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NR Network Architecture / Network Interface - N1 / N1-Mode

 

N1 interface(Reference Point) indicates the connection between UE and AMF mainly for NAS layer signaling, but the exact role of N1 interface (reference point) is a little bit ambiguous to me.

 

 

 

What is N1 Interface ?

 

According to 23.501-Figure 4.2.4-4, N1 reference point (labeled as '1') is indicated as follows. It seems obvious that N1 is the interface between UE and AMF, but there is no access network along N1. So I was wondering how UE can get access to AMF without access network.

 

 

 

With further looking at other parts of the document (like figures in section 23.501-4.2.8.2 and description at 23.501-5.3.3.1), it seems that N1 interface represent the interface as follows. At least for now, my understanding is as follows.

  • N1 is the interface between UE and AMF
  • N1 represents the combined path  UE <--> Access Network and Access Network <--> AMF
  • The Access Network can be a 3GPP based (e.g, gNB) or non 3GPP based.
  • Most of NAS signaling is going through N1 

 

There is another statement from 24.501-3.1 writing the definition of N1 NAS signalling connection as follows.

    N1 NAS signalling connection: A peer to peer N1 mode connection between UE and AMF. An N1 NAS signalling connection is either the concatenation of an RRC connection via the Uu reference point and an NG connection via the N2 reference point for 3GPP access, or the concatenation of an IPsec tunnel via the NWu reference point and an NG connection via the N2 reference point for non-3GPP access.

 

 

 

 

What is N1 mode ?

 

In addition to N1 reference point/interface with the focus on physical connections in the overal network architecture, there is another term called N1 mode which is more focused on functional point of view (especially with the perspective of NAS signaling).

 

The formal definition of N1 mode is defined in 24.501-3.1 as follows

    N1 mode: A mode of a UE allowing access to the 5G core network via the 5G access network.

 

 

UE Usage state and N1 disable/enable

 

Following table is based on 24.501 - 4.3.3.1, 4.3.4.1 and 4.9. I changed the format for better / quictly to be used as a cheat sheet -:)

UE Usage Setting

Network Capability

Registration Status

N1 mode status

Power Cycle (Off --> On)     Re-enable the N1 mode
USIM Removal     Re-enable the N1 mode
Disable ->Timer Expiration(*1)     Re-enable the N1 mode

From "data centric" to "voice centric"

"IMS voice not available" over 3GPP access only

the UE is only registered over 3GPP access

Disable the N1 mode capability for 3GPP access

From "data centric" to "voice centric"

"IMS voice not available" over both 3GPP access and non-3GPP access

the UE is registered over both 3GPP access and non-3GPP access

Disable the N1 mode capability for 3GPP access  and non-3GPP access

From "data centric" to "voice centric"

"IMS voice not available" over both 3GPP access and non-3GPP access

the UE is registered over 3GPP access only

Disable the N1 mode capability for 3GPP access

From "data centric" to "voice centric"

"IMS voice not available" over both 3GPP access and non-3GPP access

the UE is registered over non-3GPP access only.

Disable the N1 mode capability for non-3GPP access

From "voice centric" to "data centric"

N1 mode capability for 3GPP access is disabled at the UE (due to IMS voice unavailability)

 

Re-enable the N1 mode capability for 3GPP access

From "data centric" to "voice centric"

"IMS voice not available" over non-3GPP access only

the UE is only registered over non-3GPP access

Disable the N1 mode capability for non-3GPP access

From "voice centric" to "data centric"

N1 mode capability for non 3GPP access is disabled at the UE (due to IMS voice unavailability)

 

Re-enable the N1 mode capability for non-3GPP access

"voice centric"

"IMS voice not available" over 3GPP access only

the UE is only registered over 3GPP access

Disable the N1 mode capability for 3GPP access

"voice centric"

"IMS voice not available" over non-3GPP access only

 

Disable the N1 mode capability for non-3GPP access

"voice centric"

"IMS voice not available" over both 3GPP access and non-3GPP access

UE is registered over both 3GPP access and non-3GPP access.

Disable the N1 mode capability for 3GPP access and non-3GPP access

"voice centric"

"IMS voice not available" over both 3GPP access and non-3GPP access

UE is registered over 3GPP access only

Disable the N1 mode capability for 3GPP access

"voice centric"

"IMS voice not available" over both 3GPP access and non-3GPP access

UE is registered over non-3GPP access only.

