# 3GPP Technical Document Summary: Reply LS on IMS Voice over NB-IoT NTN Security

## Document Identification

- **Document Number:** S3-253797
- **Meeting:** SA3#124, Wuhan, China (13-17 October 2025)
- **Release:** Rel-20
- **Work Item:** FS_5GSAT_Ph4_SEC
- **Type:** Reply Liaison Statement

## Context

This document is SA3's response to a Liaison Statement from SA2 (S3-253118/S2-2507636) regarding security considerations for IMS voice over NB-IoT NTN (Non-Terrestrial Network) connected to EPC.

## Main Technical Contribution

### Security Overhead for Voice Media Packets over NAS

**Question from SA2 (Question 7):**

SA2 asked whether SA3 would accept eliminating the 5 bytes of NAS layer security overhead for voice media packets in the context of:
- Alternative solutions documented in TR 23.700-19
- Support of IMS voice over NB-IoT NTN connected to EPC
- Use of a specific SRB via dedicated EPS bearer for Data over NAS
- Transfer limited to voice media packets only

**SA3 Decision:**

SA3 **rejected** the proposal to eliminate the NAS layer security overhead with the following rationale:

- The majority of companies in SA3 opposed allowing non-integrity protected voice packets to be sent over NAS to the MME
- **No consensus** was reached in SA3 to eliminate the 5 bytes of NAS layer security overhead
- This maintains the integrity protection requirement for voice packets transmitted over NAS

## Implications

The decision preserves the existing NAS layer security mechanisms for voice traffic, prioritizing security over the potential bandwidth savings that would result from removing the 5-byte overhead. This is particularly relevant for NB-IoT NTN scenarios where bandwidth efficiency is typically a concern, but SA3 determined that security requirements take precedence.

## Action to SA2

SA3 requests SA2 to take this security position into account when finalizing the solutions for IMS voice over NB-IoT NTN connected to EPC.