5G NSA Call Flow :
UE (User Equipment) powers on and connects to the 4G LTE network, searching for a 5G signal. It sends an RRC Connection Request to the eNB (Evolved NodeB) with the UE’s identity.
The 4G eNB responds with an RRC Connection Setup message, containing configuration information for the UE.
UE sends an RRC Connection Setup Complete message, confirming that it’s connected to the 4G network.
The eNB sends an S1AP Initial Context Setup Request to the MME (Mobility Management Entity), including the UE’s context and 5G-related capabilities.
MME processes the request and responds with an S1AP Initial Context Setup Response, including the necessary bearers and security keys.
The eNB communicates with the 5G gNB (Next Generation NodeB) to establish a dual connection using an X2AP EN-DC
Configuration Update message, containing the UE’s context.
The gNB responds with an X2AP EN-DC Configuration Update Acknowledge message, confirming that the 5G NR (New Radio) link is established.
Now, let’s say John wants to access a high-speed video streaming service. His UE sends a Service Request message to the MME, requesting the establishment of the 5G data bearer.
The MME forwards the request to the SGW (Serving Gateway) and PGW (Packet Data Network Gateway) using a Create Session Request message.
The SGW and PGW set up the necessary data bearers and respond with a Create Session Response message, containing the 5G data bearer information.
The UE can now access high-speed data services over the 5G NR connection while still using the 4G LTE network for voice and other services.
In summary, the 5G NSA call flow involves a series of messages and IEs exchanged between the UE, eNB, gNB, MME, SGW, and PGW to establish dual connectivity, leveraging both 4G LTE and 5G NR networks for improved performance.