6.2.1 E-UTRAN Attach procedure for (v)SRVCC
23.2163GPPRelease 16Single Radio Voice Call Continuity (SRVCC)Stage 2TS
E-UTRAN attach or tracking area update procedure for 3GPP (v)SRVCC UE is performed as defined in TS 23.401 [2] or tracking area update procedure in 5GS to EPS mobility with N26 for 3GPP (v)SRVCC UE is performed as defined in TS 23.502 [45] with the following additions:
– (v)SRVCC UE includes the (v)SRVCC capability indication as part of the "MS Network Capability" in the Attach Request message and in Tracking Area Update Request message. MME stores this information for (v)SRVCC operation. The procedures are as specified in TS 23.401 [2].
NOTE 1: If the service configuration on the UE is changed (e.g. the user changes between an IMS speech/video service supported by the home operator and a PS speech service incompatible with (v)SRVCC), the UE can change its (v)SRVCC capability indication as part of the "MS Network Capability" in a Tracking Area Update message.
– SRVCC UE includes the GERAN MS Classmark 3 (if GERAN access is supported), MS Classmark 2 (if GERAN or UTRAN access or both are supported) and Supported Codecs IE (if GERAN or UTRAN access or both are supported) in the Attach Request message and in the non-periodic Tracking Area Update messages.
NOTE 2: MS Classmark 2, MS Classmark 3, STN-SR, C‑MSISDN, ICS Indicator and the Supported Codec IE are not sent from the source MME to the target MME/SGSN at inter CN-node idle mode mobility.
– HSS includes STN-SR and C‑MSISDN as part of the subscription data sent to the MME. If the STN-SR is present, it indicates the UE is SRVCC subscribed. If a roaming subscriber is determined by the HPLMN not allowed to have SRVCC in the VPLMN then HSS does not include STN-SR and C‑MSISDN as part of the subscription data sent to the MME. If the subscriber is allowed to have vSRVCC in the VPLMN then HSS also includes vSRVCC flag as part of the subscription data sent to the MME. If STN-SR and C-MSISDN are not included in the subscription data, then the MME shall not set "SRVCC operation possible" indication.
– MME includes a "SRVCC operation possible" indication in the S1 AP Initial Context Setup Request, meaning that both UE and MME are SRVCC-capable and that STN-SR and C-MSISDN are included in the subscription data.
E-UTRAN emergency attach procedure for 3GPP SRVCC UE is performed as defined in TS 23.401 [2] and above with the following clarifications:
– SRVCC UE shall include the SRVCC capability indication as part of the "MS Network Capability" in the Emergency Attach Request message, and maintained during Tracking Area Updates. MME stores this information for SRVCC operation. The procedures are as specified in TS 23.401 [2].