The operation of RLC TM Mode is very simple and easy to understand.
TM stands for Transparent Mode
Functions performed by RLC TM mode:
1. Segmentation and reassembly of RLC SDU will not be performed
2. RLC headers are not added
So from the above 2 points we can say that, RLC layer will not do any modification
As we are not doing any reassembly and ARQ operation, hence there is no overhead.
As there is no ARQ if there are any missing RLC PDU during RLC transmission, they are lost permanently.
Functions performed by RLC TM Transmitting Entity
1. When a RLC Transmitting Entity in TM mode, will forms the the TMD PDU from RLC SDU, it will not segment or concatenate the RLC SDU.
2. As it will not perform segmentation or concatenation, it will not add RLC headers in TMD PDU.
Functions performed by RLC TM Receiving Entity
1. When a RLC Receiving Entity in TM mode, will receive the TMD (Transparent Mode Data) PDU it shall deliver TMD PDU to upper layers.
Points to remember:
1. It is ideal for voice packets
2. It will not guarantees delivery of the packet
3. No RLC headers are added
4. It will not perform segmentation and reassembly of RLC SDUs.
Reference TS 36.322