Reply LS on the RAN simulation assumptions, bundling period and SPS for ULBC
This is a Reply LS from RAN2 to SA4 addressing questions related to protocol overhead, MAC header sizing, and SPS operation for voice transmission in NB-IoT NTN context.
RAN2 specifies the expected protocol overhead for UP-based voice transmission:
- PDCP: 1 byte
- RLC UM: 1 byte
- MAC header: Variable size (addressed separately)
RAN2 provides two scenarios for CP-based voice transmission:
Current Specification:
- RRC: 2 bytes
- RLC AM: 2 bytes (currently the only specified mode for SRB)
- MAC header: Variable size
Potential Future Enhancement:
- RRC: 2 bytes
- RLC UM: 1 byte (if introduced for SRB in case CP solution is selected)
- MAC header: Variable size
Note: RAN2 indicates that further feedback on expected average RoHC header size may be provided later.
RAN2 provides detailed guidance on MAC header sizing:
- Range: 1 to 3 bytes possible
- Most common scenario: 3 bytes total MAC header size is likely
- This clarifies SA4's question about whether 1 byte MAC header is realistic
RAN2 addresses the feasibility of 120ms and 240ms bundling periods for SPS operation in NB-IoT NTN:
Key Issue Identified:
- SPS periodicities of 120ms and 240ms do not divide 10240
- This creates specification challenges
RAN2 Position:
- Work on SPS for voice has not yet started in RAN2
- Supporting these non-standard periodicities would require additional specification work to resolve the identified issues
- Implies potential complexity and effort required if these periodicities are to be supported
RAN2 requests SA4 to take the provided responses into account in their ongoing work on ULBC.