and
  • There are one or more dialogs in the transferable session set supporting one session where the SC UE has completed a reliable offer / answer procedure and has an active speech media component ("recvonly" or "sendrecv" directionality).

Applicable dialogs include those for which:

  • A SIP 180 (Ringing) response to the SIP INVITE request has not been received yet in any of the existing dialogs.
  • All dialogs are early dialogs created by the same SIP INVITE request.
  • The Contact header field provided by the SC UE includes the g.3gpp.ps2cs-srvcc-orig-pre-alerting media feature tag; and
  • The Feature-Caps header field provided by the SCC AS towards the SC UE includes the g.3gpp.ps2cs-srvcc-orig-pre-alerting feature-capability indicator.

The range of SBC SRVCC pre-alerting phase support includes:

  • SRVCC support for both outgoing (originating) and incoming (terminating) calls
  • Responds to media feature tag and fcaps indicator
  • SRVCC support of EATF emergency calls
  • Handover (HO) cancelling scenarios
  • Rx interactions
  • SRVCC Error scenarios, including:
    • HO cancellation failures
    • Rx failures
    • Standard SIP transaction failures
  • ACLI, SNMP and HDR statistics for applicable, successful and failed SRVCC handovers

The diagram below shows a flow for an outgoing call that includes an SRVCC handover during the pre-alerting phase. Prior to the alerting phase, the SBC, acting as ATCF (IM/CN subsystem) receives and forwards the new INVITE to update the call to the circuit switched (CS) leg. Ultimately, the signaling between the MSC, the ATCF and the SOC AS succeeds. The components proceed through the alerting phase, eliminating the packet switched based (PS) leg and using the MSC server to manage the CS call.

Figure 18-15 Mobile Originating Call in Pre-Alerting Phase



The diagram below shows a flow for an incoming call that includes an SRVCC handover during the pre-alerting phase. Prior to the alerting phase, the SBC, acting as ATCF (IM/CN subsystem) receives and forwards the new INVITE to update the call to the circuit switched (CS) leg. Ultimately, the signaling between the MSC, the ATCF and the SOC AS succeeds. The components proceed through the alerting phase, eliminating the packet switched based (PS) leg and using the MSC server to manage the CS call.

Figure 18-16 Mobile Terminating Call in Pre-Alerting Phase