In TM mode, RLC does not perform any operations on the SDU and it is transmitted transparently.
It will only buffer data on Tx side.
No RLC header, No reordering, No segmentation, no ressembly is done.
The PDU that is transmitted is called as TMD PDU.
TM Data Transfer Procedure
A TM entity will perform below transmit operations.
1. Submit an RLC SDU without any modification to lower layers
A TM entity will perform below receive operations.
1. Deliver the TMD PDU without any modification to upper layer
RLC TM PDU
From the above image, TMD PDU will have only data field and will not consist of any RLC header.
We know that RLC layer supports 3 modes: TM/UM/AM. Now the question arises which mode to be used ?
TM/UM is used for real-time services. Because, real-time services goal is to minimize the delay and where a small amount of data loss is acceptable
AM is used for non-real-time services. Here bit of delay is tolerable but higher transmission quality is required.