Main principles for 5G Architecture
- Discrete UP functions from CP functions.
- Enable supple and efficient network slicing.
- Define procedures as services, so that their reuse is possible.
- Authorize each NF to interact with other NF directly .
- Minimize dependencies between Access network and core network.
- Assist a unified authentication framework.
- Assist capability exposure.
- Assist roaming
Architecture reference model
The 5G architecture is defined as service based and the interaction between NFs is defined in two ways as :
- A service based representation
- A reference point representation
Network functions and entities
AUSF,AMF, DN, UDSF, NEF, NRF, NSSF, PCF, SMF, UDM, UDR, UPF, AP, UE, RAN, 5G-EIR, SEPP, NWDAF
Non-roaming reference architecture
Non roaming architecture for UE’s concurrently examining two data networks using multiple PDU sessions. This architecture is for PDU sessions where two SMFs are selected for two distinct PDU sessions. Each SMF may have the ability to manage both a local and a central UPF within a PDU session.
Roaming reference architecture
Data storage architectures
The 5G system architecture permits any NF to retrieve its unstructured data from a UDSF. The 5G system architecture permits the UDM, PCF and NEF to save data in the UDR, comprising subscription data and policy data by UDM and PCF, structured data for exposure and application data for application detection, AF request information for multiple UE’s by the NEF.
UDR supported by the NEF belongs to the same PLMN where the NEF is located. UDR supported by the UDM belongs to the same PLMN where the UDM is located if UDM supports a split architecture. UDR supported by the PCF belongs to the same PLMN where the PCF is located.
The Nudr interface is defined for the network functions such as UDM,PCF and NEF to support a particular set of data saved and to read , update , cut and subscribe to notification of relevant data changes in the UDR. Data in the UDR sets exposed via Nudr to the respective NF service consumer and stored shall be standardized : subscription data, policy data , structured data for exposure, application data : packet flow descriptions for application detection and AF request information for many UE’s. The service based Nudr interface defines the content and format of 3GPP defined information elements exposed by data sets.
Service based interfaces
N1 , N2, N3, N4,N6 ,N9,N5 ,N7, N8, N10, N11, N12, N13, N14, N15 , N16 , N17 , N18 , N22, N23, N24, N27, N31 , N32, N33, N34, N35, N36, N37, N40, N50
Support of non-3GPP access
Non - 3GPP access networks shall be attached to the 5G core network via non-3GPP interworking function ( N31WF). The N31WF interfaces the 5G core network CP and UP functions via N2 and N3 interfaces. The N2 and N3 reference points are used to attach standalone non-3GPP accesses to the 5G core network control plane. A UE that examines the 5G core network over a standalone non-3GPP access shall, after UE attachment , assist NAS signalling with 5G core network control plane functions using the N1 reference point. The PLMN choice for the 3GPP support does not depend on the N31WF selection. A UE found an IPsec tunnel with the N31WF to connect to the 5G core network over untrusted non-3GPP access.
Interworking with EPC
Registration & connection management
The registration management is used to register or deregister a UE with the network, and find the user context in the network. The connection management is used to find and release the signalling connection between UE and the AMF.
Two RM states are utilized in the UE and the AMF:
In the RM‑DEREGISTERED state, the UE is not registered with the network. The UE context in AMF handles no valid location information for the UE so the UE is not reachable by the AMF. Some parts of UE context may still be saved in the UE and the AMF.
In the RM‑REGISTERED state, the UE is registered with the network. In the RM-REGISTERED state, the UE can receive services that need registration with the network.
5GS registration management state models
RM State model in UE
RM state model in AMF
Registration Area management
Registration Area management includes the functions to assign and reassign a Registration area to a UE. Registration area is controlled per access type i.e., 3GPP access or Non-3GPP access.
