PDU and SDU

Hi everyone. Can someone explain to me the difference between SDU and PDU? It’s really confusing me. For example rlc SDu and rlc pdu And pdcp sdu and pdcp pdu…

xxxSDU is the data unit entering layer xxx
xxxPDU is the data unit exiting layer xxx

So RLC SDU = PDCP PDU
And MAC SDU = RLC PDU

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Hi O.B thank you. This is assuming transmission going downward through the protocol stack? But the transmission can go upwards at the receivr node from phy to Mac to rlc to pdcp to sdap then what happens to the naming does it change? Thanks alot for any insights. @O.B

From a layer perspective, whatever is tx/rx from upper layer is sdu , and whatever is tx/rx from lower layer is pdu.

@needoos.th thank you. So it doesn’t matter if the direction is
PHY MAC RLC PDCP
Or the direction is
PDCP RLC MAC PHY?

Does SDU PDU naming change between the two?

No - example for RLC - MAC is lower layer and PDCP is upper layer - exchange with MAC in both direction is PDU , and exchange with PDCP in both direction is SDU

The RLC layer receives packets from the PDCP layer. These packets are called PDCP PDUs from a PDCP point of view and represent RLC SDUs from an RLC point of view