5G Standalone Access Registration

UE registers with the 5G Core Network (5GC) via the 5G-RAN



The transition to 5G will probably start with 4G-5G dual connectivity-based solutions. With dual connectivity, the 5G data plane is introduced in a largely 4G network [See Figure (a) below].

As 5G gains momentum, the networks will transition to an end-to-end 5G solution with UEs registering with the 5G Core Network. In standalone mode, the UE control plane, as well as data plane, is served by 5G-NR [See Figure (b) below].

4G-5G dual connectivity vs 5G standalone architecture

We have already discussed 4G-5G dual connectivity. Here we will focus on the 5G standalone access that takes advantage of the 5G service based architecture.

5G standalone access registration flow

The 5G standalone access registration signaling flow is similar to the 4G attach and default bearer establishment flow.

Click on the following image to open the PDF flow. Within the flow, click on individual messages and actions to see a detailed description.

5G standalone access registration — complete flow

The complete 5G standalone access flow is fairly complex. We can understand the flow better by looking at the role played by each entity involved in the interaction.

UE interactions

UE performs the following interactions during the standalone registration procedure:

  1. Perform the random access procedure to initiate communication with the gNB.
  2. 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.
  3. Perform NAS level authentication and initiate ciphering for the NAS messages with the 5G core.
  4. Complete the AS security procedure with the gNB.
  5. Handle the RRC Reconfiguration from the gNB. This message sets up the default PDU session. The message may add secondary cells.
  6. The UE concludes the registration procedure.
  7. Data is flowing in the downlink and uplink directions.

Click on the following image for the sequence diagram describing the UE role during a 5G registration.

5G Standalone Access — UE role

gNB interactions

The gNB acts as the conduit for communication between the UE and the 5G Core:

  1. Handle the random access request from the UE and assign resources for initiating the RRC connection.
  2. 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.
  3. The gNB transports the Registration Request from the UE to the AMF.
  4. The gNB carries the NAS signaling between the UE and the gNB. The gNB is just acting as a conduit for these messages. The NAS signaling authenticates the UE and establishes a ciphered link between the AMF and the UE.
  5. The 5G Core initiates the default PDU session setup. A Registration Accept is also received from the UE.
  6. The gNB goes on to enable security between the UE and the gNB.
  7. 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.
  8. At this point, downlink and uplink data streams are flowing between the UE and the Internet.

Click on the following image to see the eNB interactions in 5G registration.

5G Standalone Access — gNB

5G Core interactions

AMF — Access and Mobility Function

The AMF coordinates the 5G standalone registration procedure. The sequence of interactions is:

  1. Handle the Initial UE Message from the gNB. This message carries the Registration Request from the UE.
  2. On getting the Registration Request, the new AMF obtains the UE context from the old AMF.
  3. The AMF then authenticates the UE after obtaining the keys from the AUSF. NAS level security is established between the UE and the AMF.
  4. The AMF also checks the 5G-EIR to verify that registration is not being attempted from a stolen device.
  5. The AMF then obtains subscription data from the UDM.
  6. The AMF then creates a policy-association with the PCF.
  7. 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.
  8. When the gNB signals that the Initial Context setup has been completed, the AMF updates the SMF context.

Get an overview of the AMF interactions from the AMF context diagram. Learn about AMF interaction details from the AMF sequence diagram.

AMF — Access and Mobility Function Context Diagram
AMF — Access and Mobility Function Sequence Diagram

AUSF — Authentication Server Function

AUSF — Authentication Server Function Interactions

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

UDM — Unified Data Management

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

PCF — Policy Control Function

PCF — Policy Control Function Interactions
  • The new AMF creates a policy- association with the PCF. The old AMF deletes the policy- association with the PCF.
  • The PCF registers with the AMF so that it can be notified on events like location change and communication failure.

SMF — Session Management Function

The SMF serves as the control plane entity for session management. It is involved in the following interactions:

SMF — Session Management Function Interactions
  1. The Old AMF deregisters with the SMF.
  2. 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.
  3. The SMF selects the UPF to be used for the session.
  4. The SMF update the UPF using PFCP messages via the N4 control-data plane interface.
  5. The New AMF also notifies the SMF when the session is ready for uplink and downlink data transfer.

UPF — User Plane Function

UPF — User Plane Function Interactions

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.

5G standalone access message and action details

You can click on individual messages and actions in the 5G standalone access flow to see the details. The relevant 3GPP technical specification for 5G standalone access are also identified.