Disable the N1 mode capability for non-3GPP access

 

NOTE 1 > This timer based re-enable is suggested as an implementation option in 24.501 - 4.9.1

 

 

 

3GPP Statements on N1 interface/refernce point

 

For futher clarification and for those who would not agree with my interpretation -:), I quoted various descriptions directly from 3GPP document here.

 

Regarding the role N1 reference points, it is described as follows in 3GPP.

 

23.501 - 4.2.8.1 General Concepts to Support Non-3GPP Access

  • A UE that accesses the 5G Core Network over a standalone non-3GPP access shall, after UE attachment, support NASsignalling with 5G Core Network control-plane functions using the N1 reference point.
  • When a UE is connected via a NG-RAN and via a standalone non-3GPP access, multiple N1 instances shall exist for the UE i.e. there shall be one N1 instance over NG-RAN and one N1 instance over non-3GPP access.
  • N1 NAS signalling over standalone non-3GPP accesses shall be protected with the same security mechanism applied for N1 over a 3GPP access.

23.501 - 4.4.2.2 Reference point to support SMS over NAS

  • N1: Reference point for SMS transfer between UE and AMF via NAS.

4.4.4.2 Reference point to support Location Services

  • N1: Reference point between UE and AMF via NAS.

23.501 - 5.2.5 Access control and barring

  • If the UE supports both N1 and S1 modes NAS and, as defined in TS 23.401 [26], the UE is configured for Extended Access Barring (EAB) but is not configured with a permission for overriding Extended Access Barring (EAB), when the UE wants to access the 5GS it shall perform Unified Access Control checks for Access Category 1 on receiving an indication from the upper layers as defined in TS 24.501, TS 38.331, TS 36.331.
  • If the UE supports both N1 and S1 modes NAS and, as defined in TS 23.401 [26], the UE is configured with a permission for overriding Extended Access Barring (EAB), when the UE wants to access the 5GS it shall ignore Unified Access Control checks for Access Category 1 on receiving an indication from the upper layers, as defined in TS 24.501

23.501 - 5.3.3.1 General

  • Connection management comprises the functions of establishing and releasing a NAS signalling connection between a UE and the AMF over N1. This NAS signalling connection is used to enable NAS signalling exchange between the UE and the core network. It comprises both the AN signalling connection between the UE and the AN (RRC Connection over 3GPP access or UE-N3IWF connection over N3GPP access) and the N2 connection for this UE between the AN and the AMF.

23.501 - 5.3.3.2.3 CM-CONNECTED state

  • A UE in CM-CONNECTED state has a NAS signalling connection with the AMF over N1. A NAS signalling connection uses an RRC Connection between the UE and the NG-RAN and an NGAP UE association between the AN and the AMF for 3GPP access.

23.501 - 5.6 Session Management

  • PDU Sessions are established (upon UE request), modified (upon UE and 5GC request) and released (upon UE and 5GC request) using NAS SM signalling exchanged over N1 between the UE and the SMF.

23.501 - 5.6.2 Interaction between AMF and SMF

  • The single N1 termination point is located in AMF. The AMF forwards SM related NAS information to the SMF based on the PDU Session ID in the NAS message. Further SM NAS exchanges (e.g. SM NAS message responses) for N1 NAS signalling received by the AMF over an access (e.g. 3GPP access or non-3GPP access) are transported over the same access.
  • The serving PLMN ensures that subsequent SM NAS exchanges (e.g. SM NAS message responses) for N1 NASsignalling received by the AMF over an access (e.g. 3GPP access or non-3GPP access) are transported over the same access.
  • SMF handles the Session management part of NAS signalling exchanged with the UE.
  • The UE shall only initiate PDU Session Establishment in RM-REGISTERED state.
  • When a SMF has been selected to serve a specific PDU Session, AMF has to ensure that all NAS signalling related with this PDU Session is handled by the same SMF instance.
  • Upon successful PDU Session Establishment, the AMF and SMF stores the Access Type that the PDU Session is associated.

 

23.501 - 8.2.2 Control Plane Protocol Stacks between the UE and the 5GC

  • A single N1 NAS signalling connection is used for each access to which the UE is connected. The single N1 termination point is located in AMF. The single N1 NAS signalling connection is used for both Registration Management and Connection Management (RM/CM) and for SM-related messages and procedures for a UE.
  • The NAS protocol on N1 comprises a NAS-MM and a NAS-SM components.
  • There are multiple cases of protocols between the UE and a core network function (excluding the AMF) that need to be transported over N1 via NAS-MM protocol. Such cases include:
    • Session Management Signalling.
    • SMS.
    • UE Policy.
    • LCS.

 

 

 

Reference

 

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