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:TM

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

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