This blog discusses key access transfer identifiers, with more focus on the PS to CS domain access transfer. Roaming related variables are not considered in this blog.


- Single Carrier UE. An SC UE might support ICS, (IMS Service Continuity) per TS 23.237

Session Transfer:

- For each session, there is a signaling/media path

- A session transfer means that a transfer can be done for session independently at the IMS-level of where the session is anchored per TS 23.237

Dynamic Session Transfer Number (STN):

- If dynamic STN assignment is used, the STN is sent to the UE over the CS domain

- For PS to CS transfer request, the UE sends the STN to the SCC AS

- Per TS 23.003 [14] the STN takes on the value of an E.164 numbering format

Static Session Transfer Number (STN):

- Configured at the UE and used if no dynamic STN is available for PS to CS access transfer per TS 24.237

Access Transfer Update - Session Transfer Identifier (ATU-STI):

- Assigned to SCC AS per TS 23.237 and sent by the SCC AS upon successful IMS registration to the ATCF

- The ATCF can use ATU-STI to communicate with the SCC AS later on for the purpose of access transfer procedure

Session Transfer Number for SRVCC (STN-SR):

- Per TS 24.237 the STN-SR is assigned to the SCC-AS/ATCF

- For the case where the SRVCC is enhanced with the use of the ATCF, the STN-SR is used to direct signaling towards the ATCF, otherwise it is used to direct signaling to the SCC AS per TS 23.237

- According to TS 23.003, STN-SR follows the E.164 telecommunications number format

- It is used by the MSC for session transfer of the media path from the PS domain to the CS Domain

- The STN-SR is provisioned in the HSS as part of the subscriber profile information to indicate that the UE is SRVCC capable (TS 23.216)

- If the STN-SR is updated from the value that is in the HSS, then the SCC AS updates the HSS which in turns updates the MME

Correlation Mobile Station ISDN number (C-MSISDN):

- Is used to identify an anchored session in IMS

- The C-MSISDN can be equal to the UE’s MSISDN number (E.164 format)

- It is unique per IMSI and is bound to the IMS Private User Identity

Dynamic Session Transfer Identifier (STI):

- Allocated dynamically upon session creation by the SCC AS and assigned the value of the SIP dialog Id to anchor a session and communicated to the UE per TS 23.237

- The dynamic STI is communicated to the UE over the CS domain as state in 24.237

- The dynamic STI is received over the Gm interface in the PS domain when requesting access transfer to a CS domain by the UE as stated in TS 24.237

- It is used by the SCC AS to correlate the access transfer request from the CS to the PS domain

Static Session Transfer Identifier (STI):

- It is used when the dynamic STI is not available

- When used, it means that the UE is not an ICS UE per TS 23.237

- It is used to initiate transfer from the CS to the PS domain

- It can be statically configured as the tel URI (E.164 format) or a SIP URI for of a tel URI presentation format per TS 24.237

Additional Notes:

- Upon IMS registration, the SCC AS correlates the C-MSISDN and the IMSI

- The ATU-STI and STN-SR is assigned per node (STN-SR for the ATCF/SCC AS, ATU-STI for the SCC AS), while the STI is assigned by the SCC AS per session

- The STN-SR is used to indicate an ATCF if SRVC is enhanced with the use of the ATCF; otherwise it indicates an SCC AS

- STN-SR and C-MSISDN are provisioned in the HSS and provided to the MME via the HSS upon successful registration. The SCC AS updates the STN-SR in the HSS if it is different than what is currently at the HSS

- The MSC gets the STN-SR and C-MSISDN from the MME over the Sv interface for PS to CS access transfer request

- The STN-SR and the C-MSISDN are passed to the ATCF over the I1 interface