5G Standalone Access Registration

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

Nov 17, 2018 · 6 min read

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.

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.

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.

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.

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.

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.

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.


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