TM is one of the modes supported by RLC. Characteristics of TM are given below
Transparent: The TM passes transparently through the RLC layer, meaning that no header information is added by the TM entity
Single direction: The TM operates in a single direction, i.e. it is a connectionless protocol.
Segmentation: The TM can provide some limited segmentation functions where higher layer SDUs are broken into a number of RLC PDUs
Buffering and discard; The TM can provide buffering and discard functions depending upon its configuration from higher layers
Figure showing TM entities given below:
The RLC TM PDUs are sent via a number of logical channels to the MAC layer below. The logical channels used and its purpose for TM is given below
| Logical Channel | Direction | Typical Usage |
|---|---|---|
| CCCH | UE to UTRAN | Used on the uplink for initial random access (RRC connection request), Cell Update, URA Update etc., |
| DTCH | UE to UTRAN | Used for services where delay is more important than reliability, e.g. speech and video services |
| SHCCH | UE to UTRAN | (TDD mode only) Used by UE to request capacity on the PUSCH |
| BCCH | UTRAN to UE | Used to transport system information in a cell |
| PCCH | UTRAN to UE | Paging message sent to UE on downlink |
| DTCH | UTRAN to UE | Used for same purpose as uplink |
Related Topics: RLC, Channel Mapping