Indonesia Technology Services (ITS) ™ Your devices end-to-end testing partnership Contact: Kevin Japar (https://www.linkedin.com/in/kevinjapar/) Ario Junianto (https://www.linkedin.com/in/ariochip/) Why is VoNR falling back to GSM? Early users complained that when using 5G mobile phones to make calls, they often returned to 2G calls; testers arrived at the site to conduct simulation tests, reproduced the scenario, and confirmed that there was a situation where 5G terminal calls dropped to 2G; the test data was played back and decomposed, and the situation was as follows: 1. Event retrospective 10:07:04.207 The terminal initiates a service request (NR Service Request) on the 5G network and completes RRC establishment and random access; after service acceptance (NR ServiceAccept), VO Outbound call: Call-ID:..... 10:07:18.039 After dialling, the terminal initiates redirection from 5G to LTE (RRC Release 10:07:18.581 After successful redirection, random access, bearer establishment and TAU request are initiated on the LTE network. Edited by: Asimatorus Malau 1 Indonesia Technology Services (ITS) ™ Your devices end-to-end testing partnership Contact: Kevin Japar (https://www.linkedin.com/in/kevinjapar/) Ario Junianto (https://www.linkedin.com/in/ariochip/) 10:07:19.395 The LTE network allocates voice bearers (QCI=1) to the terminal and configures measurement data. 10:07:33.252 Call Setup Timeout EPSFB of terminal VONR fail. 10:07:40.162 The terminal initiates an extended service on the LTE network (Extended ServiceRequest) request; start the CS-fallback process and release the EPS bearer Contains QCI5,6,7, for RRC release (to GSM); Edited by: Asimatorus Malau 2 Indonesia Technology Services (ITS) ™ Your devices end-to-end testing partnership Contact: Kevin Japar (https://www.linkedin.com/in/kevinjapar/) Ario Junianto (https://www.linkedin.com/in/ariochip/) 10:07:41.323 The terminal initiates a call attempt in GSM, and the channel assignment is completed; the terminal initiates CSFB from 4G successfully. 10:07:55.229 The terminal ends the call, releases the GSM channel, and initiates reselection to the LTE network. Edited by: Asimatorus Malau 3 Indonesia Technology Services (ITS) ™ Your devices end-to-end testing partnership Contact: Kevin Japar (https://www.linkedin.com/in/kevinjapar/) Ario Junianto (https://www.linkedin.com/in/ariochip/) 10:07:55.935 The terminal returns to the LTE network, and the first TAU request is rejected. because the terminal is implicitly detached. 10:07:56.040 The terminal re-initiates the attachment request (AttachRequest), RRC establishment, authentication, NAS encryption, UE capability query and default bearer establishment. 10:07:56.826 Attach successfully (LTE AttachSuc); return to LTE to complete IMS registration, -----Reconnect to the 4G network for attachment update Edited by: Asimatorus Malau 4 Indonesia Technology Services (ITS) ™ Your devices end-to-end testing partnership Contact: Kevin Japar (https://www.linkedin.com/in/kevinjapar/) Ario Junianto (https://www.linkedin.com/in/ariochip/) 10:08:02.511 The terminal releases the LTE bearer to detach. 10:08:06.171 The terminal re-initiates the access process in 5G(NR); completes the process of staying in 5G(NR) 2. Key nodes of the event Edited by: Asimatorus Malau 5 Indonesia Technology Services (ITS) ™ Your devices end-to-end testing partnership Contact: Kevin Japar (https://www.linkedin.com/in/kevinjapar/) Ario Junianto (https://www.linkedin.com/in/ariochip/) Comparing the "key points of voice EPS fall back process under 5G (NR) network SA architecture" and "main process of VONR under 5G (NR) network SA architecture" (see attached article link), this "5G call drop back to 2G" has Three key nodes occurred respectively: 1.10:07:18.039 Terminal 5G blind redirection to LTE; 2.10:07:33.252 Call Setup Timeout EPSFB of terminal VONR is lost. 3.10:07:55.935 The terminal returns to the LTE network and the first TAU request is rejected. Reason: implicitly-detached. 3. Analysis of positioning problems 1. Compare the normal process of VONR of 5G (NR): After receiving the service request (ServiceRequest), the 5G network does not configure the terminal for redirection measurement, and the terminal initiates a blind redirection to LTE; and the terminal fails to complete the 5G network Registration and PDU establishment are also among the reasons. 2. After the terminal was redirected to LTE, the network allocated a wireless bearer for the voice call (QCI=1), but the call failed due to call timeout. Mobile VOLTE Terminal Technical Specifications"-- When a VOLTE call cannot be initiated in the VOLTE coverage area, it should work in CSFB voice mode. The terminal establishes a call in GSM through CSFB ----- complies with terminal regulations. 3. The terminal ends the EGSM call and returns to LTE for the first time TAU rejected (implicitly-detached); reconnects to the LTE network TAU successfully; this may be due to an abnormality in the SGs interface between the 2G and 4G networks, which fails to transfer the terminal information after the call ends. Reason for transmission back to 4G network. Edited by: Asimatorus Malau 6 Indonesia Technology Services (ITS) ™ Your devices end-to-end testing partnership Contact: Kevin Japar (https://www.linkedin.com/in/kevinjapar/) Ario Junianto (https://www.linkedin.com/in/ariochip/) 4. Summary of the problem In the VONR voice solution of 5G (NR), the context of the terminal's "fallback from 5G to 4G" and "return from 4G to 5G" needs to be transmitted through the N26 link; the voice CSFB of the 4G to 2G network is transmitted through the SGs link. Edited by: Asimatorus Malau 7 Indonesia Technology Services (ITS) ™ Your devices end-to-end testing partnership Contact: Kevin Japar (https://www.linkedin.com/in/kevinjapar/) Ario Junianto (https://www.linkedin.com/in/ariochip/) After the terminal's 5G wireless network access registration is normal, if there is an abnormality in the interoperation of 4G and 5G, you need to contact the core network engineer for inspection. After the terminal failed to establish the LTE network VOLTE this time, the terminal initiated the CSFB service process from 4G to GSM according to the specifications; completing the voice call service on the GSM network can be said to be the last resort method for the 5G terminal. Edited by: Asimatorus Malau 8
