N1N2 Message Transfer

Feature Summary and Revision History

Summary Data

Table 1. Summary Data

Applicable Product(s) or Functional Area

AMF

Applicable Platform(s)

SMI

Feature Default Setting

Enabled - Always-on

SMS over the Non-Access Stratum Procedures: Enabled - Configuration required to disable

Related Documentation

Not Applicable

Revision History

Table 2. Revision History

Revision Details

Release

Introduced ability to send SMS over the NAS procedure

2022.01.0

First introduced.

2021.04.0

Feature Description

The NF service consumer uses the N1N2MessageTransfer service operation to transfer N1 or N2 information, or both to the UE or 5G-AN, or both.

AMF now supports the following procedures:

  • Network triggered Service Request

  • PDU Session Establishment

  • PDU Session Modification

  • PDU Session Release

  • Session continuity, service continuity, and UP path management

  • Inter NG-RAN node N2 based handover

  • SMS over NAS

  • UE assisted and UE-based positioning

  • Network assisted positioning

  • UE Configuration Update for transparent UE policy delivery


Note

AMF only supports SM messages.

How it Works

This section describes how this feature works.

Call Flows

This section describes the key call flows for this feature.

N1N2 Message Transfer Request Call Flow

This section describes the N1N2 Message Transfer Request call flow.

Figure 1. N1N2 Message Transfer Request Call Flow
Table 3. N1N2 Message Transfer Call Flow Description

Step

Description

1

The peer node sends an N1N2MessageTransfer Request Call Flow message to the AMF.

2

AMF checks if the message is acceptable. If there’s an exception, the AMF rejects the message with an appropriate cause code.

3

If the UE is in ECM_CONNECTED state, AMF forwards the message to the UE or gNB. The N2 message received from the peer node determines the N2 message type. If there’s a N1 message, it’s sent as a payload to the N2 message. AMF then responds with a 200 OK to the peer node.

4

If the UE is in ECM_IDLE state and the Asynchronous Transfer flag is set, AMF stores the message in a known location in CDL. AMF adds the location header to the response and a 202 response is sent with WAITING_FOR_ASYNCHRONOUS_TRANSFER as a diagnostic. The saved message is sent to the UE as the UE transitions to ECM_CONNECTED. The AMF doesn’t page the UE in this case.

5

If the UE is in ECM_IDLE state and the SkipInd flag is set in the received N1N2TransferReq message, AMF skips sending the N1 message to UE. AMF sends a 200 OK response with N1_MSG_NOT_TRANSFERRED as a diagnostic. The message isn’t sent to the UE as the UE transitions to ECM_CONNECTED and paging isn’t done in this scenario.

6

If the UE is in ECM_IDLE and the Asynchronous Transfer flag isn’t set, AMF stores the message in a known location. AMF adds the location header to the response and a 202 response is sent with ATTEMPTING_TO_REACH_UE as a diagnostic. The saved message is sent to the UE as the UE transitions to ECM_CONNECTED.

If paging fails, AMF sends a Failure Notification to the peer node.