5G SBI and AMF Service Operations [Part1]

In the Service Based Architecture (SBA), an NF provides multiple services for other NFs through service-based interfaces (SBIs). Multiple service operations are defined for each service. as we see above, every SBI has its own 3GPP reference to describe related service, service operations, and data structure to be considered and implemented by Telco vendors and service providers.

A service operation request or response message carries a structured object that contains one or more IEs. The value of an IE depends on the data type of the IE. For example, an IE of the structured data type may contain one or more sub-IEs.

I have tried to use Namf as an example in the below figure to illustrate the relationships between services, service operations, messages, and IEs.

The following describes the services provided by the Namf interface and the service operations supported by each service.

Namf_Communication Service

Below service operations are supported by Namf_Communication service:

• UEContextTransfer: Used in a registration procedure. When the target AMF receives a Registration Request message and the UE's 5G-GUTI in this message contains the source AMF information, the target AMF invokes this service operation of the source AMF to obtain UE contexts, such as the UE SUPI and MM contexts.
• RegistrationStatusUpdate: Used in a registration procedure. The target AMF invokes this service operation of the source AMF to update a UE's registration status on the target AMF towards the source AMF. This status reflects previous UE context transfer results for the specified UE.
• CreateUEContext: Used in an inter-NG-RAN N2-based handover procedure. When the source AMF cannot serve a UE anymore, it invokes this service operation of the target AMF that has been selected during a handover. This creates UE contexts on the target AMF.
• ReleaseUEContext: Used in an inter-NG-RAN N2-based handover cancellation procedure. When the source AMF receives a Handover Cancel message from the (R)AN during a handover, the source AMF invokes this service operation of the target AMF to release UE contexts on the target AMF.
• N1N2MessageSubscribe: Used in a UE configuration update procedure. An NF service consumer (such as the LMF or PCF) invokes this service operation of the AMF to subscribe to notifications of a specific N1 message type (such as LPP or UPDP) or N2 information type (such as NRPPa).
• N1N2MessageUnSubscribe: An NF service consumer (such as LMF or PCF) invokes this service operation to instruct the AMF to stop notifying of N1 or N2 messages of a specific type (such as LPP or UPDP).
• N1MessageNotify: Used in registration with AMF re-allocation procedure or a UE configuration update procedure. In registration with AMF reallocation procedure, the initial AMF obtains URI information of the target AMF by using the network repository function (NRF), and then invokes its own Namf_Communication_N1MessageNotify service operation to provide the target AMF with information, such as RAN NGAP ID, initial AMF name, RAN ID, N1 message, UE SUPI, and MM context, and allowed NSSAI. In a UE configuration update procedure, when the AMF receives a UPDP message from a UE, the AMF invokes its own Namf_Communication_N1MessageNotify service operation to notify the policy control function (PCF) of the UPDP message, as the PCF has subscribed to N1 messages from the AMF.
• N2InfoNotify: Used in an inter-NG-RAN N2-based handover procedure or a network-assisted positioning procedure. Inter-NG-RAN N2-based handover procedure, target AMF invokes the Namf_Communication_N2InfoNotify service operation to notify the source AMF that a UE has been handed over to the target side. Network-assisted positioning procedure when the AMF receives an NRPP message from the 5G-AN, the AMF invokes the Namf_Communication_N2InfoNotify service operation to notify the LMF of the received location information.
• N1N2MessageTransfer: Used for different procedures, such as PDU session establishment, modification, and release; network-triggered service request; inter-NG-RAN node N2-based handover; and UE configuration update. During PDU session establishment, the SMF invokes this service operation of the AMF to send N1 session management information (such as the UE IP address and QoS rules) and N2 session management information (such as the CN tunnel information and QoS profile) to UEs and the (R)AN through the AMF.
• N1N2TransferFailureNotification: Used for different procedures, such as PDU session establishment, modification, and release; network-triggered service request; inter-NG-RAN node N2-based handover; and UE configuration update. The AMF uses this service operation to inform an NF service consumer (such as an SMF) that it fails to send an N1 message to a UE because UE paging fails in the previously initiated Namf_Communication_N1N2MessageTransfer procedure.
• AMFStatusChangeSubscribe: Used in an AMF-planned removal procedure.
• AMFStatusChangeUnSubscribe: Used in an AMF-planned removal procedure.
• AMFStatusChangeNotify: Used in an AMF-planned removal procedure. This service operation is used to notify each NF service consumer that has subscribed to receive notifications of AMF status changes (for example, the AMF becomes unavailable).
• EBIAssignment: Used in a UE-requested PDU session establishment procedure or a UE- or network-requested PDU session modification procedure. An NF service consumer (such as the SMF) invokes this service operation towards an NF service producer (such as the AMF) to request the AMF to allocate EPS bearer IDs towards EPS bearers mapped from QoS flows for an existing PDU session of a given UE.
• NonUeN2MessageTransfer: Used by an NF service consumer to transfer N2 information to the 5G-AN through the AMF in the following procedures:

