# Reply LS on RAN Simulation Assumptions for ULBC

## Document Information
- **Meeting:** SA2 #170 (Gothenburg, Sweden, 25-29 August 2025)
- **Document Number:** S2-2507578
- **Response to:** S4-251584/S2-2506169
- **Release:** Release 20
- **Work Item:** FS_ULBC

## Overall Description

This document is SA2's reply to SA4's liaison statement requesting clarification on RAN simulation assumptions for Ultra-Low Bandwidth Communication (ULBC), specifically regarding protocol overhead considerations for IMS voice over NB-IoT NTN via GEO satellite.

## Technical Contributions

### Protocol Overhead Options

SA4 requested SA2 and RAN2 to comment on different protocol stack options and their respective packet overhead, including:
- User Plane (UP) vs Control Plane (CP) options
- IP vs Non-IP PDN types
- Overall packet overhead including RTP/UDP/IP with RoHC, PDCP, RLC, and MAC headers
- Potential AS layer optimizations

**SA2 Response:**
- SA2 has documented multiple alternative solutions in TR 23.700-19 for "Key Issue #1: Support of IMS voice call over NB-IoT NTN via GEO satellite connecting to EPC"
- These solutions cover various options for:
  - CIoT EPS Optimisation (User Plane or Control Plane)
  - PDN type selection (IP or non-IP)
- **No conclusion has been reached yet** on the solutions for Key Issue #1
- Final protocol overhead depends on the selected solutions
- Conclusions and estimated overhead will be provided later
- Expected completion: December 2025 (SA#110) per FS_5GSAT_Ph4_ARC SID

### MAC Header Overhead Assumption

SA4 specifically questioned whether a 1-byte MAC header overhead assumption is realistic.

**SA2 Response:**
- SA2 defers this question to **RAN2** for proper assessment
- SA2 requests RAN2 to provide feedback on expected overhead for:
  - PDCP
  - RLC
  - MAC

## Actions

SA2 requests SA4 to take the provided answers into account for their ongoing work on ULBC RAN simulation assumptions.

## Key Takeaways

- SA2's work on protocol stack options for IMS voice over NB-IoT NTN is still in progress
- Multiple solutions are under consideration with different overhead implications
- Final overhead estimates cannot be provided until solution selection is complete
- Lower layer (PDCP/RLC/MAC) overhead questions are redirected to RAN2 for expert input
- Timeline for conclusions: December 2025