5G non-standalone access

Signaling flow for 5G access via LTE-5G NR dual connectivity (EN-DC)

Transition to 5G is being accelerated by enabling support for 5G bearers in the existing 4G-LTE infrastructure.

5G mobile broadband services will be available to users in a primarily 4G network via mobile terminals that support dual connectivity to 4G LTE and 5G NR base stations at the same time.

To enable dual connectivity, the 4G infrastructure will support connecting to a 5G NR base station (gNodeB). The following figure shows a 5G gNodeB connected to the 4G EPC at the data plane level. The 5G gNodeB does not connect to the MME. The gNodeB does connect to the LTE eNodeB to receive requests to activate and deactivate 5G bearers.

Adding 5G data plane to existing 4G LTE sites (credit: Samsung)

The UE can connect to the LTE and 5G NR base station. A basic setup for such a scheme is:

  1. The UE attaches to the LTE network. The UE signals to the network that it can simultaneously connect to the 4G and 5G networks.
  2. The Core Network checks if the UE is authorized to connect to 4G and 5G networks. The 4G eNodeB is notified that the UE is permitted to connect to the 5G network.
  3. The eNodeB then takes a decision to activate a bearer on the 5G gNodeB.
  4. The 4G eNodeB and 5G gNodeB communicate to set up the bearer on the 5G gNodeB.
  5. The UE is notified about the 5G bearer via the RRC Connection Reconfiguration message.
  6. The UE then connects to the 5G network while maintaining the connectivity to the 4G network.


A high-level view of the non-standalone access is presented here. Click on the image to open the flow in PDF.

5G Non-standalone access overview

5G Non-standalone access detailed flow

Now let’s dig deeper. Here we examine the EN-DC dual connectivity signaling between the 4G eNB and the 5G gNB in detail. Click the following image to open the detailed flow in PDF.

LTE-5G NR Dual Connectivity (EN-DC) detailed flow

Adding EPC details in non-standalone access flow

The 4G EPC now supports the 5G traffic plane. This requires updates to several EPC signaling messages to support signaling of the higher 5G data rates. The following flow adds EPC details in the EN-DC dual connectivity flow.

LTE-5G NR Dual Connectivity (EN-DC) flow with EPC details

Role of different entities in the EN-DC flow

We have examined the dual connectivity flow in detail, now let’s look at the message flow from the point of view of:

UE role

UE role in 4G-5G dual connectivity

5G-gNodeB role

5G-eNodeB role in Non-standalone access

LTE-eNodeB role

LTE-eNodeB role in EN-DC dual connectivity

EPC role

EPC role in setting up an EN-DC session

Summarize the 5G Non-Standalone Architecture

Now that we have covered the 5G Non-Standalone session setup flow. Let’s review what we have learned in the following video.

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The signaling flows presented here were generated from a single model defined with EventStudio System Designer. Click here to learn more.