In the INVITE sent towards SCC-AS for SRVCC what is the Value set in the Request-URI inside this INVITE?

Interview
1

Q. In the INVITE sent towards SCC-AS for SRVCC what is the Value set in the Request-URI inside this INVITE?

a) ATU-STI
b) MSISDN
c) STN-SR
d) IMSI

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:arrow_right: Correct answer: Letter c. (or a *)

* please check this comment :point_down:

When an INVITE is sent to the SCC-AS as part of the SRVCC (Single Radio Voice Call Continuity) procedure, the Request-URI is set to the STN-SR (Session Transfer Number for SRVCC).

  • The STN-SR is a temporary routing number used to reach the target MSC in the CS domain.
  • It is allocated by the MME and delivered via SIP to the IMS core.
  • The SCC-AS identifies the session transfer using this value and routes accordingly.

Q. In the INVITE sent towards SCC-AS for SRVCC what is the Value set in the Request-URI inside this INVITE?

a) :x: ATU-STI
→ Incorrect. The Access Transfer Update – Session Transfer Identifier is used in header fields but not in the Request-URI.

b) :x: MSISDN
→ Incorrect. The MSISDN identifies the user, but SRVCC routing uses a temporary routing number.

c) :white_check_mark: STN-SR
→ Correct. STN-SR is placed in the Request-URI to route the INVITE to the correct CS domain node during SRVCC handover.

d) :x: IMSI
→ Incorrect. The IMSI is not used in SIP signaling; it is an internal identifier for mobile networks.

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This is extracted from 3GPP TS 23.237:

Session Transfer Number for SRVCC (STN-SR):
The STN-SR is used during SRVCC procedures as specified in TS 23.216. It is subscription information provided to the HSS for each subscriber when SRVCC service is supported by the home PLMN. STN-SR acts as a routing number, pointing either to the SCC-AS or to the ATCF (if enhanced SRVCC is deployed). Refer to TS 23.003 for further details.

So:

  • In eSRVCC (enhanced SRVCC), the INVITE containing the STN-SR is first routed to the ATCF.
    → The ATCF then generates a new INVITE toward the SCC-AS, where the Request-URI contains the ATU-STI.
  • In basic SRVCC (non-enhanced), the INVITE with STN-SR is sent directly to the SCC-AS.

Conclusion:

The original question is ambiguous, as it doesn’t specify whether the flow is enhanced or basic SRVCC.

However, since enhanced SRVCC (eSRVCC) is the dominant implementation in most current networks, many will answer ATU-STI, as that’s what appears in the Request-URI of the INVITE received by the SCC-AS in that scenario.

Recommendation:

To be fully accurate, the question should state the SRVCC mode (basic or enhanced). Without that context, the “correct” answer is dependent on interpretation and current network deployment trends.

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