5G non-standalone access

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

Published in
6 min readSep 16, 2018


Updated on March 15, 2019

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. Within the flow, click on the messages involving the UE, the eNB or the gNB to see the full message details.

5G Non-standalone (NSA) 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

The UE notifies the 4G network that it supports dual connectivity and then switches to the 5G bearer while maintaining the link with the 4G base station. A high-level UE show is shown below. Details can be found in the EN-DC UE role flow.

  1. The UE first attaches to the 4G network and signals to the EPC that it supports dual connectivity to 4G and 5G networks (via DCNR bit in the Attach message).
  2. The UE also signals the support for dual connectivity via the UE-MRDC-Capability container included in the UE Capability Information message.
  3. When instructed, the UE starts reporting measurements on 5G neighbors.
  4. The network assigns a 5G bearer to the UE via the RRC Connection Reconfiguration message.
  5. The UE accesses the 5G network using the random access procedure.
  6. The UE then maintains simultaneous dual connectivity to the 4G eNB and the 5G gNB.
UE role in 4G-5G dual connectivity

5G-gNB role

The 5G gNB acts as a secondary node in a dual connectivity setup. The 5G gNB provides the 5G data path.

  1. The gNB handles the secondary carrier addition request from the eNB.
  2. The gNB assigns 5G radio resources to the session and replies back to the eNB.
  3. The gNB receives a Reconfiguration Complete message from the eNB. This message signals that the UE has accepted the reconfiguration request that adds a 5G carrier in a dual-connectivity mode.
  4. The gNB then handles the random access procedure from the UE.
  5. The data flow starts on the 5G NR bearers after the 4G eNB has switched the SGW path.
5G-gNB role in Non-standalone access

LTE-eNB role

The LTE eNB plays a key role in the EN-DC session setup. A simplified sequence of actions is listed here. Details can be found in the eNB sequence diagram.

  1. The eNB handles the 4G RRC connection setup from the UE.
  2. The eNB receives the first NAS message (Attach) and transports it to the MME in the Initial UE Message.
  3. The eNB acts as a conduit for the NAS signaling between the UE and the MME for the authentication and NAS security procedure.
  4. The MME then sends the Initial Context Setup Request to the eNB.
  5. The eNB performs the UE Capability handshake with the UE (if the MME had not included the UE capabilities in the Initial Context Setup Request).
  6. The eNB then performs the AS Security Procedure and the 4G default bearer.
  7. The UE sends a Measurement Report that signals good signal quality for the 5G bearers.
  8. The eNB sends the SgNB Addition Request to the 5G gNB requesting a 5G bearer.
  9. The eNB receives the SgNB Addition Request Acknowledgement that carries the NR bearer configuration that is signaled to the UE via the LTE RRC Connection Reconfiguration message.
  10. The eNB sends the SgNB Reconfiguration Complete message to the 5G gNB on the receipt of LTE RRC Connection Reconfiguration Complete message. This message carries the NR RRC Reconfiguration Complete message as payload.
  11. The eNB then starts copying the data to the gNB. It also switches the SGW so that it can now directly start sending the bearer data to the 5G gNB.
LTE-eNB role in EN-DC dual connectivity

EPC role

The EPC checks if the UE is authorized for dual connectivity access during the attach procedure. The 4G EPC also supports switching of bearers between 4G eNBs and 5G gNBs. Refer to the EN-DC EPC call flow for details.

EPC role in setting up an EN-DC session

EN-DC message details

You can click on the individual messages in the sequence diagrams to see full details for message interactions involving the UE, the eNB and the gNB. The documents defining the messages can also be accessed directly:

Non-standalone EN-DC signaling details

5G Non-Standalone Videos

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

Overview of the Non-Standalone Access (NSA) Architecture
EN-DC: 4G signaling details, 5G measurements, and 5G initial access

Learn More

5G NR Call Flows and Tutorials