Reply LS on Issues Related to Support of IMS Voice over NB-IoT NTN Connected to EPC

Document Information

• Source: RAN2
• Target: SA2 (CC: SA4, CT1, SA3, SA1, RAN1)
• Meeting: RAN2 Meeting #131bis (Prague, Czech Republic, October 13-17, 2025)
• Release: Release 20
• Work Item: IoT_NTN_Ph4-Core
• Document Number: R2-2508784
• Response to: R2-2506747/S2-2507636

Overall Description

This is a reply LS from RAN2 to SA2 addressing multiple technical questions related to the support of IMS voice over NB-IoT NTN (Non-Terrestrial Network) connected to EPC. RAN2 provides responses to specific questions raised by SA2 regarding RoHC, scheduling methods, DRB support, and SRB configurations.

RAN2 Responses to SA2 Questions

Question 1: RoHC State Fallback Trigger

SA2 Question: Does RAN2 have any observation on how many consecutive packets lost or erroneously decompressed will trigger the RoHC state fall back at the compressor when using RoHC?

RAN2 Response:
- RAN2 is not able to make any observation on this matter
- The implementation of ROHC functions (e.g., compression and decompression) falls outside of RAN2 scope

Question 3: Scheduling Methods for Dynamic Voice Packet Sizes

SA2 Question: Can RAN2 confirm whether scheduling methods for support of IMS voice over NB-IoT NTN can handle the voice packets of different sizes changing dynamically?

RAN2 Response:
- Scheduling methods may handle voice packets of different sizes through:
- Semi-persistent scheduling
- Dynamic scheduling
- Over-provisioning of resources in UL
- RAN2 will further study and discuss the details of the scheduling mechanism to avoid potential issues

Question 6: Support for More Than 2 DRBs

SA2 Question: Is it feasible to support more than 2 DRBs for a UE accessing NB-IoT in Rel-20?

RAN2 Response:
- RAN2 understands that it is technically feasible to support more than 2 DRBs for a UE accessing NB-IoT in Rel-20

Questions 9 & 10: New SRB for Voice Media with RLC UM

SA2 Question 9: Can RAN2 provide feedback on whether it is possible to define a new SRB that will be used to carry voice media?

SA2 Question 10: If the answer to the above question is yes, can RAN2 confirm whether such SRB(s) are defined, whether it is technically feasible to support and configure RLC UM for such SRB(s) for example: in NB-IoT deployments over GEO satellite, when SRBs are used to carry voice media?

RAN2 Response:
- RAN2 understands it is technically feasible to introduce SRB with RLC UM for voice packet

RAN2 Position on CP vs UP Solution

RAN2 discussed both Control Plane (CP) and User Plane (UP) solutions for supporting IMS voice over NB-IoT NTN:

• Majority preference: UP over CP solution
• Rationale: The CP solution has higher impacts on RAN2 specifications
• Note: RAN2 understands that the final decision will be taken at the checkpoint in December plenary

Actions

RAN2 respectfully asks SA2 to take the replies and information provided into consideration.