When a UE registers with the network over the 3GPP access, the AMF assigns a set of tracking locations in TAI List to the UE. When the AMF assigns a registration area, i.e. the set of tracking locations in TAI List, to the UE it may take into account various information. The 5G System assists assigning a Registration Area using a single TAI List which comprises tracking locations of any NG-RAN nodes in the Registration Area for a UE. When a UE registers with the network over the Non-3GPP access, the AMF assigns a registration area that only comprises the N3GPP TAI to the UE.
Connection management includes the functions of founding and releasing a NAS signalling connection between a UE and the AMF over N1. This NAS signalling connection is used to authorize NAS signalling exchange between the UE and the core network. It includes both the AN signalling connection between the UE and the AN and the N2 connection for this UE between the AN and the AMF.
Two CM states are utilized to reflect the NAS signalling Connection of the UE with the AMF:
The CM state for 3GPP access and Non-3GPP access are liberated of each other.
A UE in CM-IDLE state has no NAS signalling connection founded with the AMF over N1. There are no AN signalling connections , N2 connection and N3 connections for the UE in the CM-IDLE state. The AMF is capable of retrieving stored information required for initiating communication with the UE using 5G-GUTI.
A UE in CM-CONNECTED state has a NAS signalling connection with the AMF over N1. A NAS signalling connection utilizes an RRC Connection between the UE and the NG-RAN and an NGAP UE association between the AN and the AMF for 3GPP access.
5GS Connection management state models
NAS signalling connection management
NAS signalling connection management comprises the functions of initiating and liberating a NAS signalling connection.
NAS signalling connection establishment function is offered by the UE and the AMF to initiate a NAS signalling connection for a UE in CM-IDLE state. The AMF offers the list of suggested cells, NG-RAN node identifiers for paging.
When the UE in CM-IDLE state requires to transfer an NAS message, the UE initiates a Service Request, a Registration or a Deregistration procedure to initiate a NAS signalling connection to the AMF as specified in TS 23.502. If the NAS signalling connection is to be initiated via an NG-RAN node, but the AMF finds that this UE has already initiated a NAS signalling connection, the AMF shall release the old initiation NAS signalling connection by triggering AN Release Procedure.
Based on UE subscription, Mobility Pattern and network design, the AMF may keep the NAS signalling connection until the UE de-registers from the network./
Mobility Restrictions restrict mobility handling or service access of a UE. The Mobility Restriction functionality is provided by the UE (only for mobility restriction categories provided to the UE), the radio access network and the core network.
Unless otherwise stated, Mobility Restrictions only apply to 3GPP access and wireline access, they do not apply to other non-3GPP accesses. Service Area restrictions and handling of Forbidden Areas for CM-IDLE state and, for CM-CONNECTED state when in RRC Inactive state are executed by the UE based on information received from the core network. Mobility Restrictions for CM-CONNECTED state when in RRC-Connected state are executed by the radio access network and the core network. In CM-CONNECTED state, the core network provides Mobility Restrictions to the radio access network within Mobility Restriction List.
Core network assistance information for RAN optimization
Core network assistance information for RAN aids the RAN to optimize the uE state transition steering and the RAN paging strategy formulation in RRC inactive state.
If the expected UE behaviour parameters of the uE is available, the AMF may use this information for selecting the CN assisted RAN parameter values.
The expected UE behaviour parameters can be provisioned by external party via NEF.
- Expected UE activity behavior
- Expected HO behavior
- Expected UE mobility
- Expected UE moving trajectory
The 5GC supports a PDU connectivity service i.e a service that provides exchange of PDUs between a UE and a data network identified by a DNN.
The PDU connectivity service is supported via PDU sessions that are established upon request from the UE.
The subscription information for each S-NSSAI may contain a subscribed DNN list and one default DNN.
If the DNN provided by the UE is not supported by the network and ANF cannot select an SMF by querying NRF, the AMF shall reject the NAS message containing PDU session establishment request from the UE with a cause that the DNN is not supported.
PDU session types are defined : IPv4, IPV6, IPv4v6, ethernet, unstructured.
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 UE and SMF.