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The N2 interface (control interface and protocol stack)

$5G Access,Network \stackrel{N2}{\longleftrightarrow} 5GC(AMF)$

  • Management procedures: not related to a specific UE, like setting up or resetting the N2 interface
  • Procedures related to a single UE: NAS transport procedures, UE context management, PDU-session resource-related procedures, handover management, etc.
  • Uses the NG-AP protocol (regardless of access network type: 3GPP RAN or N3IWF non-3GPP), which is an application-layer protocol
  • The AMF has a unique N2 termination point for a UE
  • Since the SMF and AMF are decoupled, NG-AP supports the AMF relaying N2 SM messages between the 5G RAN and 5GC

  • SCTP guarantees transport between the Access Network and 5GC

The control-plane interface and protocol stack between the UE and 5GC

For each connection method a UE uses (3GPP, non-3GPP), the UE has an N1 NAS connection, with a unique N1 termination point located in the AMF.

Non-access stratum (NAS)
A protocol layer that originally existed in 3G/UMTS, a functional layer between the user equipment and the core network.
It has the following functions:

mobility management
call control
session management
identity management
NAS reference

N1 NAS can be used for:

  • UE registration management
  • connection management
  • session management

The NAS protocol within the N1 interface splits into: NAS-MM, NAS-SM

NAS-MM

Used for the UE to interact with NFs other than the AMF

  • session-management messages
  • UE policy
  • location services

NAS transports session-management messages, SMS, UE policy, location services, and other protocol procedures

Features

Handles the NAS procedures between the UE and AMF, including:

  • handling the UE’s registration management (RM) and connection management (CM): providing a secure message link and access control between the UE and AMF
  • applicable to other types of NAS messages (NAS-SM, SMS, etc.), which can be transmitted simultaneously with RM/CM NAS messages
  • the same NAS protocol is used for different access methods (3GPP, non-3GPP)
  • the UE has one N1 NAS connection for each access method

NAS-SM

Supports handling session management between the UE and SMF

  • establishment, modification, and release of the PDU session’s user plane
  • SM messages are established and processed within NAS-SM (the content isn’t parsed by the AMF)
  • the NAS-MM layer handles sending/receiving SM messages between the UE and AMF, and provides security protection (protecting message integrity)

The control-plane interface and protocol stack between 5GC NFs

In the 5G core, an NF Service Producer provides services to an NF Service Consumer via a service-based interface.

The 5G core defines the following control interfaces as service-based interfaces:

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- Nsmf
- Nudm
- Nnrf
- Nnssf
- Nausf
- Nnef
- Nsmsf
- Nudr
- Npcf
- N5g-eir
- Nlmf

These service-based interfaces use HTTP 2.0 and JSON as application-layer protocols; all 3GPP network functions must support TLS.
If the PLMN doesn’t provide another network-security solution, TLS must be used.
In the future, QUIC/UDP may be used as an optimized solution.

The N4 interface and protocol stack

$SMF\stackrel{N4}{\longleftrightarrow}UPF$

N4 sits between the SMF and UPF, with control-plane and user-plane functions.
N4 defines the following procedures:

Control-plane procedures

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- N4 session management:
	for the SMF to establish, update, and delete N4 session context on the UPF
- N4 reporting procedures:
	for the UPF to report events (e.g. traffic usage) to the SMF
- N4 node-level procedures:
	establishment, update, association, and release of the N4 node between SMF and UPF; the UPF reports N4 node-level events to the SMF
- SMF charging-suspend procedures:

User-plane procedures
Forwarding traffic between the SMF and UPF:
When the user is in the CM-IDLE state, the UPF can forward downlink data to the SMF, which then decides the paging policy.

PFCP $\longleftrightarrow$ PFCP
UDF $\longleftrightarrow$ UPF
IP $\longleftrightarrow$ IP
L2 $\longleftrightarrow$ L2
L1 $\longleftrightarrow$ L1
Control Plane $\longleftrightarrow$ User Plane

The user-plane interface and its protocol stack

$Access,Network\stackrel{N3}{\longleftrightarrow}UPF$
$UPF \stackrel{N9}{\longleftrightarrow}UPF$
The N3 and N9 interfaces carry data between the 5G RAN and UPF, and between UPF and UPF, respectively.