How it Works
This section describes how this feature works.
The SMS over NAS feature supports the following procedures:
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Registration procedures for SMS over NAS—This procedure involves the following steps:
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During registration, if the UE requests for SMS support and the feature are enabled at AMF, the AMF fetches the SMS subscription data and UE context in the SMSF data along with the AM and SMF selection data. The AMF also subscribes to the UDM notifications.
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If the UE has the SMS subscriptions enabled, the AMF sends the Activate Request to the SMSF.
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The AMF supports the target PLMN and instance ID-based SMSF selection based on the NRF discovery. The instance ID received from the UDM is preferred over the instance ID received from the peer AMF.
Depending on these steps, the AMF notifies the SMS status as allowed or not allowed to the UE as part of the Registration Accept message.
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Deregistration procedures for SMS over NAS—The AMF triggers the SMS Deactivation Request towards SMSF during the following scenarios:
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UE-initiated deregistration
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Network-initiated deregistration
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The SMS was activated in the previous Registration Requests and the UE did not request for the SMS support in the subsequent Registration Request.
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Whenever the SMS state at AMF changes from allowed to not allowed, and if the SMS was previously activated, the AMF sends the Deactivation Request to SMSF.
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MO SMS over NAS in CM-IDLE or CM-CONNECTED
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MT SMS over NAS in CM-IDLE or CM-CONNECTED state through 3GPP access—This procedure involves the following steps:
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As part of MT SMS, the AMF supports handling of the EnableReachability Requests from the SMSF.
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If the UE is in the CONNECTED state, the AMF immediately responds with the UE as REACHABLE.
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If the UE is in the IDLE state and the PPF flag is set, the AMF triggers the paging procedure and updates the SMSF based on the paging response.
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