5G MAC Multiplexing, Demultiplexing, and Logical Channel Prioritization
This topic explains one of the most distinctly MAC-specific behaviors in NR: deciding what fits into the available grant, in what order it is packed, and how the receiver later interprets the resulting MAC PDU.
Multiplexing and prioritization deserve their own page because they connect queue state, logical channels, control elements, and actual PDU formation. This topic becomes visible whenever traces show that not all traffic could be served equally.
| Main role | Choose and pack the right information into the available grant |
|---|---|
| Best paired with | Logical channels, BSR, PDU format, and control elements |
| Why it matters | Grant shortage makes prioritization behavior operationally important |
| Main users | Protocol stack readers, decoder users, and scheduler debuggers |
What multiplexing and prioritization do
The core behavior to picture.
Multiplexing is the process of packing selected traffic and control information into a MAC PDU. Prioritization decides which logical channels get served first when the grant cannot fit everything that is waiting.
Demultiplexing is the receiver-side interpretation of the resulting PDU, where the packed elements are separated again based on subheaders, identifiers, and parsing rules.
Why grant size changes behavior
The practical reason this topic matters.
| Situation | What MAC must decide |
|---|---|
| Grant is large enough | More channels and control information can be packed without severe tradeoffs |
| Grant is limited | Logical channel prioritization becomes visible and some information may wait |
| Control information is present | MAC must balance SDUs and control elements within the same opportunity |
Why packing order matters
The part of grant use that becomes visible in the final PDU.
When grant space is limited, packing order is not a cosmetic detail. It directly determines which service gets through now and which service waits for a later opportunity.
This is the point where logical channel priority stops being an abstract rule and becomes visible in actual MAC payload content.
How to connect this topic with BSR and the MAC PDU
The neighboring pages that complete the picture.
| Neighbor topic | What it adds |
|---|---|
| BSR | Shows what backlog existed before packing decisions were made |
| PDU format | Shows exactly how the chosen content appears in the transmitted structure |
| Control elements | Explains why some grant space may be consumed by control rather than SDU payload |
What to look for in traces
Typical decode and debugging questions.
- Which logical channels were actually served in the grant
- Whether a control element consumed part of the grant that was expected for data
- Whether backlog and BSR suggested demand that could not fit into the current opportunity
- Whether the resulting PDU matches the expected priority behavior
Why this topic deserves its own page
The standalone value.
- It is a distinct MAC behavior with decoder and troubleshooting value
- It is too detailed to explain well inside the pillar page only
- It supports examples, grant-fit tables, and real trace interpretation
FAQ
What is MAC multiplexing in 5G?
It is the process of packing selected traffic and control information into the MAC PDU for transmission.
Why does logical channel prioritization matter?
Because limited grant space means not all buffered information can always be transmitted at once.
Why should this topic be studied with the PDU page?
Because prioritization decisions become visible in the actual MAC PDU structure and parsing order.