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5G Standalone Access Registration signaling Flow is similar to the 4G attach process and default bearer establishment flow.

5g standalone access registration complete flow

UE interactions

ue role in 5g standalone access

  • It performs the random access procedure to initiate communication with the gNB. 
  • Setup the RRC connection with the gNB. 
  • A signaling bearer is set up at this point. The UE also sends a Registration Request to the 5G Core. 
  • It performs the NAS (Non-Access Stratum) level Authentication and initiates ciphering for the NAS messages with the Core. 
  • Complete the AS (Access Stratum) security procedure with the gNB. 
  • It handles the RRC Reconfiguration from the gNB.
  • This message sets up the default PDU session. The message may add secondary cells.
  • The UE concludes the registration procedure. 
  • Data is flowing in the uplink and downlink directions. 

gNB interactions 

gNB interactions in 5g standalone
  • It handles the random access request from the UE and assigns resources for initiating the RRC connection. 
  • Setup the RRC connection with the UE. SRB1 is set up at this point. Starting at this point, the gNB starts assigning downlink and uplink resources to the UE via the PDCCH.
  • The gNB transports the Registration Request from the UE to the AMF. 
  • The gNB carries the NAS signaling between the UE and the gNB. 
  • The 5G Core initiates the default PDU session setup. A Registration Accept is also received from the UE.
  • The gNB goes on to enable security between the UE and the gNB.
  • Once the security setup is completed, the gNB activates the default PDU session via the RRC Reconfiguration message.
  • It also transports the Registration complete message to the AMF.
  • At this point, downlink and uplink data streams are flowing between the UE and the Internet. 

5G Core Interactions 

AMF - Access & Mobility Function 

5G core Access and Mobility Management Function receives all the control and session-related information from the user equipment. It is responsible for the handling of connection and mobility-related tasks. It is connected to the SMF with the N11 interface. A mobile network comprises many AMF instances and each AMF is uniquely identified by GUAMI (Globally unique AMF identifier). It implements NAS (Non-Access Stratum) Ciphering and integrity protection algorithms. It is responsible for Lawful intercept, Access Authentication, and Authorization. It also performs Security context management.

  • It handles the Initial UE message from the gNB. This message carries the Registration Request from the UE.
  • On getting the Registration Request, the new AMF obtains the UE context from the old AMF. 
  • The AMF then authenticates the UE after obtaining the keys from the AUSF. 
  • NAS level security is established between the UE and the AMF. 
  • The AMF also checks the 5G-EIR to verify that registration is not being attempted from a stolen device. 
  • The AMF then obtains subscription data from the UDM. 
  • The AMF then creates a new policy associated with the PCF. 
  • Then the AMF updates the SMF context and sends an Initial Context Setup Request to activate the default PDU session. The message also carries the Registration Accept message from the AMF. 
  • When the gNB signals that the Initial context setup has been completed, the AMF updates the SMF context. 

AUSF - Authentication Server Function 

It is an Authentication Server Function that performs the authentication of the user equipment. It is located in a home network. It makes the decision on UE Authentication for authenticating data and keying material like 5G AKA and EAP-AKA used. It is used to facilitate 5G security purposes.  

The AUSF participates in the Authentication procedure. It obtains the UE Authentication information from the UDM. 

AUSF - Authentication Server Function

UDM- Unified Data management 

It is a centralized way to control network usage data. It is cloud-native and designed for 5G technology. The functions of UDM are similar to HSS (Home Subscriber Service). It generates 5G AKA Authentication vectors, user identification handling, this function stores the long-term security credentials used in Authentication for AKA. It stores subscription information.

  • The UDM provides authentication vectors during the registration procedure. 
  • The new AMF serving the user also registers with the UDM. 
  • The UDM also provides Subscriber Data Management information to the new AMF. 

PCF - Policy Control Function

It supports the unified policy framework that governs the network behavior. It provides policy rules to enforce them. To facilitate this, the subscription information is gathered from the Unified data management function. It provides policy rules for control plane functions, which include network slicing, roaming, and mobility management. 

  • The New AMF creates a policy associated with the PCF.
  • The old AMF deletes the policy associated with the PCF.
  • The PCF registers with the AMF so that it can be notified of events like location change and communication failure. 

SMF - Session Management Function

It is the fundamental element of the 5G core network architecture which is responsible for interacting with decoupled data plane, creating, updating, and removing PDU (Protocol data units). It manages the session context with the User plane function. The functions performed by SMF include UE IP address allocation and management, termination of interfaces, downlink data notifications, charging data collection, lawful intercept, roaming functionality, etc.

  • The old AMF deregisters with the SMF.
  • The new AMF updates the session management context. The SMF assigns an IP address and the tunnel ID to be used for sending uplink data. 
  • The SMF selects the UPF to be used for the session. 
  • The SMF updates the UPF using PFCP messages via the N4 control-data plane interface. 
  • The new AMF also notifies the SMF when the session is ready for uplink and downlink data transfer. 

UPF - User Plane Function 

It is the fundamental component of a 3GPP 5G Core Architecture. It represents the data plane evolution of a control and user plane separation (CUPS). The functions of UPF include QoS handling for the user plane, packet routing, and forwarding, packet inspection, lawful intercept for the user plane, traffic accounting and reporting, etc.

  • The UPF is a data plane component that handles user data.
  • The UPF is completely controlled from the SMF using the N4 interface.
  • The SMF uses the Packet FLow control protocol (PFCP) to update the data plane. 
  • The first PFCP session Modification request prepares the UPF to receive uplink data. 
  • The UPF also starts buffering the downlink data. 
  • Once the default PDU session has been set up, the SMF updates the PFCP session to start sending downlink data. 

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