5.12 Mobile Terminating call procedures to unregistered Public User Identities

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

5.12.0 General

This clause describes information flows for the procedures of Mobile Terminating call flows for unregistered IMS Public User Identities. The detection of an unregistered Public User Identity is done in HSS and if this Public User Identity has services related to unregistered state, a S‑CSCF is selected for the unregistered Public User Identity. S‑CSCF performs whatever further actions are appropriate for the call attempt to the unregistered IMS Public User Identity.

Two basic examples for "services related to unregistered" are call redirection to CS domain and voice mailbox service. Call redirection to CS domain is supported to cover the cases when the UE is not registered in IMS but can be reached via the CS domain. Then, a temporary S‑CSCF is selected and performs whatever further actions are appropriate for the call attempt.

The principle established in clause 4.3.3.4, where the Public User Identities for the same profile are allocated to the same S‑CSCF, is followed.

5.12.1 Mobile Terminating call procedures to unregistered Public User Identity that has services related to unregistered state

In Figure 5.43 below the Public User Identity is unregistered for IMS and the Public User Identity has services related to unregistered state. In this case, the HSS responds back to I‑CSCF with an indication that I‑CSCF should select S‑CSCF for this MT call to the unregistered Public User Identity of the user or provide the I‑CSCF with the previously allocated S‑CSCF name. Before S‑CSCF selection, I‑CSCF shall query HSS for the information related to the required S‑CSCF capabilities. I‑CSCF selects a S‑CSCF to invoke service logic and I‑CSCF routes the call further to the selected destination. If the S‑CSCF does not have the relevant information from the user profile then the S‑CSCF shall download the relevant information from HSS before it invokes service logic and any further actions in the call attempt. The service implemented by this information flow could be e.g. "Call Forward Unconditional".

This is shown by the information flow in Figure 5.43:

Figure 5.43: Mobile Terminating call procedures to unregistered IMS Public User Identity that has services related to unregistered state

1. I‑CSCF receives an INVITE message.

2. I‑CSCF queries the HSS for current location information.

3. HSS either responds with the required S‑CSCF capabilities which I‑CSCF should use as an input to select a S‑CSCF for the unregistered Public User Identity of the user or provides the I‑CSCF with the previously allocated S‑CSCF name for that user.

4. If the I‑CSCF has not been provided with the location of the S‑CSCF, the I‑CSCF selects an S‑CSCF for the unregistered Public User Identity of the user.

5. I‑CSCF forwards the INVITE request to the S‑CSCF.

6. The S‑CSCF sends Cx-Put/Cx-Pull (Public User Identity, S‑CSCF name) to the HSS. When multiple and separately addressable HSSs have been deployed by the network operator, then the S‑CSCF needs to query the SLF to resolve the HSS. The HSS stores the S‑CSCF name for unregistered Public User Identities of that user. This will result in all terminating traffic for unregistered Public User Identities of that user being routed to this particular S‑CSCF until the registration period expires or the user attaches the Public User Identity to the network. Note: Optionally the S‑CSCF can omit the Cx-Put/Cx-Pull request if it has the relevant information from the user profile.

7. The HSS shall stores the S‑CSCF name for that user and return the information flow Cx-Put Resp/Cx-Pull Resp (user information) to the S‑CSCF. The S‑CSCF shall store it for that indicated Public User Identity.

8. S‑CSCF invokes whatever service logic is appropriate for this call attempt.

9. S‑CSCF performs whatever further actions are appropriate for this call attempt (in the case where the S‑CSCF decides to redirect the session towards CS domain, the Mobile Termination Procedure MT#3 (clause 5.7.2a) applies).

The S‑CSCF may deregister the Public User Identity at any time (e.g. according to operator network engineering requirements) by issuing a Cx-Put2 (Public User Identity, clear S‑CSCF name) clearing the S‑CSCF name stored in the HSS. If S‑CSCF name stored by the HSS does not match the name of the S‑CSCF that originated the Cx-Put2 then the HSS will acknowledge the clearing request but take no further action.

5.12.2 Mobile Terminating call procedures to unregistered Public User Identity that has no services related to unregistered state

In the example information flow the Public User Identity of the user is unregistered and the Public User Identity has no services related to unregistered state.

This is shown in the following information flow (figure 5.44):

Figure 5.44: Mobile Terminating call procedures to unregistered Public User Identity that has no services related to unregistered state

1. I‑CSCF receives an INVITE message.

2. I‑CSCF queries the HSS for current location information.

3. HSS responds with an indication that the Public User Identity is unregistered, but no services are related to unregistered state.

4. I‑CSCF responds to the origin of the request that the user is not reachable at the moment.