5G NR NSA ENDC Dual Connectivity Signalling end to end attach Call flow

5G NR NSA ENDC (Dual Connectivity) Signalling end to end attach Call flow

In this blog we will mainly focus on 5G NR ENDC dual connectivity call flow. Till step 8, it is completely LTE attach and i will leave that for you to read and understand. You can also click this link “Basic LTE attach and ENDC cell selection” to get some basic lte attach knowledge.

5G NR ENDC End to End call flow

Step 5. RRC connection re-configuration message

The MN-eNB sends the rrc connection re-configuration message to UE which includes the attach accept and Activate Default Bearer Request. This message also contains the “5G measurement object”.  Thus, this message indicates the UE to measure the mentioned 5g channels and send the reports.

Step 9, 10, 11. 5G cell Measurement Reports

After the LTE attach completes and the default bearers established, UE starts measuring the 5G NR cells and sends the reports back to MN-eNB. UE uses PSS and SSS to calculate the signal quality.

Step 12. X2AP SgNB Addition Request

After getting the measurements reports from UE, the LTE eNodeB decides to add the 5G-NR cell as a secondary node and  eNB sends a Secondary Node Addition Request to the gNodeB. This message carries below information:

1. RRC and Radio Bearer configuration.
2. UE capabilities and security information

Step 13. X2AP SgNB Addition Request Acknowledge

The NR cell responses with SgNB addition request acknowledge. This message includes:

1. NR rrc configuration which contains the information about radio resources and bearer configuraion.

Step 14. RRC connection re-configuration

Basically, this is NR cell RRC connection re-configuration message. The 4G cell just send the gNB rrc connection message to the UE. This message actually assigns the 5G cell to the UE.

This message contains the following information:

1. Rach parameters to attach 5G cell later.
2. C-RNTI to access 5G cell.

Step 15. RRC connection re-configuration complete

The UE inform the eNB about the receipt of the RRC connection re-configuration message. This message actually meant for the NR gNB.

Step 16. X2AP SgNB Reconfiguration Complete

LTE eNB transport the rrc complete message to the NR gNB. Once the rrc re-configuration completes for the NR cell, the LTE cell starts sending data to 5g cell and 5G cell buffered the data until bearer established with the UE. 4G eNB also informs the gNB about the PDCP SN and HFN for all the bearers that will be transferred to 5G.

Step 17. S1AP E-RAB Modification Indication

eNB notify the MME that the bearers are being witched from 4G LTE to 5G NR

Step 18. GTPv2-C Modify Bearer Request

MME requests the SGW to update the bearers.

Step 19. GTPv2-C Modify Bearer Response

SGW switched the bearer from eNB to gNB and sends the acknowledgement to MME.

Step 20. GTPv2 End Marker Packet

SGW sends the end marker to MN-eNB to mark the end of packet transfer.

Step 21. X2AP End Marker Packet

LTE eNB sends the packet marker to gNB.

Step 22. S1AP E-RAB Modification Confirmation

MME sends the bearer modification confirmation to MN-eNB.

Step 23. Data will be directly sent to the gNB as we have already created bearer from network to 5G cell.

Step 24, 25, 26. UE Acquires 5G Cell

Now UE will use  PSS for frame synchronization and SSS for subframe synchronization. It will also be able to find the physical cell identity of the 5G cell using PSS and SSS. And with the PCI, the UE will decodes the MIB and it will achieve complete down link synchronization. Please remember, in case of NSA, MIB and system information will be sent to UE as a part of rrc-reconfiguration message.

Then the UE will start RACH procedure with the 5G gNB. The Random access procedure will be non contention based if the preamble was assigned in  rrc connection re-configuration message.

Step 27. After the RACH procedure, gNB stops buffering data and  starts sending data to UE. UE also gets uplink grant in RACH response, so UE can also send UL data. I will discuss 5G RACH procedure in my next blog.