1. Non-UE associated network assistance data obtaining
2. Warning request transfer procedure
3. Configuration transfer procedure
4. During configuration transfer, the source AMF invokes this service operation of the target AMF to transfer the RAN configuration information from the source NG-RAN to the target NG-RAN through the target AMF.

• NonUeN2InfoSubscribe: Used by an NF service consumer (such as the LMF, CBCF, or PWS-IWF) to subscribe to the AMF for notifying non-UE-specific N2 information of a specific type (such as NRPPa or PWS indication).
• NonUeN2InfoUnSubscribe: Used by an NF service consumer (such as the LMF) to unsubscribe to the AMF to stop notifying N2 information of a specific type (such as NRPPa).
• NonUeN2InfoNotify: Use to obtain non-UE associated network assistance data and receive PWS events from the NG-RAN. The AMF invokes this service operation to notify an NF service consumer that it has received non-UE N2 information from the 5G-AN.

Namf_EventExposure Service

Below service operations are supported by Namf_EventExposure service:

• Subscribe: Used by an NF service consumer (such as the NEF) towards the AMF to subscribe to an event for a specific UE, a group of UEs, or any UE.
• Unsubscribe: Used by an NF service consumer (such as the NEF) towards the AMF to remove an existing subscription previously created by itself on the AMF.
• Notify: Used by the AMF to send a notification to an NF service consumer if certain events included in the subscription have taken place.

Namf_MT Service:

Below service operations are supported by Namf_MT service:

• EnableUEReachability: Used in an MT SMS over NAS procedure in the idle or connected state. An NF service consumer (such as the SMSF) invokes the Namf_MT_EnableUEReachability service operation to enable the reachability of a UE.
• ProvideDomainSelectionInfo: Used by an NF service consumer (such as the UDM) to obtain the following UE information for terminating domain selection of IMS voice services.

1. Indication of supporting IMS voice over PS sessions
2. Timestamp of the last radio contact with the UE
3. Current access type and RAT type

Namf_Location Service

Below service operations are supported by Namf_Location service:

• ProvidePositioningInfo: Used in the following procedures:

1. 5GC-MT-LR procedure
2. 5GC-MT-LR procedure without UDM query
3. Location continuity for handover of an emergency session from NG-RAN
4. This service operation is invoked by an NF service consumer (such as the GMLC) to request the current geodetic and optionally civic location of a UE, and triggers the AMF to invoke services towards the LMF.

• EventNotify: This service operation is used to notify an NF service consumer (such as the GMLC) of UE location events related to emergency sessions in any of the following scenarios.

1. An emergency session is initiated.
2. A handover procedure takes place during an emergency session.
3. An emergency session is released.

Used in the following procedures:

1. 5GC-NI-LR procedure
2. Location continuity for handover of an emergency session from NG-RAN

• ProvideLocationInfo: Used by an NF service consumer (such as the UDM) to request the network-provided location information (NPLI) of a target UE.

Summary

to summarize what we discussed in this article, I have tried to merge all Namf Service Operations, their NF consumers, use cases, and related 3GPP procedure/messages in the below table.

Please try the below link for full quality:

AMF Service Operations

AMF Service Operations .pdf

AMF Service Operations

AMF Service Operations .pdf

Reference:

• 3GPP TS 23.501
• 3GPP TS 23.502
• 3GPP TS 29.518
• 3GPP TS 23.273
• 3GPP TS 23.041

I hope this article would be helpful to all Packet Core experts. please contact me for any question or comment regarding this article.