LTE: Difference between RLC AMD PDU and RLC AMD PDU segment ?

Question

Can anyone please provide some detailed information about these two?

Answer 1

RLC AMD PDU is associated with AM RLC entity. Transmitting side of AM RLC entity prepares and sends AMD PDU based on the transmitting opportunity indicated by lower layer. If receiver AM RLC entity doesn't get AMD DPU or portion of AMD PDU, it sends Status Report to peer AM RLC entity. After receiving Status Report from the receiver RLC entity, transmitting RLC entity will send AMD DPU. But based on transmitting opportunity indicated by lower layer(MAC), RLC does segmentation of the AMD DPU which is being sent to peer RLC entity, to fit into the size of RLC PDU.

So that is known as "RLC AMD PDU segment"

Comments

can we concatenate one PDU segment with other SDU in a transmission opportunity?

Answer 2

RLC_AMD_PDU is just a PDU generated in the RLC_AM mode by the process of segentation and concatination(Based on the availability of resource or TBS value provided by MAC), whereas RLC_AMD_PDU_Segment is the Retransmitting PDU which is being segmented due to the change of TBS value.
Ex: If the PDU with SN = 4 and with the TBS value = 100 is sent and due to some radio link issues it has failed to reach its peer RLC , Now the peer RLC will send the NACK_SN = 4. Obviously, the transmitter RLC entity has to retransmit this PDU, But, if the TBS value is reduced than it was before, lets say 50, Now the Tx RLC has to send 2 PDUs of same SN(4) each of 50 bytes -> "and these PDUs are called RLC AMD PDU segments".

Hope You got the Answer.
Enjoy..!